Texas Tech University
Center for Pulsed Power & Power Electronics

Authors: Nathan Fryar; Kirk Schriner; Jacob Stephens; James Dickens; Andrew Young; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10494597

Abstract: An experimental study evaluated the feasibility of replacing traditional insulating gases such as SF $_{{6}}$ with C $_{{4}}$ F $_{{7}}$ N (3M, Novec 4710) in flux compression generator (FCG) applications. Currently available data indicate that Novec 4710 could offer certain performance benefits over SF $_{{6}}$ . However, the available literature is focused on low frequency (50–60 Hz) and dc at static pressures. To evaluate the performance of Novec 4710 under the pulsed dynamic pressure and temperature conditions found in an FCG, we report a performance comparison between three sets of identical FCGs using air, SF $_{{6}}$ , and Novec 4710 as the insulating gas. The generators used in this study had a single stage, directly seeded design with an armature diameter of 25 mm and a stator diameter of 46 mm. To highlight the performance of the different gases rather than any wire insulation, the stator was constructed with uninsulated wire. Furthermore, the generators were seeded aggressively, making the performance difference between the different gases more apparent. The performance was monitored with a pair of differential Rogowski coils that captured the generators’ di/dt while also using high-speed videography to capture possible gaseous breakdown signatures. The data gathered during this study indicate that Novec 4710 performs at least as well as SF $_{{6}}$ in FCG applications, if not significantly better.

Early Access Articles

Authors: B. Esser; Z. Cardenas; J. T. Mockert; J. C. Stephens; J. C. Dickens; J. J. Mankowski; A. A. Neuber; D. Friesen; D. Hattz; C. Nelson

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10485615

Abstract: With the damaging effects of electrostatic discharges (ESDs), it is important to study them within various parameter regimes relevant to real-world scenarios. One such scenario studied here is a floating dielectric with no nearby ground plane with a grounded object approaching at a high rate of speed. One may encounter this scenario when moving objects by hand. Discharge current and radiated fields are captured for discharges drawn from polytetrafluoroethylene (PTFE) and poly methyl methacrylate (PMMA) with approach speeds ranging from 20 to 100 mm/s. Previous studies have shown that at lower speeds in metal-to-metal or metal-to-ground backed dielectric geometries, the peak current and discharge distance decrease with increasing speed. However, in the speed range studied here, an increase in distance and current are observed for increasing speed. Namely, the highest speeds coincide with the highest peak currents and discharge distances of approximately 800 mA and up to 24 mm. With no grounded backing, as opposed to setups in other ESD studies, the electric field between the dielectric and an approaching electrode has a more uniform distribution with highest fields at the electrode, which is elucidated to be the driving factor in the differences revealed in this study. Two spheres of differing diameters and a conical electrode are used to draw discharges off the dielectric surfaces. Captured radiated fields via B-dot sensor, plasma imagery via intesified charge-coupled device (ICCD), and mapping of surface potential reveal that a majority of the energy lost during a discharge goes to gas losses and radiated fields.

Early Access Articles

Authors: Luke Silvestre; Jacob Stephens; James Dickens; John Mankowski; Andreas Neuber; Ravindra P. Joshi

PDF: https://ieeexplore.ieee.org/abstract/document/10033097

Abstract: This report employs a Vlasov–Poisson model to elucidate fundamental electron phase–space mechanics of a multipactor discharge from onset to saturation. At the onset of multipactor, the electron phase–space is primarily defined by sharp features in both the physical space and energy space. With increasing electron density, space-charge effects lead to debunching of the swarm in phase–space. The temporal evolution of the electron energy distribution is studied across a single impact cycle. The average and peak-to-peak saturation values for the entire first-order multipactor regime are presented. Comparisons between the third- and fifth-order multipactors highlight the nuanced similarities and differences in the energy distribution of the multipacting system. The Vlasov–Poisson approach, which neglects collisions, is well suited for such analysis since the multipactor phenomenon occurs under near-vacuum collisionless conditions. It also overcomes difficulties associated with kinetic schemes that require adequately sampling all of the electron phase–space, including sparely populated regions, or special treatments to model strong growths in carrier densities.

Journal

Authors: Tyler Watson; James Dickens; Andreas Neuber; John Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10310794

Abstract: Flux compression generators (FCGs), are unable to drive high impedance loads efficiently. To ensure that the output from an FCG can drive a high impedance load, an impedance transformation is necessary. This impedance transformation is known as the power conditioning stage of the FCG. Air core transformers were developed to eventually be used as a power conditioning stage for an FCG. Two types of air core transformers were developed, known as the C-TACT and COAX-ACT. The design and fabrication of these types of ACTs are discussed, along with experimental results and simulation. Finally, a comparison is made between the developed ACTs.

Conferences

Authors: A. Gregory; D. Wright ; H. Spencer ; J. J. Mankowski ; J. C. Dickens ; J. Stephens; A. A. Neuber

PDF: https://pubs.aip.org/aip/rsi/article/94/5/054705/2888169/An-apparatus-for-probing-multipactor-in-X-band

Abstract: Rectangular waveguides are susceptible to avalanche-style breakdown via the multipactor phenomenon. The growth in secondary electron density produced via multipactor can damage and destroy RF components. A pulse-adjustable, hard-switched modulator powering an X-band magnetron was utilized to drive a modular experimental setup that enables testing different surface geometries and coatings. Power measurements, taken via diodes, and phase measurements, facilitated via a double-balanced mixer, were integrated into the overall apparatus enabling multipactor detection with high sensitivity and nanosecond temporal resolution. The utilized 150 kW peak microwave source with 2.5 μs pulse width and 100 Hz repetition frequency allows for threshold testing without the need for initial electron seeding. This paper includes the initial results of surface conditioning of the test multipactor gap via electron bombardment.

Journal

Authors: Max Flynn; John Agan; Andreas Neuber; Jacob Stephens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10310706

Abstract: A genetic algorithm procedure used to generate and optimize electron-neutral collision cross-sections for C4 F7N is detailed. The method creates a set of cross-sections from parameterized shapes available from literature, Born approximations, and cubic splines. The most fit cross-sections generated in this way are those which, when input to a multi-term Boltzmann equation solver, best reproduce experimental drift velocity data. The cross-sections generated in this way are compared with cross-sections calculated using the R-matrix method software Quantemol-EC.

Conferences

Authors: David Saheb; Travis Wright; John Mankowski; James Dickens; Andreas Neuber; Emily Schrock; James Schrock; Jacob Stephens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10310677

Abstract: Nonlinear transmission lines (NLTLs) are a promising technology for high power microwave (HPM) generation. However, NLTLs are also typically characterized by relatively short RF pulse widths, on the order of tens to hundreds of nanoseconds. One approach to increasing the overall average microwave power is the application of short excitations in very high pulse repetition frequency, thus yielding a high effective RF duty cycle. This research details the development of a compact pulsed power driver capable of delivering up to 3 kV high voltage excitation, with pulsed widths between 100 – 500 ns, and multi-MHz pulse repetition frequency to a PCB-based NLTL HPM source.

Conferences

Authors: Jake Slattery; Austin T. Hewitt; Brandon Bywater; John Mankowski; James Dickens; Andreas Neuber; Donald Friesen; David Hattz; Craig Noltensmeyer; Neil Koone; Crystal Nelson; Jacob Stephens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10376245

Abstract: Mitigating electric and magnetic fields induced by lightning strike events is critical for the security and longevity of electrical components. Electromagnetic interference originating from a lightning strike is categorized into the low to medium frequency ranges, commonly referred to as the “magnetic coupling’’ regime. This low-frequency characteristic brings about the problem of shielding magnetic flux lines. Such shielding can be accomplished with highly conductive materials via the generation of eddy currents or by diverting the flux lines using materials with a large relative permeability. The shielding effectiveness of materials with varying thicknesses and geometry are tested using a pulse forming network having comparable characteristics to a scaled lighting strike. A four-stage Marx generator with a peak current of $\sim$33kA and a risetime of 1.4 $\mu$s is used as the pulse forming network to emulate the scaled electric and magnetic fields. The data collected from these experiments provide information and techniques that can be implemented into the design of a portable, deployable shielding enclosure.

Conferences

Authors: Brandon Bywater; John Mankowski; James Dickens; Andreas Neuber; Matthew Capps; Jacob Stephens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10310949

Abstract: Sulfur Hexafluoride’s (SF6) high dielectric strength makes it heavily relied upon as a high voltage insulating medium. However, its atmospheric lifetime of 3,200 years and overall high global warming potential has motivated a search for alternatives. One possible alternative, C4F7N (NOVEC 4710) features a dielectric strength more than twice that of SF6 yet features an atmospheric lifetime one-hundredth that of SF6. Previous research concerning breakdown characteristics of C4F7N has been primarily limited to long-timescale DC and 60 Hz AC conditions. In this study, the breakdown strength of gas mixtures comprised of 5%, 10%, and 20% C4F7N in a N2 or CO2 background were tested under pulsed, microsecond high power microwave (HPM) conditions over a range of 16 psia to 32 psia at room temperature. To achieve this, a 3 MW S-band (2.85 GHz) high-power magnetron is used to excite a traveling-wave ring resonator, which provides a nominal effective HPM power level of ~20 MW. A stepped impedance transformer is used to further increase the electric field to approximately 160 kV/cm RMS. For comparison, the reported dielectric strength of C4F7N is compared against the dielectric strength of pure SF6.

Conferences

Authors: M. Flynn; A. Neuber; J. Stephens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10481159

Abstract: Anisotropic scattering is implemented in a multi-term Boltzmann equation solver (MultiBolt v3.1.0) for the modeling of electron-kinetic behavior considering both tabulated and analytic differential cross sections (DCSs). Collision terms for isotropic scattering, forward scattering, and screened-Coulomb scattering are presented and employed for elastic and inelastically scattered electrons as well as secondary electrons generated from ionization events. Swarm parameters (electron mobilities, velocities, reaction rates, and diffusion coefficients) are calculated and compared against experimental data to assess the influence of different scattering treatments on the macroscopic observable quantities. The assumption of isotropic and forward scattering models, as an approximation of the DCS more broadly, is evaluated for these conditions.

Conferences

Authors: M. Mounho; R. Clark; W. Brooks; M. Hopkins; A. Neuber; J. Stephens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10481136

Abstract: Anode-initiated high-voltage vacuum surface flashover is a prominent failure point in large-scale (megavolt and mega-ampere) pulsed power machines. To investigate the underlying basic mechanisms of anode-initiated vacuum surface flashover, a moderate-scale test platform was developed at the hundreds of kilovolt level to emulate the conditions in large-scale machines. This system comprises a positive 45-degree wedge of insulator placed between a hemispherical anode with a 2-cm radius and a planar cathode, approximating a normalized electric field profile found in large-scale pulsed power insulator stacks. An 8-stage Marx generator that produces a pulse up to 240 kV with a ~50 ns risetime is used to provide the high voltage excitation; experiments are conducted across 2 to 6 mm sized flashover gaps at a ~ 10−6 Torr background pressure. This study reports the statistical characterization of high voltage vacuum surface flashover, high speed and time-integrated imaging of flashover development, and temporally resolved emission spectroscopy from 200 nm to 600 nm of self-produced light emanating from these flashover events. Future steps towards an upgraded experimental platform with 600 kV capability are discussed.

Conferences

Authors: D. Wright; A. Gregory; H. Spencer; J. Mankowski; J. Stephens; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10481093

Abstract: The increased need for multi-carrier signals and higher power requirements has made it essential to study the multipactor (MP) phenomenon in practical structures. A 3-stage rectangular waveguide filter has been designed and implemented in a plug-and-play test fixture for X-band frequencies. The test source for this system is a coaxial magnetron, which yields a peak power output of 150 kW at a frequency of 9.4 GHz and a pulse width ranging from 0.25 to 2.5 μs. Global power diagnostics and phase detection methods were employed to detect MP in the system.

Conferences

Authors: Matthew Hopkins; William Brooks; Raimi Clark; Zakari Echo; Ronald Goeke; Christopher Moore; Michael Mounho; Andreas Neuber; Jacob Stephens; Jacob Young

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10200954

Abstract: This presentation describes a new effort to better understand insulator flashover in high current, high voltage pulsed power systems. Both experimental and modeling investigations are described. Particular emphasis is put upon understand flashover that initiate in the anode triple junction (anode-vacuum-dielectric).

Conferences

Authors: John Mockert; John Mankowski; Benedikt Esser; Donald Friesen; Blake Havens; David Hattz; James Dickens; Crystal Nelson; Jacob Stephens; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10310963

Abstract: A system to map surface charge densities on dielectric objects – e.g. PTFE, PMMA, and PA6 – is described to study maximum surface charge limits discharge and decay characteristics. Two electromechanical movements are used to achieve these goals: a three-axis robotic arm with an electrostatic probe and a three-axis auxiliary movement fitted with a current viewing resistor and laser displacement sensor. Dielectric objects no larger than a cylinder of eight-inch diameter and twelve inches in height are analyzed to create surface point clouds and generate a set of scan points. The arbitrary object is positioned upon a rotational axis with a laser displacement sensor oriented radially towards the rotational axis of the platform to determine the object’s physical limits.

Conferences

Authors: Michael Mounho; Raimi Clark; William Brooks; Matthew Hopkins; Andreas Neuber; Jacob Stephens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10310812

Abstract: Statistical characterization of vacuum surface flashover is achieved by electrically stressing a positive 45° wedge of cross-linked polystyrene (Rexolite) with a fast, high-voltage pulse. The insulator is placed between a hemispherical anode with a 2 cm radius extending into a parallel plate planar cathode. This configuration is intended to confine the flashover plasma to a ~1 cm wide region on the surface of the insulator for repeatable optical access. The voltage source comprises an 8-stage Marx generator that can provide a pulse up to 264 kV with a ~50 ns risetime. Two capacitive voltage dividers are used as voltage diagnostics for the system. They are placed on the high-voltage coaxial cable entering the vacuum chamber and on the ground side of the flashover gap. A current viewing resistor (CVR) is placed into the outer shield of the high-voltage coaxial cable as the current diagnostic. A DSLR camera is employed to capture a time-integrated image of the flashover event. Pulse voltage, current, and risetime statistics are collected and reported for a statistically significant sample size for a 2.5 mm gap.

Conferences

Authors: Raimi Clark; Michael Mounho; William Brooks; Matthew Hopkins; Jacob Stephens; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10310987

Abstract: Temporally resolved emission spectroscopy capturing the light emission from 200 to 800 nm is utilized to assess the behavior of pulsed anode-initiated surface flashover in vacuum. The limiting failure mechanism at vacuum-insulator interfaces tends to be surface flashover since it occurs at field thresholds lower than bulk insulator breakdown or bridging plain vacuum gaps. Of particular interest are insulator geometries whose surface is angled to the electric field such that the electrons are pulled away from the surface. This often-dubbed “positive” surface geometry is known to outperform planar or negative angle geometries under most circumstances. This research examines a positive 45° insulator system subject to conditions that emulate those experienced in the insulator stacks of large-scale pulsed power machines, such as Sandia National Laboratory’s Z-machine: voltage risetimes in the tens of nanoseconds, pressures in the single microTorr, and peak electric fields of hundreds of kilovolts per centimeter at the anode triple junction. Cross-linked polystyrene (Rexolite) insulators were tested under these conditions and investigated with spatiotemporally resolved spectroscopy. Carbon ions are identified which originate from either the insulator itself or from adsorbed gases, as well as aluminum and magnesium ions from the metal electrodes. Simulated spectral lines of the carbon ions show agreement with the measured spectra for a Boltzmann temperature of 2.75 eV, while metal species require a reduction in temperature of at least 1 eV in order to match. These results lend some insight into the non-equilibrium nature of the early stages of anode-initiated flashover.

Conferences

Authors: Silvestre, L; Shaw, ZC; Sugai, T; Stephens, J; Mankowski, JJ; Dickens, J; Neuber, AA; Joshi, RP

PDF: https://iopscience.iop.org/article/10.1088/1361-6463/ac2c38/pdf

Abstract: Multipactor is studied based on the coupled Vlasov-Poisson equation set and applied to a parallel plate geometry. This approach can be considered complementary to the particle-in-cell (PIC) methods that have provided excellent insight into multipactor behavior. However, PIC methods have limitations in terms of 'particle noise,' which can affect electron energy distribution functions and create re-scaling issues under conditions of charge growth. Utilizing our continuum Vlasov-Poisson approach yields susceptibility curves that are in line with reports in the literature, Spark3D PIC simulations, and experimental data. Playing to the strength of the Vlasov-Poisson approach, the differences between various multipactor orders are elucidated as they are observed in phase-space, revealing electron density dynamics without requiring increased computational resources due to electron growth. The method presented is general and can be extended to multi-input excitations and higher-dimensional phase-space.

Journal

Authors: S. A. Watkins, R. J. Lee, T. H. Austin, J. Mankowski, J. Brinkman, J. Dickens, A. A. Neuber

PDF: https://onlinelibrary.wiley.com/doi/full/10.1002/prep.202200010

Abstract: While polymer-bonded explosives, PBX, have reduced sensitivity to ignition from mechanical shock or heating compared to conventional explosives, the investigation of the mechanical ignition mechanisms for PBX remains vital to assessing the safety during machining and general handling. Under frictional heating, hot spot generation due to high melting point contaminants in the interface is a suspected source for increased probability of ignition. To investigate such frictional heating, samples of PBX 9501 and PBX 9502 were impacted and skidded against a tangentially moving target and thermally imaged. Temporally resolved temperature and forces were simultaneously measured with and without 300 μm silica grit at the frictional interface. A trend of increasing temperature was observed as the speed and tangential force on the sample increased. Grit particles in the frictional interface were found to act as frictional heat concentration spots owing to the grit‘s protrusion from the surface and lesser susceptibility to melting compared to the PBX. The coefficient of friction between PBX 9501 and the skidding surface was observed to be largely constant for forces and speeds at the lower end of the test range. In contrast, at high tangential speeds, the coefficient was found to be significantly lower.

Journal

Authors: Xiaoli Qiu; Benedikt Esser; Ivan Aponte; John Mankowski; James C. Dickens; Andreas A. Neuber; Ravi P. Joshi

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9925595

Abstract: The behavior of the breakdown electric field versus gap lengths (in the 1–5-mm range) and at different frequencies in the 1–80-MHz span, has been studied numerically at atmospheric pressure. Unlike previous studies of radio frequency (RF) breakdown, the role of photon-emission processes is explicitly included and shown to be important for large-area electrode configurations. Numerical analysis based on Monte Carlo calculations is used to predict the breakdown thresholds. The simulations embed a statistical photon transport model, based on random selections of emission angles and times from excited atoms, as well as photoemission from the electrodes. Simulation results compare well with experimental data from our group, but only with the inclusion of photon processes. Though both photoemission and photoionization are included in the breakdown physics, the former is identified as the dominant process. The frequency behavior of breakdown fields is also assessed with the inclusion of photons, and the results reveal a U-shaped trend with increasing values for smaller gaps.

Early Access Articles

Authors: Tyler Buntin; Matthew Abide; Andreas Neuber; James Dickens; Ravindra Joshi; John Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9893540

Abstract: Most high-power microwave (HPM) sources, such as the magnetically insulated transmission line oscillator (MILO) being developed at Texas Tech, utilize cold cathodes that generate electrons via explosive emission. Highly emissive cathodes such as the presented can generate current densities and currents greater than 1 kA/cm 2 and 10 kA, respectively, which are required for devices that can output radio frequency (RF) power greater than 100 MW. Typically, these cathodes are made of materials such as metal, silk or synthetic velvet, carbon fiber, and cesium iodine (CsI)-coated carbon fiber. In order to optimize the MILO performance, we fabricated carbon fiber velvet cathodes and compare their performance with other commercially available carbon fiber cathodes. Fabrication was done on a manual, mechanical loom using commercially available carbon fiber thread. Four carbon fiber cathodes were tested: in-house fabricated monomodal carbon fiber velvet, in-house fabricated bimodal carbon fiber velvet, in-house fabricated carbon fiber plain weave cloth, and bimodal carbon fiber velvet manufactured by ESLI Inc. Testing was performed in a vacuum chamber with variable AK gap in the high vacuum range (10−7 torr). High-speed optical imaging was performed in order to determine the uniformity of the generated plasma as well as the e-beam. Voltage and current measurements were performed to determine diode impedance and perveance.

Journals

Authors: W. Brooks; R. Clark; J. Young; M. Hopkins; J. Dickens; J. Stephens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9779121

Abstract: Surface flashover in vacuum imposes a substantial physical limit on modern, large-scale pulsed power. One of the ramifications is a minimum size requirement for new machines, which in itself becomes a hard barrier to the modernization and improvement of existing infrastructure. Pulsed power topologies require the physical mechanisms of both anode- and cathode-initiated flashover to be considered. Originally, the geometrical implications of field emission at the cathode triple junction (CTJ) motivated the usage of configurations that avoid electrons impinging on the insulating material. This will largely suppress the cathode-initiated flashover, which is best described by the secondary electron avalanche mechanism, gas desorption, and final breakdown in the desorbed gas. It depends on the cascade growth of a conducting plasma along the length of the insulator from the cathode. Mitigating the cathode-initiated flashover typically comes at the cost of a significant field enhancement at the anode triple junction (ATJ). In a typical implementation, the anode field may be three times higher than the cathode field for a given voltage, making the corresponding anode-initiated flashover much more common than otherwise. In the case of pulsed, anode-initiated flashover, experimental evidence suggests that charge is directly extracted from the insulator resulting in the insulator taking on a net positive charge advancing the anode potential. Along with accompanying gas desorption from the surface, the potential will then propagate from the anode toward the cathode until the effective length of the gap is sufficiently reduced to support flashover. The underlying physical mechanisms of cathode- and anode-directed flashover are discussed in light of previously gathered experimental data and recent experiments with pulsed, high-gradient, anode-initiated flashover.

IEEE Early Access Articles

Authors: Henry John Gaus III; David H. Barnett; Andreas A. Neuber; John J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10099398

Abstract: An experiment for imaging detonators used in explosively driven pulsed power applications with high-speed, short-exposure-time cameras will be described in the work to follow. Three, commercially available, high intensity, pulsed xenon light sources (> 107 candela intensity) yielded unsatisfactory image quality with a minimum exposure rate (~320 k frames per second). Above 320-k frames per second, the combined output intensity of the light sources was too dim. Due to this limitation, a lamp system was designed that would be capable of delivering higher light intensity to the target. Two types of lamp arrays were designed and tested. The first was a large lamp array comprised of a two high-energy flash lamps, while the second was a small lamp array comprised of many low-energy flash lamps. The large lamp array was intended for multiple shot use and was placed behind a protective sheet of polycarbonate to separate the bulbs from the detonator. The second small lamp array with low-cost flash lamps was intended for one-time use and will be placed closer to the detonator. Multiple five-stage, Rayleigh Pulse Forming Networks (PFNs) were developed to find the optimal energy for the flash lamp array. Each PFN was modeled using LTSpice circuit simulator to verify proper operation and help with optimization. Experimental measurements were taken of the PFN voltage and current outputs and compared to simulated values. A photodiode was used to measure relative light intensity from the different lamp arrays.

Conferences

Authors: William Brooks; Micah Lapointe; Landon Collier; John Mankowski; James Dickens; David Hattz; Neil Koone; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9664800

Abstract: Investigation of lightning strikes to conductors ran through long spans of rigid steel conduit was performed. An overdamped-exponential current waveform with controlled peaks and rise rates was used to inject simulated lightning strikes. The impact of the length of wire, length of conduit, grounding location/s, and load type was investigated. Breakdown of 600 V, 12 AWG, THHN insulated wire (3.23 mm OD, 2 mm conductor diameter) was observed for voltages above 45 kV. The presence of resistive loads (between wire and ground) in excess of 20 $\Omega $ or current rise times in excess of 5 kA/ $\mu \text{s}$ were found to consistently produce breakdown between wire and conduit. Practical power circuit elements such as outlets and splices were found to breakdown at voltage levels much below the wire insulation failure threshold.

Journals

Authors: A. T. Hewitt; B. Esser; R. P. Joshi; J. Mankowski; J. Dickens; A. Neuber; R. Lee; J. Stephens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9700732

Abstract: Three different isolator topologies utilizing photoconductive (PC) elements are explored for their application as a controllable attenuator for a Ka-band radar system. Network analyzer measurements are reported for each geometry in the unilluminated case, while a high-speed, high dynamic range heterodyne detection apparatus is used to measure the transient attenuation behavior of the isolators when illuminated. The electromagnetic characteristics of the illuminated isolators are demonstrated to be in good agreement with COMSOL Multiphysics simulations. Two of the isolator topologies rely on the PC element becoming highly reflective to achieve high isolation, which in turn requires high optical power and charge carrier density (~1017 cm $^{-3}$ ). For the optical power available here (100 W), the first device demonstrated a peak attenuation of 53 dB, while the second device achieved only 33 dB. In the third topology, RF propagation is parallel to the major dimensions of the PC element. As a result, superior isolation is achieved with the PC element in the primarily absorbing state, associated with significantly lower carrier concentration (~1015 cm $^{-3}$ ). This device achieved 63 dB of attenuation for only 3 W of optical power, demonstrating that PC technologies may be competitive with other isolator technologies with some notable advantages.

Journals

Authors: Nicholas Harrison; Kaylee Allen; James C. Dickens; Andreas A. Neuber; John Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10099412

Abstract: The RADAN series-based MG-4 Microwave Generator is a compact, high power microwave system developed by the Institute of Electrophysics in Ekaterinburg. The system features the RADAN high voltage generator which is a SINUS-series device featuring a Tesla transformer charger and a Blumlein pulse forming line. The MG-4 microwave head is a mm-band relativistic backward wave oscillator (BWO) that operates at 35 GHz with a 5 to 10 MW peak output power and a pulse width of 3 ns. To confirm the RF output power level of the MG-4, a commercially available RF envelope detector was employed. Analog Devices ADL6012 is a broadband envelope detector that operates from 2 GHz to 67 GHz at input powers up to $+15 \ \text{dBm}$, It also features a 500 MHz envelope bandwidth and 0.6 ns output risetime capability. The diagnostic setup features the ADL6012-EV ALZ, an evaluation board with the ADL6012 offered by Analog Devices, shielded in a fitted brass box located in the far field $(\sim 60$ cm) from the microwave output horn. The output mode of the MG-4 is nominally TM0l, but a mode convertor allows for a TE11 output mode as well. The positive and negative envelopes of the pulse are captured separately. Based on the peak differential output voltage of the positive and negative signal, the input power of the detector can be determined by the typical performance characteristics curves in the ADL6012 data sheet. Lastly, accounting for the attenuators, antennas, and free space path loss, the peak output power of the MG-4 can be accurately determined.

Conferences

Authors: L. Silvestre; R. Joshi; J. Stephens; J. Dickens; J. Mankowksi; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9813001

Abstract: The impact of secondary electron yield (SEY) variations on multipactor in a parallel plate geometry is probed. In this contribution, electron swarm dynamics are simulated via the continuum approach across a variety of SEY curves. The objective is to determine which changes and shifts in the SEY curve are most sensitive to the final multipactor outcome. For instance, will a variation in the maximum SEY yield while retaining the first crossover energy produce significant changes in the MP susceptibility? To parameterize, the probed SEY curves are approximated by suitable square, triangular, or trapezoidal shapes and fed into the Vlasov-Poisson-based MP model to evaluate the impact of chosen energy-dependent deviations on multipactor. A shaped approximation of the SEY curve has already shown to produce signifigant changes in multipacotr susceptibility especially in the 1 st order regime. Variations in the 1 st and 2 nd crossover points of the SEY curve are also compared within the same susceptibility graphs. The results of this Vlasov-Poisson method are benchmarked against commercial software, particularly Spark3D. The results obtained and implications of SEY deviations on multipactor will be presented and discussed.

IEEE Conferences

Authors: M. Flynn; A. Neuber; J. Stephens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9733141

Abstract: Low temperature plasma (LTP) models, such as fluid and global models, frequently rely on Boltzmann equation (BE) solvers for the fast calculation of rate and transport coefficients from cross section data. While multi-term BE solvers currently see strong support in the kinetic modeling community, two-term BE solvers remain the preferred choice in the greater pulsed power and plasma science community. The limitations of the two-term approximation are well known, yet it is still regularly assumed that the two-term approximation is suitable for applications in LTP models. Recent studies have demonstrated otherwise where, even in a simple 1D fluid model, the use of multi-term derived rate and transport coefficients yielded superior fluid model results compared to the same model using two-term derived rate and transport coefficients.Much of the reluctance in adopting multi-term BE solvers may be attributed to a lack of publicly available tools. Here, we report on the public release of MultiBolt v3.0.0 as a free and open-source C++ header library as well as its pre-compiled terminal-based executable and graphical user interface. We also report on the inclusion of new functionality in temperature dependence, superelastic collisions, and the object-oriented treatment of both discrete and analytic cross sections.

IEEE Conferences

Authors: Hewitt, AT; Lee, RJ; Watkins, S; Brinkman, J; Stephens, JC; Dickens, JC; Neuber, AA

PDF: https://aip.scitation.org/doi/10.1063/5.0043825

Abstract: A remote-operated apparatus for testing the detonation sensitivity of energetic materials is detailed. Using an air ram and rotating disk, the normal force and transverse velocity of the impact plane are controlled independently, enabling the exploration of varying impact conditions over a wide parameter space. A microcontroller local to the apparatus is used to automate apparatus operation and ensure temporal alignment of the impacting ram head with the rotating disk. Calculation of the firing parameters and issuing of operational commands are handled by a remote computer and relayed to the local microcontroller for execution at the hardware level. Impact forces are taken from fast strain measurements obtained from gauges incorporated into the ram head. Infrared imaging of explosive samples provides insight into the peak thermal temperatures experienced at the sample surface during the impact event.

Journal

Authors: M. Flynn, Max, A. Neuber, and J. C. Stephens

PDF: https://doi.org/10.1088/1361-6463/ac29e7

Abstract: The accurate calculation of DC breakdown voltage thresholds solely from elementary electron-neutral interactions in complex gas mixtures using a multi-term Boltzmann equation (BE) kinetic model is demonstrated. SF6:N2 mixtures in the 100 Td < E/N < 400 Td field regime are explored to benchmark the model's effectiveness. A ten-term BE model is found to yield DC breakdown voltages which, on average, agree within 3% of experimental measurements. A two-term BE model is also applied in order to characterize the error introduced in all calculations by the two-term approximation. These discrepancies are largest in pure N2 where error is greater than 10% for diffusion coefficients, within 6% for particular vibrational rate coefficients, and within 5% for breakdown voltages. However, this error falls to within 1% for most parameters and breakdown voltages in mixtures with large SF6 content.

Journal

Authors: Zachary Cardenas; Ben Esser; Ivan Aponte; James Dickens; John Mankowski; Jacob Stephens; Donald Friesen; Crystal Nelson; Neil Koone; David Hattz; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9733134

Abstract: Characterization of surface charge decay for varying materials – PTFE, Acrylic, and metals – of 100 mm diameter, radially symmetric bodies was performed. In a gaseous environment, charged materials will lose their charge depending on gas type, humidity, and other environmental factors. Eventually, the charge will be (1) neutralized by ions or electrons in the surrounding gas (2) distributed by conduction on the material surface or in its volume (3) or neutralized by ions or electrons in the material surface and volume. Metal and polymer samples are charged to a known potential in the tens of kilovolts range, and the decaying voltage is monitored with a non-contact probe until the potential reaches near zero. Humidity has an impact on both groups of materials. As a baseline, in air at 42% relative humidity in an enclosed chamber, a positive charge equivalent to 20 kV potential on a metal sphere of 50 mm radius decays to near zero within 25 minutes. In contrast, the same sphere negatively charged decays slightly faster within 20 minutes. Overall, the humidity has a significant effect on the charge decay time, such that an order of magnitude longer times (hours) are observed at low humidity. Effects of chamber size were also studied, drawing a connection to charge carriers in the air available for decay, with chambers of volume(s): 3.3 105 cm3 and 1.4 106 cm3, respectively.

Conferences

Authors: Aponte, IA; Esser, B; Dickens, JC; Mankowski, JJ; Neuber, AA

PDF: https://aip.scitation.org/doi/10.1063/5.0061663

Abstract: Unipolar (DC) and radio frequency (RF) corona at 3.3 MHz is studied at centimeter-sized gaps in a needle-plane geometry in atmospheric air at room temperature. Positive and negative corona using pure tungsten electrodes with varying tip angles revealed a lower onset voltage for the needle with the smaller included angle. The RF corona onset voltage and corresponding time delay were measured for a series of needles composed of pure tungsten or 2% lanthanated tungsten. The corona onset, established when the first instance of UV photon emission is detected via photomultiplier tube, occurred primarily during the negative half cycle of the applied RF voltage for pure tungsten needles. In contrast, with lanthanated tungsten needles, such preference was not observed. No distinguishable difference in onset voltage between pure tungsten and lanthanated tungsten was found, indicating that adding a small amount of lanthanum to tungsten has a negligible impact on the onset voltage at 3.3 MHz frequencies for electrodes at room temperature.

Journal

Authors: M. Gaddy, V. Kuryatkov, N. Wilson, A. Neuber, R. Ness, and S. Nikishin

PDF: https://www.mdpi.com/2079-9292/10/13/1600/pdf?version=1625298532

Abstract: The suitability of GaN PCSSs (photoconductive semiconductor switches) as high voltage switches (>50 kV) was studied using a variety of commercially available semi-insulating GaN wafers as the base material. Analysis revealed that the wafers’ physical properties were noticeably diverse, mainly depending on the producer. High Voltage PCSSs were fabricated in both vertical and lateral geometry with various contacts, ohmic (Ti/Al/Ni/Au or Ni/Au), with and without a conductive n-GaN or p-type layer grown by metal-organic chemical vapor deposition. Inductively coupled plasma (ICP) reactive ion etching (RIE) was used to form a mesa structure to reduce field enhancements allowing for a higher field to be applied before electrical breakdown. The length of the active region was also varied from a 3 mm gap spacing to a 600 µm gap spacing. The shorter gap spacing supports higher electric fields since the number of macro defects within the device’s active region is reduced. Such defects are common in hydride vapor phase epitaxy grown samples and are likely one of the chief causes for electrical breakdown at field levels below the bulk breakdown field of GaN. Finally, the switching behavior of PCSS devices was tested using a pulsed, high voltage testbed and triggered by an Nd:YAG laser. The best GaN PCSS fabricated using a 600 µm gap spacing, and a mesa structure demonstrated a breakdown field strength as high as ~260 kV/cm.

Journal

Authors: A. Gregory; Z. C. Shaw; D. Wright; J. Mankowski; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9733122

Abstract: The multipactor effect is detrimental in space-based RF systems through detuning, heating, and causing permanent component damage. Studying thresholds and suppression of multipactor utilizing surface geometries in structures akin to WR-90 waveguide are of specific interest in the presented work. Operating in the dominant TE10 mode, a copper stepped impedance transformer transitions the waveguide to a 1.5 mm height, providing a frequency-gap product conducive for multipactor ignition. The stepped impedance transformer housing is designed as a plug-and-play system, allowing for quick swapping different stepped impedance transformer heights or materials. Input power is injected into the test gap from a coaxial magnetron, operating at 9.4 GHz with a peak power of 250 kW and 2.5 µs pulse width.The setup will enable the measurement of base thresholds for a machined and polished copper surface (material that has undergone a simple cleaning process only), as well as thresholds for multipactor suppression geometries. The testing apparatus details, including the phase-sensitive diagnostics, are presented

IEEE Conferences

Authors: Qiu, X; Saed, MA; Mankowski, JJ; Dickens, J; Neuber, A; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/5.0029859

Abstract: Mitigation of multipactor in waveguides is of importance, and strategies have included the addition of external fields, materials engineering, or surface modifications. Here, geometry modifications of rectangular waveguide surfaces and the application of an axial magnetic field are investigated for suppressing multipactor growth. A Monte Carlo approach has been used to simulate electron dynamics. The empirical secondary electrons yield is modeled based on a modified Vaughan approach. The electric fields driving electron transport were derived from separate electromagnetic calculations to adequately include field perturbations due to the presence of surface patterns in the rectangular waveguide structure. Combinations of grooves and a DC magnetic field are shown to effectively mitigate multipactor growth at field strengths up to similar to 10(5) V/m. Finding optimal combinations for an arbitrary field and operating frequency requires further work.

Journal

Authors: J. Williams; E. Weeks; R. Clark; S. Watkins; J. Dickens; J. Mankowski; J. Brinkman; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9733143

Abstract: Explosive driven pulsed power performance benefits from modern polymer-bonded explosives. In practical application, high precision in the dimensional shape is required, and high machining speeds are desired. The limits of machining speed, driven mainly by the thermal response of the energetic material, are investigated. Specifically, the thermal response of PBX 9501 (95% HMX, 2.5% Estane, 2.5% BDNPA/BDNPF by weight) and PBX 9502 (95% TATB, 5% Kel F-800 by weight) under conventional milling methods is examined. The presented work focuses on face milling performed with dry machining on a CNC, remote-controlled milling machine. The primary parameters of interest are the spindle speed, feed rate, step size, and depth of cut, with additional consideration given to endmill diameter, milling technique, and the number of flutes. The temperature of the system is monitored via high-speed IR videography and near the cutting interface with a K-type thermocouple inserted into the endmill’s through-coolant hole. Operational forces and torques are recorded by a 6-axis force sensor mounted beneath the HE samples.An empirical relationship between temperature and the milling parameters is presented. Overall, milling regimes exist outside of DOE-STD-1212-2019 for which milling temperatures remain well below the HE critical temperatures.

IEEE Conferences

Authors: M. LaPointe; B. Esser; I. Aponte; Z. Cardenas; J. Dickens; J. Mankowski; J. Stephens; D. Friesen; C. Nelson; N. Koone; D. Hattz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9733129

Abstract: Undesired accumulation of charge on dielectric materials causing electrostatic discharges can be an issue in pulsed power systems and electrical systems in general. As such, an understanding of surface charge distribution, charge accumulation, and decay is required. An in-house no-contact electrostatic probe designed with a 2 mm resolution was fabricated to measure and map surface charge distribution. Designed as a contactless instrument, the probe consisting of a metal rod and low leakage amplifier circuit probes the electric potential near the surface. A scan of the surface then provides the raw data, a potential distribution. The actual probe response - i.e., the surface charge to voltage transfer function, is accounted for in post-processing. This is accomplished via an Inverse Wien Filter - a technique often applied in image processing - to deconvolve the probe response from the measured data. A commercially available electrostatic probe, the Trek 341B meter with a 3455ET probe, capable of measuring +/- 20 kV was compared to the in-house probe that is designed to cover a wider range from +/- 35 kV. A resolution better than the simple probe resolution is achieved through the distribution’s scanning voltage method and deconvolution. Applying repeated scans, surface charge decay was tracked on various polymer materials to determine the material and environment dependence; materials included were PA6, PTFE, and others. As an example of material dependence, samples charged to 20 kV at 65% humidity experience full charge decay in approximately 45 seconds for PA5 (152 mm dia.), while it took about 100 times longer for PTFE (51 mm dia.) to observe the same decay/redistribution of charge.

IEEE Conferences

Authors: Raimi Clark; Jacob Young; William Brooks; Matthew Hopkins; John Mankowski; Jacob Stephens; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9733139

Abstract: Early light emission provides information about the dominant mechanisms culminating in vacuum surface flashover (anode-initiated vs. cathode-initiated) for particular geometries. From experimental evidence gathered elsewhere, for the case of an insulator oriented at 45° with respect to the anode, anode-initiated flashover is believed to dominate since the field at the anode triple point is roughly three times that of the cathode. Similar to previous work performed on cathode-initiated flashover, light emission from the voltage rise through the impedance collapse is collected into two optical fibers focused on light emanating from the insulator in regions near the anode and cathode. The optical fibers are either connected to PMTs for spectrally integrated localized light intensity information or to a spectrograph used in conjunction with an ICCD camera. Challenges associated with localizing the flashover for optical diagnostics and incorporating the optical diagnostics into the high-field environment are discussed. Initial results for cross-linked polystyrene (Rexolite 1422) support the premise that flashover is initiated from the anode for these geometries, as early light from the anode leads cathode light up to photocathode saturation. Early spectroscopy results show promise for future characterization of the spatio-temporal development of emission from desorbed gas species across the insulator surface and identification of bulk insulator involvement if it occurs.

Conferences

Authors: R. M. Clark, J. Brinkman, and A. A. Neuber

PDF: https://onlinelibrary.wiley.com/doi/full/10.1002/prep.202000330

Abstract: Polymer-bonded explosive (PBX) 9502 (95 % TATB, 5 % Kel F-800 by weight) is dry-drilled on a CNC milling machine and its thermomechanical response to varying feed rates, drilling speeds, and peck depths with 4 mm and 5 mm diameter drill bits is investigated. The tested samples are affixed to a force sensor that enables recording temporally resolved cutting forces and torques, and a drill-embedded thermocouple yields local temperature data. From the data, an empirical relationship between temperature changes and feed per revolution is developed, which reveals reduced temperatures in higher feed per revolution regimes for PBX 9502. The observed relationship allows extrapolating to temperatures for other hole diameters, indicating increased temperature for smaller diameter drilling across the board. Additional testing was performed with PBX 9501 (95 % HMX, 2.5 % Estane®, 2.5 % BDNPA/BDNPF by weight), albeit over a reduced parameter space, which revealed the opposite behavior for the feed per revolution temperature dependence. It is concluded that both PBX 9502 and PBX 9501 can be dry-drilled efficiently beyond the limits of presently applicable US-DOE standards, where cutting interface temperatures remain far below material critical temperatures. Finally, data reveals that coolant usage in the drilling process for these materials provides a wide safety margin.

Journal

Authors: Esser, B; Shaw, ZC; Dickens, JC; Neuber, AA

PDF: https://aip.scitation.org/doi/10.1063/5.0009836

Abstract: A high power, pulsed RF source designated for use in multipactor research is described. Four gallium nitride high electron mobility transistors from Cree/Wolfspeed, capable of 700 W in long pulse mode (500 W rated output), are combined to achieve a maximum rated output of 2.8 kW with a pulse length of similar to 100 mu s. Custom splitters/combiners are used due to the power levels considered in addition to a custom power and sequencing control system to ensure the proper biasing and sequencing of the relatively delicate depletion mode GaN devices. With high efficiency and small size, gallium nitride devices present a good solution for lab based sources, and this paper aims to provide information helpful in the construction of such a source. The multipactor phenomenon itself is studied within a high impedance waveguide section-achieved with a tapered impedance transformer-placed in a WR284 traveling wave ring resonator, which increases the effective power up to a factor of 20, or similar to 40 kW.

Journal

Authors: I. A. Aponte; B. Esser; J. C. Dickens; J. J. Mankowski; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9717642

Abstract: DC and RF breakdown at 3.3 MHz was studied in centimeter size gaps − 1 − 10 mm – with comparison to small gaps in literature and Monte Carlo simulations in atmospheric conditions. As a point of reference, DC breakdown using stainless-steel Bruce-profile electrodes were measured to compare with RF measurements. RF breakdown with a slow rising envelope ($\sim 5\ \text{mV}/\mu\mathrm{s}$) yielded approximately 80% of DC measurements (∼25 kV/cm in 5 mm gap), which agrees with Monte Carlo simulations and results found in literature. Increasing the envelope rise time to greater than $1000\ \mathrm{V}/\mu \mathrm{s}$ yielded ∼120% of DC measurements (∼37 kV/cm in 5 mm gap). Comparisons with Monte Carlo simulations which included photon processes – theorized to be critical to obtaining accurate results – furthered understanding of the processes involved in pre-ionization before breakdown occurs.

IEEE Conferences

Authors: T. Sugai; Z. Shaw; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9717525

Abstract: Multipactor is a resonant nonlinear electron multiplication effect that may occur in high power microwave devices at very low pressures, such as those operating in particle accelerators and satellite subsystems. Its effects range from signal degradation to the damage and destruction of microwave components. Thus, multipactor physics has been studied through theoretical analysis, numerical simulation, and experiment. Previously, we developed a direct electron observation system using an Electron Multiplier Tube (EMT) and succeeded to directly detect multipactoring electrons in the center of the broadwall of rectangular waveguides ^{1, 2}. Here, we provide a method for evaluating the electric charge density and secondary emission yield (SEY) in waveguides. The experimentally obtained EMT signal is analyzed with the extensive usage of the numerical simulation software Spark3D. The software was utilized to analyze multipactor onset in waveguide structures, where the electric field distribution without multipactor was carefully simulated, employing high-frequency solvers. The EMT signal and the charge density were simulated for the same conditions as the experiment. As a result, a calibration line indicating the proportional relation between the EMT voltage and the charge density, which is independent of some conditions, i.e., input power and gap size, was obtained. Further, after adjusting the SEY curve imported to Spark3D, the rising shape of the experimental EMT signal pulses fit with the simulated one, and the experimental threshold power for the EMT signal generation was consistent with the simulated multipactor threshold power. Since the simulation matches the experiment in threshold power and signal shape, one expects that the charge density and SEY curve deduced from the simulation are accurate.

IEEE Conferences

Authors: Sugai, T; Shaw, Z; Dickens, J; Neuber, A

PDF: https://aip.scitation.org/doi/10.1063/5.0011641

Abstract: Multipactor is a resonant nonlinear electron multiplication effect that may occur in high power microwave devices at very low pressures, such as those operating in particle accelerators and satellite subsystems. In this research, multipactor of a rectangular waveguide was analyzed using the commercially available, numerical simulation software Spark3D. The electromagnetic wave in the simulation was a TE10 mode-2.85GHz wave of varying power, fed into the impedance transformer waveguide. At the lowest threshold, multipactor is generated in the minimum height region in the impedance transformer and nowhere else. More precisely, the multipacting electrons have a continuous energy distribution since the emitted secondary electrons carry a random initial velocity distribution. We observed that there are cases where the impact electron energy decreases despite an increase in power due to growing non-resonance of the microwave field and electron oscillations, resulting in not only two threshold points where secondary emission yield (SEY)=1 but several more. As a consequence, it was uncovered that when the average SEY in the highest field region is close to or less than one, multipactor may be caused in a lower field region where the SEY is effectively higher than one. The numerical results are compared with data from the experiment. While there is some deviation between the thresholds obtained from Spark3D and the experiment, the results at higher power levels are consistent with the experiment in the view of the SEY for each power level.

Journal

Authors: M. Flynn; A. Neuber; J. Stephens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9717463

Abstract: Insulating gases, such as SF6, and N2, are modeled using a multi-term Boltzmann equation model. Owing to their large vibrational cross-sections, modeling such gases using the conventional two-term Boltzmann approach leads to near singularities in the electron phase space at the velocity-space origin. Thus, a large error in predicted in swarm parameters for two-term Boltzmann models, even at low-to-moderate reduced electric fields.

Conferences

Authors: Qiu, X; Diaz, L; Sanati, M; Mankowski, J; Dickens, J; Neuber, A; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/5.0010389

Abstract: Secondary electron emission from copper is probed utilizing Monte Carlo simulations that take account of elastic scattering based on the Mott theory and inelastic collisions based on energy-dependent energy loss functions. The loss function and stopping power were obtained through first-principles density functional theory. Angular assignment of electrons following elastic scattering or the creation of secondaries is shown to affect the energy-dependent secondary electron yield (SEY). A good match of the simulation results (with a peak SEY of similar to 180% at around 300eV and less than 10% deviation over the 0 to 1000eV energy range) to available experimental data is shown based on an energy and momentum conservation scheme. Also, the distribution of delay times for the generation of secondaries, the SEY behavior at different incident angles, the energy distribution of emergent secondaries, and their creation profiles as a function of depth are computed to provide a more complete picture of the governing mechanisms and predicted responses.

Journal

Authors: L. Silvestre; R. Joshi; J. Stephens; J. Dickens; J. Mankowski; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9717616

Abstract: Multipactor discharge is a resonant phenomenon that can be initiated in vacuum under RF excitation, giving rise to charge growth over time. The electron dynamics under such collisionless conditions has been researched by kinetic Monte Carlo and magnetohydrodynamic models in the past. As an alternative, we develop and present studies of a Vlasov equation based numerical model to calculate multipactor susceptibility in common microwave structures [1]. In contrast to kinetic models, utilization of the Vlasov equation permits the continuous treatment of the electron distribution in phase space, thereby capturing all statistical outcomes in a single calculation. To address the computational demand of the Vlasov equation, parallel computing techniques are utilized.

IEEE Conferences

Authors: T. Buntin; M. Abide; J. Dickens; A. Neuber; R. Joshi; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9717481

Abstract: Most high power microwave sources, such as the Magnetically Insulated Transmission Line Oscillator (MILO) being developed at Texas Tech, utilize cold cathodes that generate electrons via explosive emission. These highly emissive cathodes can generate current densities and currents greater than kA/cm² and 10 kA, respectively which are required for devices that can output RF power greater than 100 MW. Typical explosive emission cathode material includes metal, velvet, carbon fiber, and CsI coated carbon fiber. In order to optimize the MILO performance, we have begun fabricating carbon fiber velvet and comparing the performance with other commercially available materials. Fabrication was done on a manual, mechanical loom using commercially available carbon fiber thread.

IEEE Conferences

Authors: Aponte, IA; Esser, B; Shaw, ZC; Dickens, JC; Mankowski, JJ; Neuber, AA

PDF: https://aip.scitation.org/doi/10.1063/1.5119152

Abstract: Radio frequency (RF) breakdown in air at a frequency of particular relevance to ionospheric heating—3.3 MHz, close to the low end of the applicable frequency range—is studied at centimeter-sized gap distances and compared to the literature for small gaps. To establish a reference point, Paschen's early DC breakdown study utilizing two brass spheres of 1 cm radius was replicated following the original procedure and subsequently extended to examine RF breakdown. Various electrode combinations were tested with brass cathodes creating the highest variance in the datasets among DC tests. The greatest variation in RF breakdown arose when either electrode was brass. Gap distances of 1–10 mm were tested for both DC and RF with the slow-rise time (5 mV/μs) RF breakdown occurring at approximately 80% of the DC breakdown value, a value corroborated by Monte Carlo breakdown simulations. Pushing the envelope rise time of the applied RF voltage into the microsecond regime yielded an RF voltage of roughly 20% above the DC breakdown value accompanied by a distinct increase in breakdown amplitude fluctuations. Illuminating the gap electrodes with deep ultraviolet (280 nm and below) minimized the breakdown amplitude fluctuations due to photoemission at the electrodes as expected. Finally, to address the conditions found in real-world geometries with sharp corners or protrusions, RF corona behavior utilizing tungsten needles above a ground plane is measured. The obtained results help us define the operation limits of high-power antennas at 1–10 MHz frequencies.

Journal

Authors: B. Esser; Z. Shaw; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9717900

Abstract: A pulsed 2.85 GHz RF source design with high average power is presented for use as the principle exciter in an experiment to directly detect multipactor. With a rated output power of 700 W each in long pulse mode (∼ 100 □s), four Cree GaN HEMTs are used to achieve a maximum of 2.8 kW rated output. A fifth HEMT is used to drive the four output devices with approximately 40 W each. A free running VCO with low tuning sensitivity is used which ensures a stable frequency output despite a noisy environment. Buffering and initial amplification is provided by an LNA with a TTL RF switch used to generate the pulses. With a rise time of 35 ns, this switch is fast enough to create reasonably square pulse edges. A microwave amplifier with 45 dB of gain provides the majority of the gain in the system and the power necessary for the final GaN stages, approximately 9 W. An in-line micrometer adjusted, continuously variable attenuator between LNA and TTL switch provides level control.

IEEE Conferences

Authors: William Brooks; David H. Barnett; W. A. Harrison; David Hattz; John Mankowski; James Dickens; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9089302

Abstract: A 3-D Monte Carlo-type random walk model was constructed for the assessment of lightning attachment probabilities to small structures. The simulation assumed buildings had a negligible impact on the propagation of lightning. A purely stochastic propagation model based on a previously proposed gas and charged particle diffusion process was employed. The attachment was based on the electrogeometric model in which striking distance is determined by return stroke peak current. This model allows for hundreds of thousands of samples to be evaluated in the window of a few minutes on readily available consumer computing hardware. Using this model, it became possible to enable characterizing building protection as a probability distribution of striking distance. Such was done to provide a deeper understanding of the impact of building protection design choices than is readily available from binary testing. The model was calibrated for minimum input resolution, which is found to be insensitive to variations in step length and moderately insensitive to variations in propagation angle distribution, resulting in normalized errors of less than 15% (rms). A parametric sweep of geometric features was performed for a large (100 m $\times $ 50 m) rectangular building with catenary wire protection. For heights of less than 30 m, lightning was found to bypass protection structures and strike to the building itself at rates that were insensitive to variations in building height. The extent to which the protection may be recessed from the building's perimeter was found to have a significant impact. Variations in building aspect ratio were found to be of limited impact except for cases of extreme aspect ratio where competition with the ground appears to have resulted in much better lightning protection performance.

Journals

Authors: Zachary C. Shaw; Benedikt Esser; James C. Dickens; John J. Mankowski; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9520510

Abstract: A planar test cell was designed and implemented to observe the multipactor effect in waveguide structures. This plug and play device allows for multiple geometries to be machined and easily replaced within the test structure. A direct detection method was used to observe the multipactor effect while the upper and lower thresholds were measured for a 2.1 mm gap at 2.85 GHz. While there is an obvious lower limit to multipactor (2 kW), there was no observable upper limit even at powers over 200 kW. This is attributed to the transverse electric field distribution in the dominant TE10 mode which is not taken into account in most multipactor theoretical models.

Conferences

Authors: Shaw, ZC; Silvestre, L; Sugai, T; Esser, B; Mankowski, JJ; Dickens, JC; Neuber, AA

PDF: https://aip.scitation.org/doi/full/10.1063/5.0012833

Abstract: Multipactor in WR-284-like geometries is measured utilizing local and global detection techniques. To emulate conditions one may find in a waveguide filter structure while maintaining the fundamental microwave mode, a standard rectangular waveguide geometry with the reduced waveguide height set to 2.1 or 5.5mm was adopted. Two high power RF sources were used to investigate a large range of input power (few kWs to MWs): a solid state source using GaN HEMTs allowing for larger pulse widths than standard magnetrons (100 mu s as opposed to similar to 4 mu s) and a MW level S-band coaxial magnetron for the high power end. Particular interest was taken in capturing the lower and upper limits of multipactor threshold. Lower multipactor thresholds for finite pulse duration are governed by the appearance of one or more electrons in the multipactor gap during the applied pulse as well as a minimum power (electric field) level that affects a secondary electron emission yield above unity. As shown, such initial electrons(s) may easily be seeded via an external UV source illuminating the gap. However, wall collisions of excited metastable molecules may be another source of electrons, an observation based on the experiment and prior research. A multipactor upper threshold was non-existent in the experiment, even at powers over 200kW within a 2.1mm test gap, which numerically yielded a gap transit time significantly shorter than one half-period of the GHz wave. This is attributed to the electric field distribution within the waveguide structure, which results in the multipactor's spatial position moving to more favorable locations within the test gap.

Journal

Authors: J. Williams; D. Barnett; J. Dickens; A. Neuber; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9717487

Abstract: The development of a solid state high-voltage pulse generator for a spark-gap triggered Marx generator is discussed. The control board features a microcontroller to provide added flexibility and safety. A PIC16 microcontroller and two fiber-optic cables are critical to the design. Optically isolated serial communication between the device and a remote PC is used to control every aspect of the output pulses. This allows for more specific configuration of the pulser and provides remote adjustment of the pulse generator. The design uses the microcontroller's digital I/O pins and SPI/UART peripherals to send and receive operating data over fiber-optic, control the output pulse repetition rate, interface with a digital potentiometer to change the signal amplitude, and activate components that send optically isolated signals to disable/enable the high voltage power supply. In the case of microcontroller failure, the design includes a backup circuit that reconfigures the fiber-optic receiver to trigger an output pulse instead of interpreting UART communication signals. This permits the operator to still trigger the Marx generator remotely in a “single-shot” mode. The successful operation of this device involves configuring the PIC16 to send a 4 kV pulse at repetition rates up to 1 kHz. The flexibility provided by the microcontroller allows customization of several aspects of the output pulse, leaving the design useful in almost any application requiring high voltage pulses.

IEEE Conferences

Authors: S. Feathers; J. Stephens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8515278

Abstract: The investigation of a high output power, arc lamp exciplex ultraviolet (UV) source for pulsed power applications is presented. The arc lamp generates up to 550 W from XeF* exciplex radiation at 351 nm, totaling to nearly 0.15-mJ total radiated UV energy over the duration of the UV pulse. With an ellipsoidal reflector, the arc lamp produces 400 W/cm2 and up to 0.1 mJ of UV light onto a 1-cm2 area. A complete experimental investigation of the arc lamp for both XeCl* (308 nm) and XeF* (351 nm) exciplex sources operated under varying excitation and pressure conditions is reported. As an application, the arc lamp is successfully utilized as an illumination source for an intrinsically triggered, wide bandgap SiC photoconductive semiconductor switch (PCSS), where a PCSS ON-state resistance of 500 Ω is achieved.

Journals

Authors: D. H. Barnett; K. Rainwater; J. C. Dickens; A. A. Neuber; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8613091

Abstract: This paper focuses the tunability of a reflex triode virtual cathode oscillator (vircator). The vircator cathode is a bimodal carbon fiber (CF) material, while the anode is polished pyrolytic graphite. These materials have ideal operating characteristics for use within a vircator. These materials have high operating temperatures greater than 1000 K which support large current densities of ~200 A/cm2. A 12-stage, 158-J pulse-forming network (PFN)-based modular Marx generator is used to drive the vircator at 350 kV, 4 kA with ~100-ns pulsewidth at a pulse repetition frequency up to 100 Hz. The 12 stages of the Marx are constructed from a PFN using five, 2.1 nF, high-voltage ceramic capacitors in parallel. The Marx is broken into six modules each containing two stages. The Marx modules are machined from acetyl copolymer commonly called Delrin to provide rigidity and strength. Each Marx module includes air supply lines machined directly into each block, allowing external airlines to connect to each module chamber, rather than every spark gap. After the Marx erects, the energy is used to drive the virtual cathode oscillator (vircator) where subsequent frequency generation is manipulated through a new rectangular waveguide used as the resonant cavity. The new design has three parts of the cavity that can be changed; the bottom plate, back wall, and anode-cathode gap (A-K) distance. Each of these parts moves via linear actuators, two on the bottom plate, one on the A-K gap, and linear bellows for the back wall. The square waveguide cavity is welded into a circular stainless steel sleeve and is housed within a 10-in circular vacuum chamber. The anode is stationary in the vacuum chamber and connects to the Marx generator through a nickel shaft that feeds through the back wall, circular sleeve, and the rectangular waveguide. The anode is pyrolytic graphite, while the cathode is CF. The waveguide bottom plate, back wall, and cathode move around the stationary anode. This allows the height of the resonant cavity and the back wall distance from A-K gap to be independently changed of each other.

IEEE Journals

Authors: H. K. A. Nguyen; J. Mankowski; J. C. Dickens; A. A. Neuber; R. P. Joshi

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8599158

Abstract: Multipactor growth in rectangular waveguides is probed based on a kinetic approach. Unlike most studies relying on the Vaughan model, a probabilistic approach for random multiple secondary particle emissions is used. Spread in electron emission velocities, the angular dependence of secondary emission yields, and an external radio frequency (RF) driving field due to a TE10 mode, were all built in. The calculations predict the secondary emission yield for copper, probe the population growth dynamics, and obtain the susceptibility diagram. Despite a maximum field at the waveguide center from the RF excitation, maximum electron densities are predicted at locations symmetrically displaced from the center. The secondary electron yield (SEY) characteristics, its local maxima, and the role of oblique incident angles, collectively lead to multipactor finding its place at off-center locations.

Journal

Authors: T. Buntin; M. Abide; D. Barnett; J. Dickens; A. Neuber; R. Joshi; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9009681

Abstract: A low-impedance MILO is being developed at Texas Tech University, and a compact Marx generator was designed to drive it. The target design goals of the Marx are an output voltage greater than 500 kV and an output current greater than 40 kA. Risetime needs to be sub 150 ns and the pulsewidth must be greater than 100 ns. These performance goals were determined from PIC simulation of the MILO such that an RF efficiency (>10%) and RF peak power (> 1 GW) can be achieved. Tests using smaller 3 and 4 stage Marx generators with the same topology as the final design were used to determine a per-stage inductance of approximately 120 nH. From this derived inductance, multiple configurations were simulated to decide upon the ideal design for the desired performance goals. From these simulations, an 18-stage Marx with 2 capacitors per stage was chosen as the most optimal design, and from simulations into a 12 Ohm load a number of the criteria can be met with this configuration. The simulated peak voltage and current are 570 kV and 48 kA, respectively, while pulse risetime and pulsewidth are 170 ns and 540 ns, respectively. The designed Marx is being experimentally validated to confirm the findings of the simulation, firing into an approximately 12 Ohm water load to represent the low-impedance MILO that is being designed.

IEEE Conferences

Authors: Shaw, ZC; Garcia, A; Powell, M; Dickens, JC; Mankowski, JJ; Neuber, AA

PDF: https://aip.scitation.org/doi/10.1063/1.5089764#:~:text=Electron%20multipliers%20ABSTRACT%20Apparatus%20which%20is%20used%20to,GHz%20with%20up%20to%20approx.%201%20MW%20power.

Abstract: Apparatus which is used to directly observe electrons in microwave vacuum components was designed and implemented into a WR-284 like waveguide operated at 2.85 GHz with up to approx. 1 MW power. To generate desired electric field levels for driving secondary emission, the waveguide structure is manipulated by reducing the test section height to 6 mm from the standard WR-284 rectangular waveguide height of 34 mm. Both test and standard sections were operated in the dominant TE10 mode. A 1 mm aperture was cut into the broadside wall of the waveguide section enabling a portion of electrons in the waveguide to enter a properly biased electron multiplier tube mounted atop of the test section. Waveforms are presented showing the direct measurement of electrons, providing a local detection method with nanosecond temporal resolution. Future work will incorporate the test setup for multipactor studies. Published under license by AIP Publishing.

Journal

Authors: L. Collier; T. Kajiwara; J. Dickens; J. Mankowski; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8770271

Abstract: Solid-state semiconductor switches are emerging as an attractive choice for the fast switching of compact, repetitive, and pulsed power systems. In particular, the high voltage and fast switching capabilities of SiC MOSFETs are well suited for many applications when appropriately gated. For instance, the turn-on and turn-off characteristics of such devices are strongly dependent on the gate driving circuitry. Traditional commercial gate drivers, typically utilizing push-pull or totempole driving topologies, are often not well suited for fast, high current switching with rise times on the order of 10-20 ns, as the driving performance is highly dependent on the combined RLC characteristics of the driving circuitry and the switching device. The proposed gate drive topology utilizes a current-carrying inductor to rapidly charge the MOSFET gate-source capacitance. A high-voltage inductive kick generates the necessary potential to drive the inductor current into the gate through the parasitic gate impedance. As the energy stored in the drive inductor is continuously variable, it can be adjusted such that the gate voltage settles to a lower value, typically 20 V, after the initial kick to prevent excessive gate-source overvoltage. With an inductive drive current of ~23 A, a peak dI/dt of 25 kA μs-1 was achieved for the tested bare SiC MOSFET die. Additionally, a peak dI/dt of 13 kA μs-1 was achieved with the TO-247 packaged device.

IEEE Journals

Authors: Aponte, IA; Esser, B; Shaw, ZC; Dickens, JC; Mankowski, JJ; Neuber, AA

PDF: https://aip.scitation.org/doi/10.1063/1.5119152

Abstract: Radio frequency (RF) breakdown in air at a frequency of particular relevance to ionospheric heating-3.3 MHz, close to the low end of the applicable frequency range-is studied at centimeter-sized gap distances and compared to the literature for small gaps. To establish a reference point, Paschen's early DC breakdown study utilizing two brass spheres of 1 cm radius was replicated following the original procedure and subsequently extended to examine RF breakdown. Various electrode combinations were tested with brass cathodes creating the highest variance in the datasets among DC tests. The greatest variation in RF breakdown arose when either electrode was brass. Gap distances of 1-10 mm were tested for both DC and RF with the slow-rise time (5 mV/mu s) RF breakdown occurring at approximately 80% of the DC breakdown value, a value corroborated by Monte Carlo breakdown simulations. Pushing the envelope rise time of the applied RF voltage into the microsecond regime yielded an RF voltage of roughly 20% above the DC breakdown value accompanied by a distinct increase in breakdown amplitude fluctuations. Illuminating the gap electrodes with deep ultraviolet (280 nm and below) minimized the breakdown amplitude fluctuations due to photoemission at the electrodes as expected. Finally, to address the conditions found in real-world geometries with sharp corners or protrusions, RF corona behavior utilizing tungsten needles above a ground plane is measured. The obtained results help us define the operation limits of high-power antennas at 1-10 MHz frequencies.

Journal

Authors: B. Esser; J. J. Mankowski; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8789007

Abstract: A modification of a previously introduced electrically small antenna is presented with tuning methods for continuous band coverage for ionospheric heating (∼3–10 MHz). Consisting of a small loop antenna inductively coupled to a capacitively loaded loop (CLL), the design may be tuned ±50% of the center of the band by simply adjusting the capacitance of the CLL. Abandoning the use of lossy materials for tuning such as solid dielectrics or ferrites, the antenna is greater than 80% efficient across its tuning range. A tenth scale prototype with electromechanical geometry tuning is tested for frequency range and tuning capability especially at the low-frequency end where port reflection losses tend to dominate. Tuning of the small loop antenna-CLL coupling is used to mitigate this matching issue, which was demonstrated on the physical antenna model. Experimentally, a tuning range of 33.5–117.5 MHz is achieved with low reflection achievable across the range.

AGU Journals

Authors: M. Powell; Z. Shaw; J. C. Dickens; J. J. Mankowski; A. A. Neuber; C. Scribner

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9009615

Abstract: Pure N2 and SF6, as well as their mixtures, are evaluated for high electric field breakdown tested at pressures ranging from 750 torr to 1650 torr at 2.85 GHz. Previous research concerning the breakdown characteristics of pressurized SF6 and SF6 mixtures at S-Band frequencies is limited, likely due to the high electric fields required to breakdown pressurized SF6. A stepped impedance transformer is used in conjunction with a traveling wave resonator to obtain the high electric field amplitudes necessary to break down the gases. Starting with the output from a 3.5 MW coaxial magnetron the electric field amplitude in the test piece at the center of the stepped impedance transformer yielded a maximum of about 150 kV/cm RMS field. Using Pure SF6 as a baseline, the measured breakdown field shows a mostly linear dependence upon pressure in a range from 750 torr to 1350 torr, while some leveling out tendency is observed at pressures greater than 1350 torr. Since pure N2 exhibits a much lower breakdown threshold, ~ 60%, compared to pure SF6, mixing the two gases also results in a lower effective breakdown threshold; however, the reduction in the electric field breakdown threshold is not strictly proportional. For example, a 60/40 SF6 to N2 mixture resulted in a 90% breakdown field while a 20/80 mixture still yielded about 80% in the high-pressure regime.

IEEE Conferences

Authors: R. Lee; A. Hewitt; R. Clark; H. Hudyncia; T. Buntin; D. Barnett; J. C. Dickens; J. J. Mankowski; W. A. Harrison; E. Tucker; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9009758

Abstract: The impact of mechanical stresses on polymer bonded high explosives, HE, is investigated. High-Speed photography in the visible spectrum, VIS, as well as mid-wave infrared (MWIR) of HE during small diameter drilling and controlled skidding is presented. Controlled drilling into the HE enables recording the size and temperature of shavings under varying feed and speeds. Even at very high drill speeds, the HE phase transition temperature of approx. 180 degree Celsius is rarely exceeded. The MWIR signals radiated are recorded with FLIR's X6901sc High-speed MWIR camera, which uses InSb technology, with a wavelength range from 3.0 to 5.0 μm, and up to 1,004 fps at a resolution of 640 × 512 in the temperature range of interest. High-speed recording in the visible is obtained utilizing Phantom's VEO710s high-speed camera at a higher frame rate of 7,400 fps at a resolution of 1280 × 800 in the VIS. Observing the HE-grit interaction in the MWIR poses a great challenge, for IR is blocked by many glasses.

IEEE Conferences

Authors: R. Clark; R. J. Lee; A. T. Hewitt; T. Buntin; D. Barnett; J. C. Dickens; W. A. Harrison; E. Tucker; J. J. Mankowski; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9009974

Abstract: The drilling behavior of polymer bonded high explosives (HE) is investigated by varying drilling parameters and analyzing resultant forces and thermal response. A modified drill press enables remote operation and precise control of cutting speed, feed, and depth. To acquire temperature at the cutting interface a K-Type thermocouple is inserted in the coolant holes of thru-coolant drill bits and epoxied flush with the drill's flank face, which yielded a sensing accuracy of ±1°C and a resolution of 0.48°C. Cutting forces and torques are acquired with a sampling speed of up to 2,000 Hz. The comparison of downward directed forces across cutting operations is indicative of which speed and feed rate combinations limit excessive stressing of the HE, while cutting axis torques give indication in the case of drilling obstructions such as insufficient chip clearance. Drilling conditions in excess of the existing US DOE-STD-1212-2012 limitations are tested to determine safe but efficient machining limits for these materials. Drilling speed, feed rate, and peck depth are varied for drilling cycles with a 5 mm diameter drill bit, and further cycles are performed to determine the effect of increasing cut diameter. In peck drilling, clearance of chip from the drill flute is crucial and governs the drill's temperature rise.

IEEE Conferences

Authors: M. LaPointe; L. Collier; T. Kajiwara; J. Dickens; J. Mankowski; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9009986

Abstract: An innovative gating scheme for wide bandgap semiconductor switches is investigated to fully exploit recent advances of SiC MOSFET properties in hold-off voltage (from single digits to tens of kV) and low on-state resistance (tens of mΩ). Robust gate driving techniques are required to achieve fast risetimes on the order of 10-20 ns. Further, due to the high dI/dt, and subsequent inductive kickback, parasitic inductance may drastically affect the performance of commercially available totem-pole gate drivers. Further, traditionally packaged MOSFETs exhibit additional degradation of switching characteristics due to the introduction of parasitics primarily due to their lead geometry.

IEEE Conferences

Authors: M. Abide; T. Buntin; D. Barnett; J. Dickens; R. Joshi; A. Neuber; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9009939

Abstract: The development of a low-impedance magnetically insulated transmission line oscillator (MILO) driven by a compact Marx generator developed by Texas Tech University is discussed. The goals of the project aim to develop a MILO operating within the S-Band that can provide an RF peak output power of greater than 1 GW with greater than 10% efficiency. The device design followed a set of base design equations that were applied to a CST Studio Suite (CST) for a Particle-in-Cell, PIC, simulation to model the MILO. These simulation results then inform changes to the model to optimize the prospective performance of the device. The simulations were developed to account for realistic material properties that were then applied to critical surfaces of the device. Additionally, a circuit simulation was included to model a Marx generator feeding the input of the MILO to simulate the eventual experimental setup. Current results verify an expected RF peak power of approximately 4.5 GW at 2.5 GHz operating in the TM01 mode when excited with an input signal that has a peak voltage of 600 kV while providing a peak current of 58 kA. The simulation confirms the design should perform within these constraints.

IEEE Conferences

Authors: Landon Collier; Tyler Buntin; James Dickens; John Mankowski; John Walter; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8561207

Abstract: The diffusion of transient magnetic fields through the walls of a hollow conductive shell is an important phenomenon of interest throughout a variety of pulsed power applications. Basic solutions do exist for cylindrical geometries in the limiting case that the skin depth is much larger than the wall thickness; however, in many pulsed applications, the transient skin depth is often similar to the conductor thickness. As the underlying thin-wall assumption begins to breakdown, the production of complex eddy current distributions in the conductor walls results in deviation from these simplified analytical solutions of the diffused field. Precise calculation of these current distributions is essential for many applications including inductive shielding and magnetic field diagnostics near conductors. Electromagnetic simulations using the finite-element method provide a more accurate picture of the diffusion process in this regime. A high magnetic field testbed facilitates measurement of the diffused fields in order to verify simulation accuracy. The effect of material conductivity, wall thickness, and conductor geometry on the diffusion process is examined.

Journals

Authors: T. Klein; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9009641

Abstract: A pulse charger module was designed and tested for use in a larger system Each pulse charger module is powered with a 12 V Lithium-ion battery and set to charge a nF sized capacitor up to 100 kV in less than 10 μs. This is achieved by initially charging a μF sized capacitor to 3 kV, then switching a thyristor to discharge this capacitor into a step-up pulse transformer to charge the load capacitor. A PIC 18F26K80 8-bit microcontroller in each pulse charger module will be used to control the module, communicate with other modules and a computer, and monitor voltages. The modules are programmed to automatically detect the total number of modules well as communication delays between each module at startup, allowing for synchronous triggering and induvial identification and control. Each module is kept in a low power mode when not in use, and fiber optic communication is used throughout such that electrical isolation between modules and the master computer is ensured.

IEEE Conferences

Authors: Qiu, X; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5056766

Abstract: Thin nanoscale coating of metal electrodes by graphene promises to be a useful approach for suppressing the secondary electron yield and potential multipactor. Recent calculations showed reductions by as much as 50% for graphene over copper electrodes for energies below 125 eV, with results in good agreement with experimental data. Here, the resistance to possible degradation of this structure, in response to incoming atomic projectiles, is gauged based on molecular dynamics simulations. Our results for surface irradiation by carbon atoms (as an example) on nanoscale graphene coatings indicate a defect threshold of about 35 eV, lower surface damage for thicker layers, negligible sputtering, and defects less than 6 angstrom in dimension for energies up to 300 eV. The electrode structure is shown to be robust with better resistance to damage than metal alone. Published under license by AIP Publishing.

Journal

Authors: A. R. Chowdhury; S. Nikishin; J. Dickens; A. Neuber; R. P. Joshi; R. Ness

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8662228

Abstract: Time-dependent photocurrent response in semi-insulating GaN is simulated with a focus on the "Lock-On" phenomenon. A one-dimensional, time-dependent model based on drift-diffusion theory is used. The model was first tested for GaAs and shown to yield good agreement with data, before applying it to GaN simulations. The main findings are that compensated GaN with deeper traps nearer the midgap at higher densities, and/or multiple levels around the mid-gap would aid in driving the PCSS towards Lock-On. The initial average threshold field for Lock-On in GaN is predicted to be around 150 kV/cm, though this would be strongly dependent on the trap parameters of a sample.

IEEE Journals

Authors: F. Hegeler; A. Neuber; C. Hatfield

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8689446

Abstract: Recounts the career and contributions of Lynn LaMar Hatfield.

Journal

Authors: J. Culpepper; A. Miller; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9009753

Abstract: It is desired to integrate a photoconductive semiconductor switch (PCSS) capable of holding off and switching 100 kV into a package with small parasitic inductance such that sub-nanosecond rise time is still achievable at current amplitudes of hundreds of amperes. A GaAs based PCSS is utilized, which makes it necessary to address the filamentary nature of the current, which may lead to a shortening of device lifetime. In order to design a practical package, COMSOL based 2D electric field simulations have been utilized to aid in shaping the field between the PCSS semiconductor, the electrodes, and the high voltage encapsulant. To deal with the unavoidable high field stresses in the small package, the switch is brought to voltage within a few microseconds only, and then closed. Thus, keeping the duration of voltage stress very short, and the risk of self-triggering due to leakage current low.

IEEE Conferences

Authors: W. Brooks; D. Barnett; J. J. Mankowsi; J. C. Dickens; A. A. Neuber; W. A. Harrison; D. Hattz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9009756

Abstract: A stochastic model of downward lightning propagation and attachment was developed based on existing models of the propagation of a negative leader as a gas diffusion problem. Attachment was modelled using the rolling sphere concept. The resulting simulation enables Monte Carlo methods of assessing the efficacy of geometric lightning protection schemes. Protection afforded to a 100 m by 50 m building was investigated to elucidate the relative impact of selected protection configuration parameters. The results were found to be insensitive to variations in building height for configurations of less than 30 m. Variation in footprint area found protection for large buildings generally exhibits worse performance relative to smaller structures. This trend was examined in detail for a regular, square arrangement of lightning rods. Variation in building aspect ratio was found to have a pronounced impact on protection performance and is attributed to non-preferential striking to the available geometries. A method for arriving at the expected rate of a given building being stuck is outlined which expects the aforementioned structure to be struck at a rate of about four times per 10 000 years.

IEEE Conferences

Authors: B. Esser; Z. Shaw; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9575563

Abstract: A pulsed RF source for multipactor research capable of providing 2 kW at 2.85 GHz to a multipactor test cell is described utilizing state-of-the-art GaN HEMTs from Cree/Wolfspeed and integrated GaN amplifier from Qorvo/Tri-Quint. A Mini-Circuits ZX95-2920CA+ VCO is used as the signal generator with low phase noise and modulation capable with a bandwidth of 55 MHz. An inline attenuator provides the operator power control in the range of approximately 42 to 64 dBm (16 to 2800 W). A high-speed RF switch creates a 100-microsecond pulse with a rise time of 25 ns and a typical switching time of 35 ns. A single Tri-Quint TGA2585-SM provides the majority of the gain in the system, 32 dB, and the necessary power for the final output stage (6 W). Four CGHV31500F amplifiers operating in parallel comprise the output stage providing the bulk of the power needed, 500 W each for a total of 2 kW with 12.5 dB gain each, with a single unit providing the necessary input power before splitting to the four finals (50.5 dBm, ~ 112 W). A custom PCB was designed to properly bias the GaN stages and prevent device failure due to improper bias sequencing. Custom power splitters and combiners had to be used due to the high-power levels being considered with directional couplers on the output to monitor output (forward) power and reflected (reverse) power during operation. This will provide valuable insight into characteristics of the plasma cloud generated during the multipactor event.

IEEE Conferences

Authors: Chowdhury, AR; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.4972968

Abstract: Simulation studies of the electrical response characteristics of 4H-SiC switches containing traps are reported in the absence of photoexcitation. The focus is on trap-to-band impact ionization and the role of hole injection from the anode. Simulations show that hole-initiated ionization can be more important than the electron-initiated process. The results also underscore the role of hole injection at the high applied voltages. Our one-dimensional, time-dependent model yielded reasonable agreement with measured current-voltage data spanning over three orders of magnitude, but only when impact ionization was taken into account. Finally, the simulations predicted undulations in the device conduction current density with respect to time, due to the dynamic interplay between impact ionization, spatial electric field values, and occupancies of the trap levels.

Journal

Authors: Hieu K. A. Nguyen; John Mankowski; James C. Dickens; Andreas A. Neuber; Ravi P. Joshi

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9575287

Abstract: Multipactor in a rectangular waveguide is studied using numerical simulations. Particular attention is given to the secondary electron emission characteristics including their energy spectrum (hence velocity spread) and angular distribution. Elastically scattered, rediffused and true secondary electrons are all comprehensively included based on the Furman-Pivi model [1] for the TE10 mode. The focus is on small waveguides and lowest order resonance conditions.

Conferences

Authors: D. H. Barnett; A. A. Neuber; J. C. Dickens; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8936727

Abstract: This study serves to describe three-dimensional particle-in-cell (PIC) simulations of a tunable reflex-triode virtual cathode oscillator (vircator). Experimental data from the compact hard-tube reflex-triode vircator developed at Texas Tech University (TTU) is used to validate simulated results. The vircator developed at TTU is capable of burst-mode operation at pulse repetition rates (PRFs) up to 100 Hz for a period of one second. A pulse energy of 158 J drives the vircator, and 600 kV (open circuit) pulse forming network (PFN) based Marx generator. The vircator is comprised of a bimodal, carbon fiber cathode and a pyrolytic graphite anode, with the ability to quickly change the distance between the anode-cathode (A-K) gap, back wall distance, and bottom plate distance between experiments. The PIC simulations have been performed using CST PIC Solver, by Dassault Systemes. The models detail virtual cathode formation and the subsequent extraction of radiated microwave power for a variety of cavity geometries. A working three-dimensional, relativistic, electromagnetic, particle-in-cell model of a vircator allows for quick, predictive results relative to building an experimental setup. The model is used to determine the necessary driving voltages, A-K gap distances, and cathode current densities to extract microwave radiation at a desired. Simulated results aid in identifying mode contributions. Voltage, current, and microwave data are presented and compared against experimental results at different operating conditions.

IEEE Conferences

Authors: Nguyen, HKA; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5019360

Abstract: The suppression of secondary electron yield (SEY) which can possibly lead to multipactor is an important goal for several applications. Though some techniques have focused on geometric modifications to lower the SEY, the use of graphene coatings as thin as a few monolayers is a promising new development that deserves attention either as a standalone technique or in concert with geometric alterations. Here we report on Monte Carlo based numerical studies of SEY on graphene coated copper with comparisons to recent experimental data. Our predicted values are generally in good agreement with reported measurements. Suppression of the secondary electron yield by as much as 50 percent (over copper) with graphene coating is predicted at energies below 125 eV, and bodes well for multipactor suppression in radio frequency applications. (c) 2018 Author(s).

Journal

Authors: Woodrum, RB; Barnett, DH; Dickens, JC; Neuber, AA

PDF: https://onlinelibrary.wiley.com/doi/10.1002/prep.201700285

Abstract: This paper presents the work performed on dry-lathing PBX 9501 to gather and analyze cutting force as well as temperature data during the machining process. The data is compared to present USA federal-regulation-constrained machining limits of high explosives. The effects of machining parameters depth of cut, surface meters per minute, and feed per revolution on cutting force and cutting interface were evaluated. Cutting tools of tip radius 0.013cm and 0.127cm were tested to determine the effect of the tool shape on the machining process. Empirically, a pronounced dependence of the maximum tool temperature on the depth of cut and surface meters per minute was found, while the dependence on the feed per revolution was found much weaker. It is elucidated that rapid, shallow cuts optimize machining time for a billet of PBX 9501 while minimizing temperature increase and cutting force.

Journal

Authors: B. Esser; J. C. Dickens; J. J. Mankowski; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9575799

Abstract: Ionospheric Heating, traditionally performed utilizing large fixed location arrays, while effective, limits research efforts to those fixed locations. As one may envision, a more compact transportable array will provide additional research opportunities at latitudes previously unexplored. For instance, closer to the equator where the Earth's magnetic field is primarily parallel to the ground. An electrically small inductively coupled antenna is considered for its suitability in such an array. A Small or Semi Loop Antenna (SLA) inductively couples to a Capacitively Loaded Loop (CLL) providing high efficiency and natural 50 $\Omega$ port matching. The CLL, of hinged petal design, consists of a large parallel plate capacitor, and half cylinder inductive sections which are hinged at the base to allow for tuning in the range of ionospheric heating (3 - 10 MHz) with a plate included angle of 0 - 16°. A prototype antenna was designed and evaluated at 1/10^th scale - for ease of research efforts - with tuning range of 30 - 100 MHz including capability of tuning the coupling between SLA and CLL to achieve good source matching across the tuning range, particularly at the low and high ends. This tuning method may also be used to compensate for mutual impedance effects in array operation, improving array performance. A gain of approximately 5 dBi is observed when placed upon a sufficiently large ground plane. Tuning of the antenna prototype is achieved via stepper motor driven system to remotely adjust both the CLL and SLA angles continuously.

IEEE Conferences

Authors: I. A. Aponte; B. Esser; Z. Shaw; J. C. Dickens; J. J. Mankowski; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9575875

Abstract: RF breakdown at a frequency of particular relevance to ionospheric heating - 3.3 MHz, close to the low end of the applicable frequency range - is studied at centimeter sized gap distances and compared to literature for small gaps. Paschen's famous DC breakdown study utilizing two brass spheres of radius 1 cm was recreated following the original procedure, from which the data was used to compare to a study of RF breakdown. Through testing it was found that brass as an electrode material exhibits a large standard deviation in breakdown voltage and as such stainless-steel Bruce profile uniform electric field electrodes were substituted. Steel's resistance to surface ablation provided for extremely low standard deviation of measurements and hence good repeatability. Additionally, unlike brass, steel is not contaminated through reaction with gaseous elements such as carbon, oxygen and nitrogen. Electrodes were polished to remove any field enhancements and cleaned of polishing compound such that dielectric inclusions were largely avoided.

IEEE Conferences

Authors: M. Powell; Z. Shaw; J. C. Dickens; J. Mankowski; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9575375

Abstract: The design and implementation of an experimental setup are demonstrated for the breakdown testing of pressurized $\text{SF}_{6}$ (1–3 atm) at 2.85 GHz. Sulphur hexafluoride $(\text{SF}_{6})$ is a common insulating gas used to mitigate breakdown in high power microwave systems. While this gas is known for its high dielectric strength, available data for the breakdown of pressurized SF 6 and $\text{SF}_{6}$ mixtures at S- Band frequencies is limited. The obtained breakdown characteristics are compared to mixtures of $\text{SF}_{6}$ and Nitrogen $(\mathrm{N}_{2})$ for similarities in breakdown fields in a pressurized environment.

IEEE Conferences

Authors: Nicholas A. Wilson; Jared A. Culpepper; Vladimir Kuryatkov; Matthew Gaddy; James C. Dickens; Sergey Nikishin; Richard Ness; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9575438

Abstract: The suitability of commercially available wide bandgap GaN material for the fabrication of photoconductive semiconductor switches, PCSS, was investigated. A variety of PCSSs were fabricated utilizing diverse GaN samples, which were shown to exhibit significantly diverse physical properties. That is, sample characterization techniques such as cathodoluminescence (CL), photoluminescence (PL), secondary ion mass spectrometry (SIMS), Current-Voltage behavior, and scanning electron microscopy (SEM) were applied to characterize the samples prior to processing.

Conferences

Authors: Chowdhury, AR; Dickens, JC; Neuber, AA; Ness, R; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5013248

Abstract: The time-dependent photoconductive current response of semi-insulating GaAs is probed based on one-dimensional simulations, with a focus on the lock-on phenomenon. Our results capture most of the experimental observations. It is shown that trap-to-band impact ionization fuels local field enhancements, and photon recycling also plays an important role in pushing the device towards lock-on above a 3.5 kV/cm threshold field. The results compare well with actual data in terms of the magnitudes, the rise times, and the oscillatory behavior seen at higher currents. Moving multiple domains are predicted, and the response shown depended on the location of the photoexcitation spot relative to the electrodes. Published by AIP Publishing.

Journal

Authors: Nguyen, HK; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5004995

Abstract: Calculations of electron impact ionization of nitrogen gas at atmospheric pressure are presented based on the kinetic Monte Carlo technique. The emphasis is on energy partitioning between primary and secondary electrons, and three different energy sharing schemes have been evaluated. The ionization behavior is based on Wannier's classical treatment. Our Monte Carlo results for the field-dependent drift velocities match the available experimental data. More interestingly, the field-dependent first Townsend coefficient predicted by the Monte Carlo calculations is shown to be in close agreement with reported data for E/N values ranging as high as 4000 Td, only when a random assignment of excess energies between the primary and secondary particles is used.

Journal

Authors: J. P. Verboncoeur; N. Behdad; J. H. Booske; J. C. Dickens; R. M. Gilgenbach; M. Gilmore; N. M. Jordan; R. P. Joshi; Y. Y. Lau; J. Mankowski; D. Morgan; A. A. Neuber; S. Portillo; E. Schamiloglu; P. Zhang

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9575295

Abstract: Multipactor onset, growth, associated space charge effects, and transition to ionization breakdown due to ambient or desorbed gases represent key stages of single and multifrequency RF -driven phenomena that inhibit performance in space-based and terrestrial vacuum electronics devices. Performance degradation through space charge detuning and interference with gain is expected for medium duration pulses, and ion generation and damage for longer pulses. In this research, combined theoretical, computational, and experimental approaches are applied.

IEEE Conferences

Authors: Z. Shaw; B. Esser; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9575386

Abstract: An experimental setup was designed and implemented for the study of the multipactor effect in a rectangular waveguide under high vacuum. The experiment and test piece were constructed using WR 284 waveguide sections operating at S- Band frequencies. Multipactor effects are to be detected via an Electron Multiplier Tube (EMT). The test setup includes sections of standard WR 284 waveguide which will be in line with a custom test piece. Design of the test setup incorporates a standard piece of waveguide with a custom steel collar which can be used to insert different tapers, and even periodic structures into the hollowed out broadside wall. A solid state RF transmitter with 2.8 kW peak power and a center frequency of 2.85 GHz is used to inject an RF signal into the test section.

IEEE Conferences

Authors: Z. Shaw; A. Hewitt; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9575943

Abstract: A multi -channel phased array is used to synthesize signals of varying frequencies through apertures in EM shielded structures. Bipolar impulses with a GHz frequency spectrum are emitted from multiple antennas and used as basis functions to pass through the aperture and undergo constructive and destructive interference at a specific point in space. Experiments based on a small antenna array demonstrate that this technique enables synthesizing a below GHz signal (~ 400 to 800 MHz) in a shielded volume with an open aperture size that would heavily attenuate a direct sub-GHz electromagnetic wave. While the experimental results demonstrate that this technique is effective at UHF frequencies, it is certainly not limited to this frequency band.

IEEE Conferences

Authors: M. Czerniak; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8373136

Abstract: As the semiconductor industry makes devices and integrated circuits (ICs) that are increasingly complex, a consequence has been that the number of processing steps is increasing, from 400 at 90nm to > 1000 in state-of-the-art designs [1], shown in Figure 1. Furthermore, despite many process steps becoming more utility (and especially electrical power) - efficient, the increasing number of times a wafer visits process chambers has resulted in the energy use per cm2 wafer area increasing, reversing the trend of previous years. Figure 2 illustrates this phenomenon, which is also exacerbated by the use of double and quadrupole patterning, 3D device stacking and the use of EUV in HVM (which reduces the number of process steps but is utility-intensive).

IEEE Conferences

Authors: Gaddy, M; Kuryatkov, V; Meyers, V; Mauch, D; Dickens, J; Neuber, A; Nikishin, S

PDF: https://link.springer.com/article/10.1557/adv.2018.234

Abstract: Characterization of three vendor's bulk semi-insulating GaN:Fe wafers, grown by either hydride vapor phase epitaxy or the ammonothermal method, was performed using: scanning electron microscopy, secondary ion mass spectroscopy, high resolution X-ray diffraction, cathodoluminescence, photoluminescence, and high voltage testing. Although the Fe doping level is significantly different for each growth method, both are promising for the fabrication of PCSS devices operating in the lock-on mode.

Journal

Authors: Benedikt Esser; Daniel Mauch; James Dickens; John Mankowski; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8679867

Abstract: An electrically small antenna is evaluated for use as the principle radiating element in a mobile ionospheric heating array. Consisting of a small loop antenna inductively coupled to a capacitively loaded loop, the electrically small antenna provides high efficiency with the capability of being tuned within the range of ionospheric heating. At a factor 60 smaller in area than a High-Frequency Active Auroral Research Program element, this antenna provides a compact, efficient radiating element for mobile ionospheric heating. A prototype antenna at 10 MHz was built to study large-scale feasibility and possible use with photoconductive semiconductor switch-based drivers. Based on the experimental study, the design has been extrapolated to a small 6 × 4 array of antennas. At a total power input of 16.1 MW this array is predicted to provide 3.6-GW effective radiated power typically required for ionospheric heating. Array cross talk is addressed, including effects upon individual antenna port parameters. Tuning within the range of ionospheric heating, 3–10 MHz, is made possible without the use of lossy dielectrics through a large capacitive area suited to tune the antenna. Considerations for high power operation across the band are provided including a method of driving the antenna with a simple switcher requiring no radio frequency cabling. Source matching may be improved via adjustment of the coupling between small loop antenna and capacitively loaded loop improving |S11| from −1 to −21 dB at 3 MHz.

Journals

Authors: D. H. Barnett; K. Rainwater; J. C. Dickens; A. A. Neuber; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8291279

Abstract: This paper presents the electrical and mechanical hardware considerations of a compact, 160 J modular pulse forming network (PFN) Marx generator used to drive a high-power microwave (HPM) source that is a time variant load at a PRF of 100 Hz. The modular Marx generator is designed to produce an open-circuit output voltage of 600 kV from a 50 kV source using twelve stages. Each stage of the Marx was constructed from a PFN fashioned from five, 2.1 nF, high voltage capacitors in parallel. Each Marx module was machined out of acetyl copolymer or Delrin© to provide optimal strength, rigidity, and a dielectric constant that closely matches transformer oil. These Marx modules include air supply lines that are machined directly into each block of Delrin© allowing airlines to connect to each module chamber rather than every spark gap. Each module has two electrode inserts placed into the sealed pressure vessel contained within the module. After the Marx erects, the energy is directed into the virtual cathode oscillator (vircator) where subsequent frequency generation is manipulated through a rectangular waveguide contained within a new resonator cavity design. The new design allows the bottom wall, back wall, and anode cathode gap to be moved by two linear actuators, a linear bellows, and another linear actuator, respectively. The cavity is contained within a 10-inch circular vacuum chamber with a round stainless steel sleeve running from the back wall to the linear bellows. Contained within the round sleeve is a rectangular waveguide where the bottom wall and the cathode are housed. The anode is connected to the Marx generator via a nickel shaft that feeds through the back wall into the circular sleeve and into the rectangular waveguide. The anode made from pyrolytic graphite, remains stationary while the bottom wall, and carbon fiber velvet cathode move relative to its position. The benefit of this design is the height and depth of the cavity resonator can be controlled independently of each other while still allowing the A-K gap to be manipulated on its own.

IEEE Conferences

Authors: Shaw, Z; Feilner, W; Esser, B; Dickens, JC; Neuber, AA

PDF: https://pubmed.ncbi.nlm.nih.gov/28964249/

Abstract: A software controllable system which generates and transmits user defined RF signals is discussed. The system is implemented with multiple, modular transmitting channels that allow the user to easily replace parts such as amplifiers or antennas. Each channel is comprised of a data pattern generator (DPG), a digital to analog converter (DAC), a power amplifier, and a transmitting antenna. All channels are controlled through a host PC and synchronized through a master clock signal provided to each DAC by an external clock source. Signals to be transmitted are generated through the DPG control software on the PC or can be created by the user in a numerical computing environment. Three experiments are discussed using a two-and four-channel antenna array incorporating Chebyshev tapered TEM horn antennas. Transmitting distinct sets of nonperiodic bipolar impulses through each of the antennas in the array enabled synthesizing a sinusoidal signal of specific frequency in free space. Opposite to the standard phased array approach, each antenna radiates a distinctly different signal rather than the same signal simply phase shifted. The presented approach may be employed as a physical layer of encryption dependent on the position of the receiving antenna. Published by AIP Publishing.

Journal

Authors: Meyers, V; Chowdhury, AR; Mauch, D; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://iopscience.iop.org/article/10.1088/1361-6463/aa59aa/pdf

Abstract: We report on the intensity-dependent behavior of the absorption coefficient (alpha) in semiinsulating 4H-SiC material. Data from as-received samples show a monotonic decrease in a with incident energy density, with a pronounced change in slope at around 10 mJ cm(-2). Annealed samples, on the other hand, exhibit an experimental trend of increasing alpha with intensity. Qualitative explanation of the observed behavior is presented that probes the possible role of spontaneous and stimulated emission for as-received samples. With annealing, trap related recombination is strongly reduced leading to higher carrier densities and increased free-carrier absorption with incident intensity. The role of band-filling and permittivity changes are shown to be inconsequential, while changes in internal fields could contribute to decreases in absorption.

Journal

Authors: R. Rodriguez-Molina; D. Mauch; V. Meyers; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8496332

Abstract: Results detailing the current handling capability of Gold bond wires (1 mil diameter), and Aluminum ribbon (1mil x 10 mil) under pulsed conditions are presented. Gold wire bonds were formed through the ball-bonding technique, and the Aluminum wire was bonded through the wedge bonding technique. Both wires were bonded to gold plated bond pads. Varying peak current densities ranging from $2 \mathrm {x}10 ^{4}\mathrm {A}$ cm $^{-2}$ to $2 \mathrm {x}10 ^{7}\mathrm {A}$ cm $^{-2}$ were applied to bond wires at pulse-widths ranging from 50 ns to $500 \mu \mathrm {s}$. In addition, the effect of surrounding medium (SF6, HV epoxy, transformer oil) on the current capability was investigated. Failure modes observed were investigated with SEM imaging, and the evolution of these failure modes during current pulsing was captured with high-speeding imaging. Finally, the obtained results are compared with the experimental and theoretical results obtained from previous investigations conducted on exploding wires.

IEEE Conferences

Authors: N. A. Wilson; D. L. Mauch; V. E. Meyers; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8496006

Abstract: Summary form only given. A very compact system integrating a high voltage pulser (up to 250 V) and high power UV LED (365 nm wavelength) was developed for triggering SiC photoconductive semiconductor switches (PCSS). The relationships between LED drive current and forward voltage to optical power were established for currents ranging from 0 A to 100 A at pulse widths ranging from 100 ns to 5 μs. The maximum optical power observed was 25 W. An optical system composed of a parabolic reflector and short focal length lenses was used to focus the emitted light onto a lateral geometry PCSS. The observed delivery efficiency was 63% onto a target 11 mm in diameter. Using the developed system, a SiC PCSS could be successfully triggered under high impedance load conditions.

IEEE Conferences

Authors: H. K. Nguyen; A. Chowdhury; J. C. Dickens; R. P. Joshi; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8496212

Abstract: Breakdown of air at atmospheric pressure in response to AC fields in gaps larger than 1 cm was simulated. Most previous literature concerning breakdown in this regime has focused on much smaller gaps1.

IEEE Conferences

Authors: Zhang, Z; Giesselmann, M; Mankowski, J; Dickens, J; Neuber, A; Joshi, RP

PDF: https://scholars.ttu.edu/en/publications/evaluation-of-high-field-andor-local-heating-based-material-degra-14

Abstract: A molecular dynamics ( MD) model is used to study the potential for mass ejection from a metal nanoprotrusion, driven by high fields and temperature increases. Three- dimensional calculations of the electric fields surrounding the metal emitter are used to obtain the Maxwell stress on the metal. This surface loading is coupled into MD simulations. Our results show that mass ejection from the nanotip is possible and indicate that both larger aspect ratios and higher local temperatures will drive the instability. Hence it is predicted that in a nonuniform distribution of emitters, the longer and thinner sites will suffer the most damage, which is generally in keeping with the trends of a recent experimental report ( Parson et al 2014 IEEE Trans. Plasma Sci. 42 3982). A possible hypothesis for mass ejection in the absence of a distinct nanoprotrusion is also discussed.

Journal

Authors: Buntin, T; Collier, L; Dickens, J; Mankowski, J; Walter, J; Neuber, A

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8291285

Abstract: Transient magnetic diffusion through conductors of thickness comparable to the skin depth is investigated. Since an analytical solution is unavailable in this case, such magnetic diffusion results must be determined via simulation or experimentation. In the experimental approach, a sinusoidal current with peak values in the range of 20-30 kA (approx. 7 kHz ringing frequency) is passed through a two turn coil generating a sinusoidally varying magnetic field. A hollow structure with metallic walls of controlled thickness is placed roughly 10 cm away from the exciting coil. The focus of this investigation is on the transient skin depth, which occurs during the first half-wave of the signal, as that is most relevant for pulsed power applications. A calibrated B-dot probe placed inside the structure facilitates measurement of the diffused field. As expected, experimental data shows that magnetic field diffusion through the wall is not instantaneous, causing a delay before the diffused field is measured inside the test structure. The impact of cracks and holes in the conductor on the speed and magnitude of the magnetic field diffusion is elucidated. Results for materials of different conductivities are compared and analyzed for the transition between thin and medium walled cases. FEM simulations are validated alongside these experimental results and used to access a larger parameter space.

Conference Paper/Presentation

Authors: Nguyen, HK; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.4990699

Abstract: The behavior of the breakdown electric field versus frequency (DC to 100 MHz) for different gap lengths has been studied numerically at atmospheric pressure. Unlike previous reports, the focus here is on much larger gap lengths in the 1-5 cm range. A numerical analysis, with transport coefficients obtained from Monte Carlo calculations, is used to ascertain the electric field thresholds at which the growth and extinction of the electron population over time are balanced. Our analysis is indicative of a U-shaped frequency dependence, lower breakdown fields with increasing gap lengths, and trends qualitatively similar to the frequency-dependent field behavior for microgaps. The low frequency value of similar to 34 kV/cm for a 1 cm gap approaches the reported DC Paschen limit. Published by AIP Publishing.

Journal

Authors: Nguyen, HK; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.5004995

Abstract: Calculations of electron impact ionization of nitrogen gas at atmospheric pressure are presented based on the kinetic Monte Carlo technique. The emphasis is on energy partitioning between primary and secondary electrons, and three different energy sharing schemes have been evaluated. The ionization behavior is based on Wannier's classical treatment. Our Monte Carlo results for the field-dependent drift velocities match the available experimental data. More interestingly, the field-dependent first Townsend coefficient predicted by the Monte Carlo calculations is shown to be in close agreement with reported data for E/N values ranging as high as 4000 Td, only when a random assignment of excess energies between the primary and secondary particles is used. Published by AIP Publishing.

Journal

Authors: B. Esser; J. C. Dickens; J. J. Mankowski; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8496347

Abstract: An electrically small antenna (ESA) is evaluated for its potential future use in a Transportable Ionospheric Heating (TIH) array. Consisting of a Small Loop Antenna (SLA) which inductively couples to a Capacitively Loaded Loop (CLL) the antenna provides a high-Q natural match to a $50 \Omega $ source. The capacitance of the CLL may be adjusted to tune the antenna in the range of ionospheric heating of approximately 3 – 10 MHz. Several methods are evaluated to achieve this tuning including a horizontal sliding plate design, and a hinged petal design.

IEEE Conferences

Authors: Meyers, V; Mauch, D; Dickens, J; Neuber, A

PDF: https://aip.scitation.org/doi/10.1063/1.4978768

Abstract: The intensity-dependent light absorption in bulk high-purity semi-insulating 4H-SiC at above band gap photon energies has been studied. In particular, 3.49 eV (355 nm) UV absorption of 160 lmthick samples of varying recombination lifetimes in the intensity range of 1 mJ/cm(2) -30 mJ/cm(2) is addressed. The effective absorption coefficient was found to vary up to 30% within this range. Assuming deep level trapping, interband absorption, and free carrier absorption as dominant processes, a four energy level model reproduces the experimentally observed absorption behavior. While nonlinearities in the optical absorption behavior of SiC have been studied previously as function of wavelength alpha(lambda), temperature alpha(T) and, to a very limited extent, at below bandgap optical intensities, the presented elucidates the UV intensity-dependent nonlinear absorption behavior, alpha(I), of SiC at above bandgap photon energies. Published by AIP Publishing.

Journal

Authors: V. Meyers; D. Mauch; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8291268

Abstract: Intensity-dependent nonlinear light absorption in bulk 4H-SiC at the above-bandgap energy of 3.49 eV (λ = 355 nm) is studied. Characterization and understanding of such nonlinear optical behavior in 4H-SiC forms the basis efficiency improvements and design of optoelectronic SiC devices, including photoconductive semiconductor switches. It is noted that previous research performed elsewhere had focused primarily on nonlinearities at below-bandgap energies, while little had been explored above-bandgap. In this study, absorption of short laser pulses with fluences ranging from 1 mJ/cm2 to 30 mJ/cm2 incident on 160 μm-thick high purity semiinsulating 4H-SiC samples of varying recombination lifetimes is addressed. Sample bulk recombination lifetimes vary from 0.5 ns to 100 ns displaying the range of effects from growth, electron irradiation, and annealing. The effective absorption coefficient varies significantly within this range as an apparent function of bulk recombination lifetime. A four-level time-and space-dependent finite difference time domain (FDTD) model taking into account electron trapping, interband absorption, and free-carrier absorption was constructed that yielded further insight into the absorption dynamics. For instance, the importance of free carrier absorption and deep-level trapping in the nonlinear absorption behavior is elucidated.

Conferences

Authors: L. Collier; J. Dickens; J. Mankowski; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7949116

Abstract: The performance of an all solid-state linear transformer driver (LTD) is evaluated based on experimentally verified behavior of a single stage. The single-stage LTD utilizes a low-profile design with robust thyristor switches and high-energy-density mica capacitors to minimize overall system inductance. Subnanosecond jitter is achieved with simultaneous thyristor triggering. The stage is magnetically coupled to a secondary winding through a central nanocrystalline core. A dc current source, decoupled with a large inductance, actively resets the core between pulses. The overall result is a low-impedance (<;1 Ω per stage) pulse generator that rivals the performance of traditional Marx systems with the improved reliability, increased lifetime, and fast rep-rate capabilities of solid-state switches. The stage is tested with charging voltages up to 8 kV into various loads and compared with simulations based on an analog behavioral thyristor switch model previously developed at Texas Tech University. The simulation is expanded into a full-scale, multistage LTD simulation and compared with a previously constructed Marx generator.

IEEE Journals

Authors: Wilson, N; Mauch, D; Meyers, V; Feathers, S; Dickens, J; Neuber, A

PDF: https://pubmed.ncbi.nlm.nih.gov/28863629/

Abstract: The electrical and optical characteristics of a high-power UV light emitting diode (LED) (365 nm wavelength) were evaluated under pulsed operating conditions at current amplitudes several orders of magnitude beyond the LED's manufacturer specifications. Geared towards triggering of photoconductive semiconductor switches (PCSSs) for pulsed power applications, measurements were made over varying pulse widths (25 ns-100 mu s), current (0 A-250 A), and repetition rates (single shot-5 MHz). The LED forward voltage was observed to increase linearly with increasing current (similar to 3.5 V-53 V) and decrease with increasing pulse widths. The peak optical power observed was >30 W, and a maximum system efficiency of 23% was achieved. The evaluated LED and auxiliary hardware were successfully used as the optical trigger source for a 4H-SiC PCSS. The lowest measured on-resistance of SiC was approximately 67 k Omega. Published by AIP Publishing.

Journal

Authors: Barnett, DH; Rainwater, K; Dickens, JC; Neuber, AA; Mankowski, JJ

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8291276

Abstract: This study focuses on achieving wide tunability of a compact reflex triode virtual cathode oscillator (vircator). The cathode is of a bimodal carbon fiber (CF) material paired with a pyrolytic graphite anode. These materials display ideal operating characteristics which include but are not limited to, long lifetime > 10(6) shots, high operating temperatures > 1000 K, and large current densities similar to 200 A/cm(2). A 12 stage, 158 J pulse forming network (PFN) based Marx generator serves to drive the vircator at 350 kV, 4 kA with similar to 100 ns pulsewidth. The operating frequency of interest is in the range of 1-6 GHz, where tunability is achieved by varying the length of the anode-cathode (A-K) gap, the length from the back wall to the A-K gap, and/or the distance from the bottom of the cavity to the A-K gap. The primary focus in this research was to increase the achievable frequencies by placing a square waveguide within a sealed vacuum tube. This allows the bottom part of the waveguide to be easily adjusted while still maintaining the waveguide integrity. The resulting microwave frequencies are shown along with the system performance.

Conference Paper/Presentation

Authors: Z. Shaw; W. Feilner; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8496174

Abstract: A low power prototype antenna array with multi-channel transmission capabilities is incorporated to synthesize sinusoidal signals in free space from non-periodic source signals. Synthesis of a specified continuous signal is suitable in applications for spatially targeted plasma generation at high power. The system operates with an external 2.4 GHz clock source which allows for a 0.417 ns step resolution. Each antenna transmits an amplitude scaled set of non-periodic bipolar impulses $( \sim 2$ ns wide) which enables synthesizing an arbitrary signal at a specific point in free space only limited by the available number of antennas. A specific bipolar impulse set is found through the particle swarm optimization, PSO, technique with appropriate goal function selection. While the optimization is simply carried out in the time domain, the respective delay between antennas is matched in the experiment through electronic delays and/or physical antenna positioning.

IEEE Conferences

Authors: B. Esser; S. R. Beeson; J. C. Dickens; J. J. Mankowski; T. M. Antonsen; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7929366

Abstract: A tunable electrically small antenna (ESA) designed to be naturally resonant at 100 MHz is evaluated for its range of tuning and feasibility for use in a mobile ionospheric heating (MIH) setup. The overarching goal is to match the ionospheric heating performance of the 180 element array at the high frequency active auroral research program (HAARP), which occupies approximately 1.2 × 105 m2 of land in Gakona, Alaska. While each HAARP crossed dipole element occupies 440 m2 of land and is tunable in the range of 2.7-10 MHz using automatic matching networks, the presented ESA approach is aimed toward enabling the fabrication of a transportable MIH array platform capable of high continuous wave (cw) power, albeit with a linear dimension five to ten times smaller than that of an equivalent dipole antenna. It is elucidated that the capacitively tuned ESA is continuously tunable to a frequency about 50% lower than that of the ESA's base frequency, albeit the resonant antenna structure carries a fractional bandwidth of merely 1%-2%.

IEEE Journals

Authors: Vincent Meyers; Daniel Mauch; Vladimir Kuryatkov; Sergey Nikishin; James Dickens; Andreas Neuber; Richard Ness

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8291269

Abstract: Photoconductive semiconductor switches (PCSS) made from bulk, semi-insulating GaN have been fabricated and tested under pulse-charged conditions. Switching response and photocurrent efficiency of GaN PCSSs triggered by sub-10 ns, 355 nm laser pulses is reported. It is shown that fast rise time (<;300 ns) voltage pulses can be used to charge a GaN PCSS to fields well beyond the DC breakdown field strength of GaN and improve switching performance. GaN's wide band gap, breakdown field strength, and electron mobility make it a material superior to SiC and far superior to GaAs for PCSS applications, though historically these materials have dominated PCSS research due to their relative ease of fabrication. Recent improvements to crystal quality and wafer size have allowed GaN and more recently semi-insulating GaN to play an increasing role in high-power and high-voltage solid state devices.

Conferences

Authors: K. Rainwater; D. Barnett; C. Lynn; J. Dickens; A. Neuber; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8012819

Abstract: This paper presents the electrical and mechanical design considerations of a compact, 160 J modular pulse forming network (PFN) based Marx generator used to drive a high-power microwave (HPM) source with a time variant load at a PRF of 100 Hz. The modular Marx generator is designed to produce an open circuit output voltage of 600 kV from a 50 kV source using twelve stages. Each stage of the Marx was constructed from a PFN created with five, 2.1 nF, high voltage capacitors in parallel. Each Marx module was machined out of acetyl copolymer or Delrin to provide optimal strength, rigidity, and a dielectric constant that closely matches transformer oil. These Marx modules include air supply lines that are machined directly into each block of Delrin allowing airlines to connect to each module chamber rather than every spark gap. The spark gaps are comprised of two electrode inserts placed into the sealed pressure vessel contained within the Marx modules. The electrode inserts are of a sleeve-electrode design, which allows the user to insert the electrode into the sleeve, then the sleeve into the Marx module. The benefit of this design is the ability to adjust the electrode gap spacing without compromising the high pressure seal. Two continuous charging inductors run between each PFN and underneath the Marx modules. Due to high voltages generated by the Marx, outer field shaping rings are used to reduce the field stress across the induction coils, resulting in longer lifetime. The inductors are also of modular design allowing for individual coils to be replaced in the event of failure. Output voltage and current waveforms from a 60 Ohm water load are presented.

Conferences

Authors: K. Eldridge; A. Fierro; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7462994

Abstract: Destructive and constructive interference of multiple time-shifted and amplitude-adjusted higher frequency signals (wavelet signals) is exploited in order to reproduce a desired signal at a given point in the far-field regime of radiating antennas. The number of individual wavelets is intentionally kept small in keeping with a realistic antenna array size, where each antenna would emit wavelets at conceivably very high power levels. Wavelet decomposition theory is coupled with particle swarm optimization to determine the necessary time shifts and amplitude adjustments of the wavelet signals. In this application, the reconstructed signal can be specified by a desired frequency or arbitrary shape. A pyramidal horn antenna array is used in the analysis of the far-field propagation of the wavelet signals due to its relatively large bandwidth and known analytical electric field solutions. It is found that when the wavelet signals are appropriately superpositioned and added in the far field, the desired signal may be reconstructed with the quality of reconstruction mostly governed by the intentionally low number of wavelets. The reconstructed signal is solely found on the centerline while the signal drastically changes off the centerline or at distances too close or too far from the antenna array.

IEEE Journals

Authors: M. B. Walls; A. Fierro; J. Dickens; J. Mankowski; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7543491

Abstract: An accurate model for simulating the transient turn-ON performance of thyristor-type switches is desirable for the development of solid-state pulse generators. The existing thyristor models are not suitable since they are either impractical to implement in an SPICE simulator, do not accurately model transient turn-ON performance, or are dependent on external circuit parameters that may not be known during the design phase, such as the load and discharge capacitance. An empirical model is developed and presented for Silicon Power's CCS SC 14N40 thyristor. The process of model determination is detailed, and waveforms obtained from the experiment and an SPICE circuit simulation that implements the thyristor model are discussed.

IEEE Journals

Authors: Chowdhury, AR; Dickens, JC; Neuber, AA; Joshi, RP

PDF: https://aip.scitation.org/doi/10.1063/1.4972968

Abstract: Simulation studies of the electrical response characteristics of 4H-SiC switches containing traps are reported in the absence of photoexcitation. The focus is on trap-to-band impact ionization and the role of hole injection from the anode. Simulations show that hole-initiated ionization can be more important than the electron-initiated process. The results also underscore the role of hole injection at the high applied voltages. Our one-dimensional, time-dependent model yielded reasonable agreement with measured current-voltage data spanning over three orders of magnitude, but only when impact ionization was taken into account. Finally, the simulations predicted undulations in the device conduction current density with respect to time, due to the dynamic interplay between impact ionization, spatial electric field values, and occupancies of the trap levels. Published by AIP Publishing.

Journal

Authors: Takeshi Ihara; Daniel Mauch; James Dickens; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7534406

Abstract: Summary form only given. Development of a switch capable of operation at high voltages, high repetition frequency, with long lifetime is essential for furthering pulsed power applications. Photoconductive semiconductor switches (PCSSs) possess inherent optical isolation and extremely low switching jitter (~10 ps), and have also been experimentally shown to be capable of switching high voltages (up to 50 kV) and currents with very fast rise and fall times (<; 1 ns)[1-2]. In this paper, we report the breakdown characteristics of a SiC PCSS triggered spark gap obtained via measurement of the voltage and current, and simulation of the electric field distribution. The triggered spark gap is composed of sphere-to-sphere electrodes and a field distortion electrode, which is kept at mid-potential in the center plane between the two spherical main electrodes. A normally open PCSS is connected between one of the main electrodes and the mid-plane (trigger) electrode, whose center bore diameter is varied from 5 to 15 mm. With application of the optical pulse to the PCSS, the trigger electrode is temporarily connected to the main electrode, effectively doubling the electric field between the trigger and opposite electrode, leading to main gap closure. In essence, while bulk SiC PCSS switching currents demand very high optical power input, the synergy of bulk SiC PCSS and traditional spark gaps enables the triggering of large current flows at very modest optical powers with low jitter. Overall, the obtained results reveal that incident laser energy and mid-plane electrode geometry heavily influence the breakdown characteristics of the spark gap including jitter time, and breakdown voltage.

Conferences

Authors: Johnson, JM; Reale, DV; Krile, JT; Garcia, RS; Cravey, WH; Neuber, AA; Dickens, JC; Mankowski, JJ

PDF: https://aip.scitation.org/doi/10.1063/1.4947230

Abstract: In this paper, a solid-state four element array gyromagnetic nonlinear transmission line high power microwave system is presented as well as a detailed description of its subsystems and general output capabilities. This frequency agile S-band source is easily adjusted from 2-4 GHz by way of a DC driven biasing magnetic field and is capable of generating electric fields of 7.8 kV/m at 10 m correlating to 4.2 MW of RF power with pulse repetition frequencies up to 1 kHz. Beam steering of the array at angles of +/- 16.7 degrees is also demonstrated, and the associated general radiation pattern is detailed. Published by AIP Publishing.

Journal

Authors: S. Feathers; J. Stephens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7534367

Abstract: Summary form only given. A pulsed UV arc lamp discharge is driven by short pulses (100 ns FWHM) with peak current levels up to 550 A, with a repetition frequency up to 1 kHz. The arc lamp incorporates an ellipsoidal reflector which efficiently focusses > 60% of the total light output onto an approximately 100 mm2 area, appropriate to trigger a large area photoconductive semiconductor switch, PCSS. Experiments using xenonchloride (XeCl*, 308 nm) and xenon-fluoride (XeF*, 351 nm) were conducted. Using nanosecond electrical excitation pulses, excimer emission in both gases was observed at differing absolute output power levels. With XeCl* (HCl 0.06%, Xe 1.5%, Ne 98.44%) and XeF* (NF3 0.12%, Xe 0.36%, Ne 99.52%), XeF* exhibited both higher power and efficiency (~0.01%) compared to XeCl*, which, however, carried comparatively low levels of HCl in the available gas mixture. The optical output energy, temporal shape of the optical power, and efficiency over varying pulse-width, current, and gas pressure were measured. Overall, the efficiency was observed to increase with decreasing pulse-width, and the peak optical power was found to increase with increasing pressure. Using XeF*, the experiments yielded an average peak power just above 300 W with the maximum peak power observed being 400 W. To demonstrate possible future application, the UV output of the 1 kHz repetition rate lamp was utilized to successfully modulate a SiC photoconductive semiconductor switch.

Conferences

Authors: Barnett, DH; Rainwater, K; Lynn, CF; Dickens, JC; Neuber, AA; Mankowski, JJ

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7534308

Abstract: This study focuses on the achieving wide tunabiltiy of a compact reflex triode virtual cathode oscillator (vircator). The cathode is of a bimodal carbon fiber (CF) material paired with a pyrolytic graphite anode. These materials where chosen for their operating characteristics which including but not limited to, long lifetime > 106 shots, high operating temperatures > 1000 K, and large current densities ~200 A/cm3 . A 12 stage, 158 J pulse forming network (PFN) based Marx generator serves to drive the Vircator at 350 kV, 4 kA with ~100 ns pulsewidth. The operating frequency of interest is in the range of 1-6 GHz, where the tunability is achieved by varying the size of the anode-cathode (A-K) gap, the length from the back wall to the A-K gap, or/and the distance from the bottom of the cavity to the A-K gap. The primary focus in this experiment was to increase the achievable frequencies by placing a square waveguide within a sealed vacuum tube. This allows the bottom part of the waveguide to be easily adjusted while still maintaining the waveguide integrity. The resulting microwave frequencies are shown along with the systems performance.

Conference Paper/Presentation

Authors: A. R. Chowdhury; D. Mauch; R. P. Joshi; A. A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7505601

Abstract: We focus on a simulation study to probe the mitigation of electric fields, especially at the edges of metal contacts to SiC-based photoconductive switches. Field reduction becomes germane given that field-induced failures near contacts have been reported. A dual strategy of extending metal contacts to effectively spread the electric field over a larger distance and to employ HfO2 as a high-k dielectric, is discussed. Simulation results show that peak electric fields can be lowered by up to ~67% relative to a standard design. Finally, our calculations predict that the internal temperature rise for a ~7-ns laser pulse and applied voltages around 20 kV (typical experimental values) would also be effectively controlled.

IEEE Journals

Authors: Fierro, A; Stephens, J; Beeson, S; Dickens, J; Neuber, A

PDF: https://aip.scitation.org/doi/full/10.1063/1.4939475#:~:text=Here%2C%20a%20method%20applicable%20to%20plasma%20simulations%20is,of%20discretely%20tracking%20photons%20and%20their%20corresponding%20wavelengths.

Abstract: The self-produced light emission from pulsed plasma discharges and its impact on plasma development are challenging to characterize through simulation and modeling, chiefly due to the large number of radiating species and limited computer memory. Often, photo-processes, such as photoionization or photo-emission of electrons, are implemented through over-simplifying approximations or neglected altogether. Here, a method applicable to plasma simulations is implemented in a Particle-in-Cell/Monte Carlo Collision model, which is capable of discretely tracking photons and their corresponding wavelengths. Combined with the appropriate cross sections or quantum yields, a wavelength dependent model for photo-ionization or photo-emission may be implemented. Additionally, by resolving the wavelengths of each photon, an emission spectrum for a region of interest may be generated. Simulations for a pure nitrogen environment reveal that the calculated emission profile of the second positive system agrees well with the experimental spectrum of a pulsed, nanosecond discharge in the same spectral region. (C) 2016 AIP Publishing LLC.

Journal

Authors: P. M. Kelly; C. F. Lynn; J. M. Parson; J. Dickens; A. Neuber; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534045

Abstract: Summary form only given. The results from the development of a three-dimensional particle-in-cell (PIC) model investigating frequency tunability of a reflex-triode virtual cathode oscillator (vircator) are presented. These efforts have focused upon achieving stable frequency output at many different frequencies in the S, L, and C-bands, from a single tube, using the ICEPIC (Improved Concurrent Electromagnetic Particle-In-Cell) code from the Air Force Research Laboratory (AFRL). Experimental data from the hard-tube vircator at Texas Tech University (TTU), which operates at background pressures less than 10-9 Torr and utilizes a bimodal carbon fiber cathode and pyrolytic graphite anode, is used to validate simulation results. Additionally, the vircator at TTU is capable of changing the accelerating voltage, the anode-cathode (A-K) gap distance, and the distance of the cavity backwall relative to the position of the A-K gap on the fly without breaking tube vacuum, all in an effort to achieve greater frequency tunability and output power. However, this creates a very large, time-intensive experimental parameter space and makes a simulation model attractive for exploring additional output capabilities. ICEPIC results from a large combination of A-K gap distances, backwall distances, driving voltages, and cavity diameters are presented and compared to experimental results, highlighting frequency tunability of the system from a single tube.

IEEE Conferences

Authors: A. R. Chowdhury; H. K. Nguyen; R. P. Joshi; J. C. Dickens; J. J. Mankowski; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534285

Abstract: Summary form only given. Breakdown of air at atmospheric pressure in high frequency uniform electric fields and large gaps is discussed. In the high frequency band of a few MHz to few tens of MHz, the breakdown threshold voltage is lowered from its DC value due to enhanced space charge from ions that become trapped in the gap.1 While there is some literature concerning breakdown in this frequency range, it does not consider gaps larger than 1 cm.²A fluid model is developed to simulate plasma development in a baseline 6 cm gap primarily to explore power limitations for high power, electrically small antennas, which are operated cw at MHz frequencies. The ion densities are obtained from a drift-diffusion model, though data for the ionization, electron collision, and attachment parameters were obtained from Monte Carlo simulations, while ion diffusion and drift velocities were taken from the literature. As expected, the Monte Carlo simulations reveal that the EEDF follows any change in the electric field on the picosecond timescale at atmospheric pressures, much faster than any variation due to the externally applied electric field. Results from the simulation for gap lengths varying from the 6 cm baseline and air pressures are obtained, analyzed, and also compared with available reports.3

Conference Paper/Presentation

Authors: Reale, DV; Parson, JM; Neuber, AA; Dickens, JC; Mankowski, JJ

PDF: https://aip.scitation.org/doi/10.1063/1.4942246

Abstract: A stripline gyromagnetic nonlinear transmission line (NLTL) was constructed out of yttrium iron garnet ferrite and tested at charge voltages of 35 kV-55 kV with bias fields ranging from 10 kA/m to 20 kA/m. Typically, high power gyromagnetic NLTLs are constructed in a coaxial geometry. While this approach has many advantages, including a uniform transverse electromagnetic (TEM) mode, simple interconnection between components, and the ability to use oil or pressurized gas as an insulator, the coaxial implementation suffers from complexity of construction, especially when using a solid insulator. By moving to a simpler transmission line geometry, NLTLs can be constructed more easily and arrayed on a single substrate. This work represents a first step in exploring the suitability of various transmission line structures, such as microstrips and coplanar waveguides. The resulting high power microwave (HPM) source operates in ultra high frequency (UHF) band with an average bandwidth of 40.1% and peak rf power from 2 MW to 12.7 MW. (C) 2016 AIP Publishing LLC.

Journal

Authors: D. L. Mauch; V. E. Meyers; R. P. Joshi; A. A. Neuber; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534404

Abstract: A comprehensive picture of the relationship between optical fluence, optical wavelength, system load, and photocurrent efficiency (PE) in SiC photoconductive semiconductor switches (PCSSs) is presented. Variation of the optical wavelength (300-380 nm) and optical fluence (0.2-200 J m-2) was accomplished with a Nd:YAG pumped optical parametric oscillator (7 ns FWHM) and a broadband variable attenuator. The PE was found to typically be in the range of 1-2 %, depending on wavelength, and the bulk PCSS on-state voltage driven by external circuit parameters. Features of the high electric field stress behavior (> 200 kV/cm) of the bulk PCSS were captured with high fidelity in a 1D drift-diffusion model with a self-consistent Poisson solver including trap assisted tunneling, Poole-Frenkel, and barrier lowering with enhanced tunneling effects. In addition, trap to band impact ionization as well as Coulombic and repulsive trapping potentials were included.

IEEE Conferences

Authors: Z. Shaw; W. Feilner; C. Lynn; A. A. Neuber; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8012905

Abstract: A two antennae, software controllable phased array was designed and fabricated to study the generation and transmission of short, nanosecond, nonperiodic pulses. This system allows transmitting a train of appropriately scaled and time-shifted bipolar signals to generate received signals with major frequency components adjustable from approximately 600 MHz to 1.5 GHz. The main components in the system include two digital to analog converters (DACs), two data pattern generators (DPG), and two power amplifiers which drive two TEM Horn antennas. The horn antennas are based on a Chebyshev taper design and a Microstrip-type balun is utilized for the transition from the coaxial feed. This approach yielded a reasonably flat frequency response in a wide range from 0.2 to 3 GHz. Thus far, 100 ps synchronization between channels was achieved, and signals of varying shape and amplitude have been received via the shifting and inverting of Gaussian input pulses defined by the user generated data input vectors. This paper presents an experimental evaluation of the hardware used to generate the multichannel array, the ability to steer the signals and generate signals of varied frequency via superposition in free space.

IEEE Conferences

Authors: B. Esser; J. Dickens; J. Mańkowski; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534197

Abstract: An electrically small, three dimensional metamaterial inspired antenna at MHz frequencies is evaluated for its potential future use as the principle radiating element in a Mobile Ionospheric Heating (MIH) installation. The design consists of a small loop antenna (SLA) which inductively couples to a capacitively loaded loop (CLL) providing a high Q with a natural match to a 50 Ω source. At approximately 2.5 × 3 × 1.2 m in size the antenna is significantly smaller than the element, 21 × 21 × 16 m, used in the High-frequency Active Auroral Research Program (HAARP) array designed for ionospheric modification. With a gain upwards of 7 dBi when mounted above a sufficiently large ground plane, and up to 90 % efficiency, this electrically small antenna, ESA, provides a compact, efficient radiating element as a standalone radiator or as part of an array. It is demonstrated that tuning from approximately 3 to 10 MHz is possible via adjustment of the capacitance in the CLL and the mutual inductance between the SLA and CLL. A full-scale prototype antenna was fabricated and evaluated to obtain experimental radiation efficiencies and patterns at 500 W power levels, which compare favorably to simulations. Scaling for array operation revealed that it is conceivable to generate 3.6 GW ERP power in equatorial zones from a transportable platform based on the ESA. Such power levels suffice effecting ionospheric modification and, for instance, ELF wave generation in the range of 1-70 Hz.

IEEE Conferences

Authors: A. Majzoobi; R. P. Joshi; A. A. Neuber; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7570265

Abstract: Particle-in-cell simulations are performed to analyze the role of secondary electron emission (SEE) on the efficiency, the output power and the leakage currents of 12-cavity, 12-cathode Rising-Sun magnetrons with diffraction output. The simulation results seem to indicate that the role of SEE would be fairly negligible. Small changes are predicted, linked to deviations in the starting trajectories of secondary electrons following their generation and the lower fraction of electrons in clusters with a synchronized rotational velocity. Overall, a peak power output of about 2.48 GW is predicted at a magnetic field of 0.45 T, with efficiencies as high as 75%. Furthermore, deviations in the output power with SEE are predicted to occur at shorter times, but would not be an issue for pulses greater than 25 ns in duration.

Journal

Authors: V. Meyers; D. Mauch; J. Mańkowski; J. Dickens; R. Joshi; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534405

Abstract: Nonlinearity of optical absorption in semi-insulating bulk 4H-SiC has been investigated. Of interest was the optical bleaching behavior of 4H-SiC at and just above the band edge in the range 3.11-3.33 eV (wavelength 380-355 nm). Results of experiments on 200 μm and 490 μm thickness samples indicate partial bleaching in the optical fluence range from 70 W/cm2 to 1.8 kW/cm2, and the absorption coefficient was found to vary by approximately 10% within this range. These experimental findings are supported by simulation results obtained from a first order semi-empirical rate based model linking excitation-induced change in density of states with the absorption coefficient over the range of tested power densities. As expected, this effect scales with photon energy. Characterization of 4H-SiC absorption behavior under varying fluence will aid in design optimization of a Photoconductive Semiconductor Switch (PCSS).

IEEE Conferences

Authors: P. M. Kelly; J. Dickens; A. Neuber; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534046

Abstract: This study describes a three-dimensional particle-in-cell (PIC) simulation for a reflex-triode virtual cathode oscillator (vircator). In particular, these efforts have focused upon the development of a robust, consistent model using the ICEPIC (Improved Concurrent Electromagnetic Particle-In-Cell) code from the Air Force Research Laboratory (AFRL). The vircator operates at background pressures less than 10^-9 Torr and utilizes a bimodal carbon fiber cathode and pyrolytic graphite anode. Experimental data from the hard-tube vircator at Texas Tech University (TTU) is used to validate simulation results. A working, three-dimensional model of a reflex-triode vircator allows for better understanding of the physical processes responsible for microwave generation and thus enables the development of a more efficient and more customizable system. Simulation results detail the virtual cathode formation and the subsequent extraction of radiated microwave energy. Rather than relying on a non-directional isotropic radiation pattern for the radiated power, the true effective radiated power (ERP) from a three-dimensional, frequency-dependent radiation pattern has been extracted from the ICEPIC model. Furthermore, contributions from higher-order modes, particularly in the upper C-band regime, lead to frequency hopping and decreased microwave output power. Simulated results aid in identifying mode contributions and developing schemes to minimize contributions from undesirable modes. ICEPIC results are presented and compared against experimental results at several different operating conditions.

Conference Paper/Presentation

Authors: Nicholas A. Wilson; Daniel L. Mauch; James C. Dickens; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7534040

Abstract: Summary form only given. The electrical and optical characteristics of a high power UV LED (365 nm wavelength) were evaluated under pulsed operating conditions for pulsed power applications. Measurements were made over varying pulse width (30 ns-100 μs), current (0 A-250 A), repetition rate (single shot -1 MHz), and temperature (23° C-80° C). Diagnostics used included a calibrated photodiode operating in the linear regime for transient optical power measurements, a grating / high speed ICCD based spectrograph for transient spectral analysis, and multiple 10:1 standard oscilloscope probes configured differentially for electrical measurements. A red shift was observed in the output spectrum of the LED with increasing temperature and increasing pulse-width. LED forward voltage was observed to increase linearly with increasing current (≈ 3.5 V-5.2 V) and decrease with increasing pulse-width. The peak optical power observed was > 13 W and a maximum efficiency of 22 % was observed. The evaluated LED and auxiliary hardware were successfully used as the optical trigger source for a SiC photoconductive semiconductor switch (PCSS) under high impedance conditions.

Conferences

Authors: C. Lynn; D. Barnett; K. Rainwater; A. Neuber; J. Dickens; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534307

Abstract: A stand-alone vircator based high power microwave system has been designed as an effects test source. The goal of this system is to cover the frequencies from 4.0 GHz to 6.0 GHz. To date a working prototype has been manufactured and field tested. The current system is capable of producing microwaves at frequencies of 4.06, 4.27, 4.45, 5.83, 5.95 and 6.06 GHz at field levels in excess of 100 kV/m, measured at 3 meters. Additionally, the source and all subsystems are capable of burst mode operation for a duration of 2 s at 500 Hz pulse repetition frequency (PRF). The wide range of tunability was achieved by varying the A-K gap as well as the location of the A-K gap inside the cavity. The A-K gap is varied by a bellows sealed linear actuator which is attached to the cathode. The position of the A-K gap within the cavity is altered by moving a liner and microwave reflector (which form a cavity) within the main vacuum tube. However, the experimentally observed frequencies obtained with the current system leaves a gap from 4.45 to 5.6 GHz. In order for the test system to produce microwaves within this frequency range, a new tube is under development. The cavity of the new tube has a smaller cavity which should push the resonant frequencies into the desired range (from 4.45 GHz to 5.6 GHz). This presentation discusses the experimental results obtained with the new tube and compares that with the previously built and tested source.

Conference Paper/Presentation

Authors: A. A. Neuber; D. L. Mauch; V. E. Meyers; B. Esser; R. P. Joshi; J. C. Dickens; J. J. Mankowski; T. M. Antonsen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7534401

Abstract: A transportable ionospheric heater, TIH, research design is presented that will enable plasma studies of the ionosphere in latitudes that are presently inaccessible by fixed installations such as HAARP (High Frequency Active Auroral Research Program). The equatorial latitude with close to zero vertical magnetic fields is especially of interest for basic plasma physics studies as well as rf communication enhancement. To achieve a power level in the ionosphere of at least 70 dBW ERP in a footprint significantly smaller than HAARP the radiated power needs to be substantially increased. This minimum ERP is achievable in a 4 × 4 antenna array with 370 kW input power per element with about 25 m by 25 m footprint vs. HAARP's equivalent 365 m by 365 m (360 antenna elements total, 10 kW maximum per antenna). Maximum ERP, up to 95 dBW, may be achieved with the TIH on a 115 m by 70 m platform, a factor 17 reduced size from HAARP. Tunable, Electrically Small Antennas, ESAs are employed to overcome the maximum power limitations of the HAARP dipole based antennas. This demands a step-up from 10 kW to several 100 kW cw power in the 3 to 10 MHz band, which is required to effectively heat the ionosphere. Driving the ESAs necessitates a tunable rf source in the same power and frequency regime, where a more traditional rf tube or all solid state approach may be pursued. The focus of the driver related research has been on photoconductive solid state switching, PCSS, in a direct drive mode that incorporates the driver into the antenna itself. A full size ESA operating at 9.5 to 10 MHz has been demonstrated at 500 W cw power levels and ~ 90% efficiency, driven by a single SiC switch mimicking the full power PCSS operation. The challenges and physics limitations of scaling the switch, the tunable ESA antenna design, as well as their coupling are presented. The significant progress made towards a transportable ionospheric heater as it relates to the physics of the PCSS switching efficiency, electrical breakdown in the MHz regime in large gaps, lower power experiments, and numerical simulations is discussed.

IEEE Conferences

Authors: W. Feilner; Z. Shaw; C. Lynn; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8012903

Abstract: Arbitrary transient electric field shapes are generated in free space utilizing a set of transient signals with proper shape, amplitude, and time shift. Akin to wavelets in signal processing, brief, non-periodic oscillations are superimposed at a pre-selected location in space to effect destructive and constructive interference. With a properly chosen signal set, an entirely different frequency or shape is generated. Two methods have been employed to find optimum signal sets, the Discrete Wavelet Transform (DWT) and Particle Swarm Optimization (PSO). While the DWT approach dictates constant time step and rectangular matching between wavelets, PSO is not restricted in this manner, allowing for more flexibility in choosing amplitudes and signal delays.

IEEE Conferences

Authors: J. Johnson; D. Reale; D. Barnett; R. Garcia; W. Cravey; J. Parson; A. Neuber; J. Dickens; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296894

Abstract: This paper details the design, fabrication and performance of a coaxial ferrimagnetic nonlinear transmission line (NLTL), four element array, high power microwave (HPM) system operated at a 1 kHz repetition rate. Prime power is delivered from an 802L TDK Lambda power supply which charges a 5.2 nF capacitor bank up to -40 kV. The capacitors are discharged through a center pin trigatron spark gap. The trigger generator is optically isolated and battery powered for noise immunity and portability. It produces a 20 kV positive polarity pulse with a 20 ns risetime. The high dV/dt (1 kV/ns) is desirable to reduce jitter inherent to spark-gap switching. After the spark-gap switch has closed, the pulse is split four ways. The four pulses propagate through four adjustable delay lines for synchronization of the individual outputs. The four delay lines connect directly into four 76 cm NLTLs with NiZn ferrites where SF6 is the insulating dielectric. Each NLTL is terminated into a custom fabricated, Rexolite-filled, TEM horn antenna via a zipper balun. Lastly, a LabVIEW based control system automates the whole system using a National Instruments cRIO controller. Experimental observations will include in-line D-dot measurements of voltage waveforms and radiated D-dot field measurements.

IEEE Conferences

Authors: J. Shaver; D. Mauch; R. Joshi; J. Mankowski; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296867

Abstract: 4H-SiC Photoconductive Semiconductor Switches (PCSSs) have shown significant promise for use in pulsed power related switch applications. This simulation uses the finite difference method, parallelized using a NVIDIA graphical processing unit and the CUDA framework, to solve the system of partial differential equations that model the semiconductor physics involved in the high voltage blocking state of the photoconductive switch. By taking into consideration material properties (mid-band gap trap energy level and concentration, etc.), we are able to gain an understanding of how changes in these parameters affect the space-charge-limited (SCL) currents observed in the high voltage blocking state. This subsequently allows for a fundamental understanding of the parameters controlling the high voltage switching capability of photoconductive switches. Results of the simulation are presented.

IEEE Conferences

Authors: Jonathan M. Parson; Curtis F. Lynn; Mike C. Scott; Steve E. Calico; James C. Dickens; Andreas A. Neuber; John J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7052371

Abstract: Operation of repetitive high-power microwave (HPM) sources is predominantly limited by thermal properties of anode and cathode materials. This letter presents a reflex-triode virtual cathode oscillator (vircator) capable of operating at 500 Hz at current densities between 100-200 A/cm2 for multiple burst durations of 1-2 s. Stable vircator operation under such a thermally punishing environment is facilitated by the use of a thin pyrolytic graphite anode. The results presented focus on two anode-cathode (A-K) gap spacings: 11 and 21 mm, which produce stable microwave radiation at 4.6 and 1.6 GHz, respectively. Characteristic voltage, current, and microwave waveforms in conjunction with short-time Fourier transforms, frequency spectrographs, and HPM power density data for 1000 and 500 pulses at 1.6 and 4.6 GHz, respectively, are presented.

Journals

Authors: Beeson, S; Dickens, J; Neuber, A

PDF: https://aip.scitation.org/doi/10.1063/1.4914043

Abstract: A 4-port S-band waveguide structure was designed and fabricated such that a signal of any amplitude (less than 1 MW) can be switched from a normally closed state, <0.5 dB insertion loss (IL), to an open state >30 dB IL by initiating plasma in a gas cell situated at the junction of this waveguide and one propagating a megawatt level magnetron pulse. The 90/10 switching time is as low as 20 ns with a delay of similar to 30 ns between the onset of the high power microwave pulse and the initial drop of the signal. Two ports of this device are for the high power triggering pulse while the other two ports are for the triggered signal in a Moreno-like coupler configuration. In order to maintain high isolation, these two sets of waveguides are rotated 90 degrees from each other with a TE111 resonator/plasma cell located at the intersection. This manuscript describes the design and optimization of this structure using COMSOL 4.4 at the design frequency of 2.85 GHz, comparison of simulated scattering parameters with measured cold tests (testing without plasma), and finally the temporal waveforms of this device being used to successfully switch a low power CW signal from 2W to <5 mW on a sub-microsecond timescale. (C) 2015 AIP Publishing LLC.

Journal

Authors: D. H. Barnett; J. M. Parson; C. F. Lynn; P. M. Kelly; J. C. Dickens; A. A. Neuber; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296790

Abstract: The paper presents the design and operational characteristics of an optically isolated, compact, modular pulse generator for spark-gap triggering applications. The pulse trigger generator is capable of operating at pulse repetition frequencies (PRFs) > 1 kHz for short bursts with variable pulse magnitudes and risetimes. The trigger generator utilizes a transformer, magnetic switch and an IGBT primary switch. It has been successfully used to trigger a trigatron-driven 10-stage, Marx generator driving a high power load. For portability and noise immunity, the trigger generator is optically isolated from its low voltage control and powered via a lithium ion polymer battery pack. A constant current dc-dc power supply charges the high voltage circuitry of the trigger generator and enables continuous operation with two modules. For operation, a large capacitor is initially charged and used as a buffer energy source. The intermediate charge storage in conjunction with a command-triggered MOSFET, provides the ability to quickly re-charge a trigger capacitor between each pulse of the burst. Circuit topology, experimental data, including voltage and current waveforms, and jitter of the overall system are discussed at various PRFs.

Conference Paper/Presentation

Authors: Shu Lin; Sterling Beeson; Yongdong Li; Chunliang Liu; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7179681

Abstract: Summary form only given. The mechanism of plasma formation under high power pulse (HPM) excitation in gases (nitrogen and argon) at atmospheric pressures is studied utilizing a fluid model verified against experimental data. A 2-D approximation was introduced to model the cylindrically shaped gas volume and the associated electric fields. Thus, the fluid equations and Poisson equation for space charge effects are updated in x and y directions only (setting the gradient in the z-direction to 0). The goal of this numerical simulation was to accurately predict the plasma formation delay time under different gas types and pressures based on the calculated evolution of the plasma conductivity.An S-band TE111 resonator with a built-in quartz gas isolation tube in the center was fabricated1. A 4 MW, 4 μs pulse in the dominant TE10 mode provided by a 2.85 GHz magnetron propagates through it. In the fluid model, the effect of HPM on plasma formation is modelled with particle heating as well as elastic and inelastic collisions driven by the incident electric field in the vertical (normal) coordinate. The amplitude distribution of the HPM electric field is obtained from numerical simulation of the resonator using commercial EM software. Since the focus is on the onset of plasma formation, simulation is stopped before a large plasma density develops up to the moment when the transmitted power is reduced by 10% (-0.5 dB), which corresponds to an average plasma conductivity of 0.02 S/m. Both measurement and simulation cover a gas pressure range from 25 to 700 torr with delay times for N2 and Ar from 22 to 204 nanoseconds. The development of the particle densities and temperatures during the simulation is presented to reveal the dominant mechanism of plasma formation in atmospheric gases. The delay times for different gas types and pressures are in good agreement for low gas pressures (<; 200 torr). Deviations between the model and experiments at higher pressure are found to be primarily due to the onset of plasma filamentation.

Conferences

Authors: K. Eldridge-Looker; A. Fierro; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296929

Abstract: Large antenna sizes present a severe limitation in the generation of low frequency signals. To alleviate this issue, a series of high frequency sources (and thus, smaller antenna sizes) may be combined to produce a lower frequency output in the far-field. This reconstruction technique applies to signals of arbitrary shapes in addition to frequency lowering. Due to the large parameter space, wavelet decomposition theory is coupled with particle swarm optimization to define appropriate time shifts and amplitude adjustments to the high frequency sources (wavelets) in order to synthesize an output signal with the desired frequency or shape. Fifteen individual high frequency wavelets (f = 1 GHz) are utilized in this application to produce a final output signal in the far-field of 600 MHz. The radiation of a single wavelet is simulated using a broadband Chebyshev TEM horn antenna in order to perform far-field constructive and destructive interference analysis of all 15 wavelet signals. Simulation of the TEM horn antenna shows reasonable wavelet signal fidelity with minimal reflections from the antenna aperture as the pulse is propagated to the far-field. Examination of the far-field electric field enables accurate temporal depiction of the reconstructed signal from the 15 pulsed sources at any given point in space. By adjusting the source array geometry, the location of the desired signal (determined by the particle swarm optimization) can be narrowed down to a single location. Overall, combining particle swarm optimization, wavelet decomposition theory, and electromagnetic wave propagation enables the accurate reconstruction of far-field temporal electric fields from the combination of 15 wavelet sources and verification of the desired signal location.

IEEE Conferences

Authors: Esser, B; Beeson, S; Dickens, J; Mankowski, J; Neuber, A

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7296897

Abstract: A tunable, metamaterial-inspired, electrically small antenna topology is evaluated for a possible future use as the principle radiating element in a mobile Ionospheric Heating (MIH) system. The RF source signal is fed via a 50 Omega coaxial cable into a small semi-loop antenna (SLA). This inductively couples to a capacitively loaded loop (CLL) providing a natural 50 Omega match to the source. The resonant frequency of the antenna can be adjusted by varying the capacitance of the CLL via inserting a large permittivity dielectric. A simplified circuit model is used to show that the resonant frequency can be tuned between 40 - 100 MHz. Also, the maximum power handling capabilities achievable with this antenna topology at frequencies relevant to ionospheric heating (similar to 10 MHz and below) are estimated.

Conference Paper/Presentation

Authors: Tiskumara, R; Joshi, RP; Mauch, D; Dickens, JC; Neuber, AA

PDF: https://aip.scitation.org/doi/pdf/10.1063/1.4929809

Abstract: A model-based analysis of the steady-state, current-voltage response of semi-insulating 4H-SiC is carried out to probe the internal mechanisms, focusing on electric field driven effects. Relevant physical processes, such as multiple defects, repulsive potential barriers to electron trapping, band-to-trap impact ionization, and field-dependent detrapping, are comprehensively included. Results of our model match the available experimental data fairly well over orders of magnitude variation in the current density. A number of important parameters are also extracted in the process through comparisons with available data. Finally, based on our analysis, the possible presence of holes in the samples can be discounted up to applied fields as high as similar to 275 kV/cm. (C) 2015 AIP Publishing LLC.

Journal

Authors: A. Majzoobi; R. P. Joshi; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7297005

Abstract: Intense electron emission from cathodes that provide very high current densities (several kA/cm2) is necessary for a various pulsed power applications. This contribution presents a quantitative analyses of the following processes and inherent physics: (a) Local field enhancements at micro-protrusions, (b) role of ion/ions near the emitting surface in lowering and thinning the potential barrier which increases emission. (c) localized heating at cathode tips that could produce hot-electrons and hot-phonons, ultimately leading to localized melting. Temperatures are predicted to possibly reach the cathode melting point on the nanosecond time scales. This is in keeping with the explosive emission phenomenon that is well known.

IEEE Conferences

Authors: Curtis F. Lynn; Jonathan M. Parson; Michael C. Scott; Steve E. Calico; James C. Dickens; Andreas A. Neuber; John J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7104123

Abstract: The thermal behavior of several electrically conducting solids under high incident electron fluence in high vacuum was evaluated. At electron energies of up to ~200 keV, the depth-dose relationship for electron penetration into the materials was considered, and the resulting energy deposition profile from the surface was revealed to extend to a maximum of ~175 μm below the surface depending on the anode material. Black body radiation is considered as the major mechanism that balances the power deposited in the material on the timescales of interest. Comparing the radiated power density at the sublimation temperature for different materials, metallic/nonmetallic, revealed that pyrolytic graphite anodes may radiate over 20 times more power than metallic anodes before failure due to sublimation. In addition, transparent pyrolytic graphite anodes (with a thickness on the order of several tens of micrometer) potentially radiate up to 40 times that of metallic anodes, since heating by the electron beam is approximately uniform throughout the thickness of the material, thus radiation is emitted from both sides. Experimental results obtained from titanium and pyrolytic graphite anodes validate the thermal analysis.

Journals

Authors: S. L. Holt; C. F. Lynn; J. M. Parson; J. C. Dickens; A. A. Neuber; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7297029

Abstract: Fast capacitor charging is a power electronics application with unique challenges requiring both low voltage, high current operation and high voltage, low current operation from the same system at different points in the charge cycle. Such dynamically differing operating points create distinct challenges in obtaining high efficiency throughout a charge cycle. In addition, the power supply must be protected from negative voltage swings when attached to a high rep-rate power modulator. This paper presents the design and testing of a rapid capacitor charger designed for high rep-rate command charging of a Marx generator. The output stage uses a large inductor to delay discontinuous conduction during the early portion of the charge cycle and greatly improve efficiency. The design goals and tradeoffs will be discussed and simulation results will be compared to experimental data.

IEEE Conferences

Authors: Meyers, V; Mauch, D; Mankowski, J; Dickens, J; Neuber, A

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7296862

Abstract: The optical properties of bulk semi-insulating GaN:Fe are obtained to assess its future suitability as a high voltage photoconductive semiconductor switch (PCSS). The material properties of GaN: Fe hold significant promise to improve devices for pulsed power and other applications. Growth techniques of bulk GaN: Fe, which have hitherto been largely insufficient for commercial applications, are nearing the point that anticipatory characterization research is warranted. In this paper, the optical constants of bulk GaN: Fe (refractive index, absorption coefficient, and off-state dielectric function) were determined by optical reflection/transmission analysis. The results of this analysis are compared with a similar treatment of bulk 4H-SiC as well as possible elements of PCSS housing: Sylgard 184 elastomer, and EFI 20003/50013 electrical potting epoxy. The data presented provide foundational material characterization to enable assessment of the feasibility of GaN: Fe as a practical high voltage PCSS material. Beyond basic materials research, these properties inform design optimization in PCSS construction and implementation.

Conference Paper/Presentation

Authors: D. Thomas; D. Mauch; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296950

Abstract: A method of characterizing mid-bandgap defect states in high purity semi-insulating 4H-SiC through leakage current analysis for optimization of SiC photoconductive switches is presented. The method utilizes two custom IV curve tracer systems to measure leakage currents through the material under various voltage/current conditions. The first system is used under low current conditions and is capable of measurements from 0 to 45 kV at currents ranging from 0 to 3 mA with pA resolution. A second system measures the transient discharge of a charged capacitor bank through the material. Due to power dissipation concerns, the second system is used for currents higher than 0.1 mA. Voltage/current measurements in this region (>0.1 mA) are of interest due to the information concerning defect states near the conduction band. These shallow defect states are detrimental to switching performance while offering little benefit to voltage hold-off. From the combined data of these two systems, characteristics of the defect states are extracted and presented. We further elucidate the effect of contact annealing temperature on shallow trap levels in electron-beam irradiated material (2*1018 1/cm2).

IEEE Conferences

Authors: Rocha, E; Parson, JM; Lynn, C; Dickens, JC; Neuber, A; Mankowski, J; Queller, T; Gleizer, JZ; Krasik, YE

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7179763

Abstract: This study focuses on the performance of a bimodal carbon fiber (CF) cathode and a carbon-epoxy multicapillary (CEM) cathode with microwaves at 50kV/m at 2m lasting 100 ns in a compact reflex triode virtual cathode oscillator (vircator). It was previously revealed that the CEM cathode was able to produce uniform emission distribution for a long duration1 . To further uncover the location(s) where plasma is forming, an intensified CCD camera is used to image both cathodes with high spatial and temporal resolution. A titanium grade 1 (TiG1) anode that is 70% transparent is used to minimize outgassing in the system resulting in the cathode being the primary outgassing constituent2 . One data set for each cathode was taken. Each data set contains diode current, voltage, and microwave fields over the course of 10,000 shots. A performance baseline is shown by comparing the evolution of these data over the course of the experiments. An 8 stage, 168 J pulse forming network (PFN) based Marx generator serves to drive both cathodes at 250 kV, 4 kA with ~175 ns pulsewidth, achieving current densities of ~200 A/cm3 . The operating frequency of interest is set in the range of 1-2 GHz, where the tunability is achieved by varying the size of the anode-cathode (A-K) gap. In all experiments the gap is fired at a 10 Hz pulse repetition rate. The characteristics of one type of cathode over the other, depending on the operating conditions, are discussed in detail.

Conference Paper/Presentation

Authors: Evan Rocha; Patrick M. Kelly; Jonathan M. Parson; Curtis F. Lynn; James C. Dickens; Andreas A. Neuber; John J. Mankowski; Tal Queller; Joseph Gleizer; Yakov E. Krasik.

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7163638

Abstract: This paper evaluates the performance of a bimodal carbon fiber cathode and a carbon-epoxy multicapillary cathode operating within a reflex-triode sealed-tube virtual cathode oscillator (vircator). The experimental results reveal that both cathodes exhibit similar emission behavior, although with some significant operational differences. An eight-stage 84-J pulseforming network-based Marx generator serves to drive both cathodes at 250 kV and 3-4 kA with a ~70-ns pulsewidth. Both cathodes undergo conditioning over 10000 pulses to determine gas evolution as well as electrical changes over time. Gas evolution of both cathodes is observed using a residual gas analyzer to determine individual gas constituents. A comparison of diode voltage, diode current, RF output, and outgassing data for both cathodes during vircator operation over 10000 pulses is presented to quantify cathode performance in a sealed-tube vircator. Changes in cathode surface morphology, from virgin to postmortem, are discussed. Data for various anode-cathode gap distances, from 3 to 15 mm, are presented. The evolution of voltage and current inputs to the vircator is discussed.

Journals

Authors: D. Mauch; J. Dickens; V. Kuryatkov; V. Meyers; R. Ness; S. Nikishin; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296989

Abstract: Semi-insulating Gallium Nitride is evaluated as a candidate material for use as a high voltage photoconductive semiconductor switch (PCSS) for pulsed power applications. The GaN:Fe samples used for this investigation were commercially available, bulk, semi-insulating samples measuring 10 mm × 10 mm × 475 μm. Their optical and crystallographic properties were determined utilizing cathodoluminesence, photoluminescence, RHEED, as well as microwave reflection techniques for carrier lifetime studies. Experimental results are presented elucidating the potential of GaN:Fe sustaining high potential differences in both lateral and vertical geometry devices. For instance, electric field hold-off exceeding 100 kV/cm was observed in lateral geometry with mm sized gaps. In addition, a process for the homo-epitaxial growth of GaN:Si was developed in order to facilitate the fabrication of high quality ohmic contacts. Lastly, experimental results evaluating the on-state performance and photo-current efficiency of a GaN:Fe based PCSS are presented.

IEEE Conferences

Authors: D. Mauch; C. Hettler; W. W. Sullivan; A. A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7151899

Abstract: The power output, forward voltage, conversion efficiency, and spectral characteristics of a 365 nm ultraviolet light-emitting diode (LED) were measured for applications of triggering wide-bandgap photoconductive switches for pulsed power applications. Pulsed currents through the LED ranged from 125 mA up to 2.2 A at widths from 10 μs up to several seconds. Using time-resolved electroluminescence spectroscopy, peak emission was observed to occur at 368.5 nm for short pulses with a red-shift to 371.8 nm for pulses 8 s in duration. A peak light output of 4.1 W was measured for short pulses (<;50 μs) of 2.12 A, corresponding to six times the rated output specification. The LED was used to trigger a high-voltage photoconductive semiconductor switch (PCSS) at voltages up to 6 kV into a high-impedance load. The 365 nm LED is a promising candidate for optical triggering of PCSS devices.

IEEE Journals

Authors: S. Feathers; A. Fierro; S. Beeson; J. Stephens; J. Dickens; A. Neuber; K. M. Williamson

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7799673

Abstract: A 3 microsecond risetime, 300 μs fall time, 25 kV high voltage pulse, chosen to emulate lightning discharges is used to investigate breakdown in close proximity to a high permittivity rutile dielectric (Er ~ 100) cylinder with 1 cm diameter. A needle-plane electrode structure is utilized with 1.05 cm electrode separation. The dielectric location is varied from immediately under the needle, to more than a 1 cm away from the needle axis. Basic system design and simulation are presented, along with simulations of the electric field distribution for various dielectric locations. The dependence of breakdown voltage on dielectric proximity is presented in detail. For instance, a minimum in breakdown voltage is observed when the dielectric cylinder is placed slightly off-center from the needle's center axis. Using high speed imaging, discharge inception and growth are presented with ~10 ns resolution.

IEEE Conferences

Authors: W. H. Cravey; D. V. Reale; R. S. Garcia; J. M. Johnson; A. A. Neuber; J. C. Dickens; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296987

Abstract: This paper discusses the design of a gas flow system for a 1 kHz repetition rate trigatron based spark gap. The system requires a flow rate high enough to clear the gas from the gap in under 1 ms while also maintaining the required operating pressure. The gap is a trigatron based design with an alumina insulated tungsten trigger pin. A 20 kV positive polarity pulse, with a 20 ns risetime, is applied to trigger the main gap which is charged to −40 kV. A brass guard ring is employed to shield the walls of the containment structure from contaminants in order to achieve a longer operational lifetime. Dry air and Nitrogen gasses are tested at various pressures and flow rates. Design considerations, such as inlet and outlet sizing and gap geometry, are taken into account to ensure a high air flow is achieved between the electrodes.

IEEE Conferences

Authors: D. Mauch; W. Sullivan; A. Bullick; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7108031

Abstract: Several generations of high power, lateral, linear mode, intrinsically triggered 4H-SiC photoconductive semiconductor switch designs and their performance are presented. These switches were fabricated from high purity semi-insulating 4H-SiC samples measuring 12.7 mm × 12.7 mm × 0.36 mm and were able to block dc electric fields up to 370 kV/cm with leakage currents less than 10 μA without failure. Switching voltages and current s up to 26 kV and 450 A were achieved with these devices and ON-state resistances of 2 Ω were achieved with 1 mJ of 355 nm laser energy (7 ns FWHM). After fewer than 100 high power switching cycles, these devices exhibited cracks near the metal/SiC interface. Experimental and simulation results investigating the root cause of this failure mechanism are also presented. These results strongly suggest that a transient spike in the magnitude of the electric field at the metal/SiC interface during both switch closing and opening is the dominant cause of the observed cracking.

IEEE Journals

Authors: E. Cordero; S. Holt; J. Dickens; A. Neuber; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296920

Abstract: High power pulsed systems often require high current 3-phase electrical service or large generators to meet prime power requirements, limiting the portability of the systems. A high power battery system offers a useful alternative for mobile applications. Of all battery chemistries, Lithium-ion polymer (LiPo) has become popular in consumer electronics due to its high energy density, low self-discharge rate and lack of memory. Unfortunately, the chemistry still has limitations. Overcharging or overheating of a LiPo cell may result in ignition and over-discharge can destroy the cell. To prevent these hazards, overcharge and undercharge conditions must be monitored at the cell level rather than the battery level because the charge and discharge efficiency varies from cell to cell causing cell voltages within a battery to diverge during normal operation. A battery management system designed to monitor and maintain a large battery designed for pulsed power applications is presented in this poster. The design of this battery management system is presented and its implementation in a multi-cell, high voltage battery capable of high current pulsed operation. A hazard analysis of high power batteries and the implemented safety system is also provided.

IEEE Conferences

Authors: C. F. Lynn; J. M. Parson; P. Kelly; D. H. Barnett; A. A. Neuber; J. C. Dickens; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296907

Abstract: Virtual cathode oscillators (vircators) can be easily tuned by altering the current density of the electron beam. This can be accomplished by changing the voltage applied to the a-k gap, or by physically changing the gap separation. Previous results with an adjustable a-k gap vircator achieved discrete tuning at frequencies of 1.5 GHz, 2.1 GHz, 4.1 GHz, 4.6 GHz, and 5.9 GHz. To achieve better tunability an adjustable cavity reflector was incorporated into the vircator design. Initial testing of the new vircator design has shown increased output power, as well as the ability to greatly increase the number of frequencies that can be tuned. Sweeping the A-K gap with the cavity reflector in two positions yielded 10 different frequencies of operation. Also sweeping the cavity reflector with the a-k gap set at 8.0mm yielded another unique frequency. Currently 11 unique frequencies have been identified, and the peak field measured from this vircator increased from ~40 kV/m to ~69 kV/m (> 70% increase in field). This paper illustrates the design of the vircator and presents some of the microwave data obtained. Additionally, a summary of all of the frequencies obtained and the peak field measured are included.

IEEE Conferences

Authors: Johnson, JM; Reale, DV; Cravey, WH; Garcia, RS; Barnett, DH; Neuber, AA; Dickens, JC; Mankowski, JJ

PDF: https://aip.scitation.org/doi/10.1063/1.4927719

Abstract: Implementing nonlinear transmission line (NLTL) technology in the design of a high power microwave source has the benefits of producing a comparatively small and lightweight solid-state system where the emission frequency is easily tuned. Usually, smaller in physical size, single NLTLs may produce significantly less power than its vacuum based counterparts. However, combining individual NLTL outputs electrically or in free-space is an attractive solution to achieve greater output power. This paper discusses a method for aligning a four element NLTL antenna array with coaxial geometry using easily adjustable temporal delay lines. These delay lines, sometimes referred to as pulse shock lines or pulse sharpening lines, are placed serially in front of the main NLTL line. The propagation velocity in each delay line is set by the voltage amplitude of an incident pulse as well as the magnetic field bias. Each is adjustable although for the system described in this paper, the voltage is held constant while the bias is changed through applying an external DC magnetic field of varying magnitude. Three different ferrimagnetic materials are placed in the temporal delay line to evaluate which yields the greatest range of electrical delay with the least amount of variability from consecutive shots. (C) 2015 AIP Publishing LLC.

Journal

Authors: J. Stephens; D. Mauch; S. Feathers; J. Mankowski; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7179942

Abstract: Summary form only given. Using voltage pulses, 10s-100s of nanoseconds in length, microdischarges (MDs) are driven with up to 1 kV, and current levels from 30-150 A. Time-averaged input power levels from 10 W-1+ kW are achieved depending on the selection of pulse width and pulse repetition rate (1 kHz-1 MHz). Using an argon-hydrogen mixture, intense VUV radiation is generated at 121.6 nm (10.2 eV, Lyman-alpha). With this source, instantaneous power levels in excess of 60 watts have been achieved with several watts time-averaged power in the VUV. Additional work utilizing XeCl* (308 nm) and XeF* (351 nm) excimer sources is also reported. Achieved power levels, instantaneous and time-averaged, efficiency, and impurity content are reported. As an application, the MD-UV source is utilized as an alternative to a high power laser for the triggering of high voltage photoconductive SiC switches.

IEEE Conferences

Authors: Barnett, DH; Parson, JM; Lynn, CF; Kelly, PM; Taylor, M; Calico, S; Scott, MC; Dickens, JC; Neuber, AA; Mankowski, JJ

PDF: https://aip.scitation.org/doi/pdf/10.1063/1.4913903

Abstract: This paper presents the design and operation characteristics of a solid-state high voltage pulse generator. Its primary utilization is aimed at triggering a gaseous spark gap with high repeatability. Specifically, the trigger generator is designed to achieve a risetime on the order of 0.1 kV/ns to trigger the first stage, trigatron spark gap of a 10-stage, 500 kV Marx generator. The major design components are comprised of a 60 W constant current DC-DC converter for high voltage charging, a single 4 kV thyristor, a step-up pulse transformer, and magnetic switch for pulse steepening. A risetime of <30 ns and pulse magnitude of 4 kV is achieved matching the simulated performance of the design. (C) 2015 AIP Publishing LLC.

Journal

Authors: Stephens, J; Fierro, A; Trienekens, D; Dickens, J; Neuber, A

PDF: https://iopscience.iop.org/article/10.1088/0963-0252/24/1/015013/pdf

Abstract: Utilizing nanosecond high voltage pulses to drive microdischarges (MDs) at repetition rates in the vicinity of 1 MHz previously enabled increased time-averaged power deposition, peak vacuum ultraviolet (VUV) power yield, as well as time-averaged VUV power yield. Here, various pulse widths (30-250 ns), and pulse repetition rates (100 kHz-5 MHz) are utilized, and the resulting VUV yield is reported. It was observed that the use of a 50 ns pulse width, at a repetition rate of 100 kHz, provided 62 W peak VUV power and 310 mW time-averaged VUV power, with a time-averaged VUV generation efficiency of similar to 1.1%. Optimization of the driving parameters resulted in 1-2 orders of magnitude increase in peak and time-averaged power when compared to low power, dc-driven MDs.

Journal

Authors: Majzoobi, A; Joshi, RP; Neuber, AA; Dickens, JC

PDF: https://aip.scitation.org/doi/full/10.1063/1.4932634

Abstract: Particle-in-cell simulations are performed to analyze the efficiency, output power and leakage currents in a 12-Cavity, 12-Cathode rising-sun magnetron with diffraction output (MDO). The central goal is to conduct a parameter study of a rising-sun magnetron that comprehensively incorporates performance enhancing features such as transparent cathodes, axial extraction, the use of endcaps, and cathode extensions. Our optimum results demonstrate peak output power of about 2.1 GW, with efficiencies of similar to 70% and low leakage currents at a magnetic field of 0.45 Tesla, a 400 kV bias with a single endcap, for a range of cathode extensions between 3 and 6 centimeters. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Journal

Authors: J. Stephens; A. Fierro; S. Beeson; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7179992

Abstract: HBSummary form only given. Using a self-matched transmission line pulse generator, high voltage pulses with magnitude 10-25 kV and ~25 ns pulse width are produced to excite low temperature plasma (LTP) formation. In using a short pulse, plasma formation is initiated, but the external excitation is removed before spark formation is completed, thus enabling a study of the LTP during its developmental phase. Using imaging in the visible spectrum and electrical diagnostics, this has been confirmed.To observe the 120 nm - 150 nm range, discharges are guided along the surface of an MgF2 observation window. On the opposing side of the window an evacuated spectrograph with intensified CCD enables the observation of VUV emission. Since the LTP is intentionally limited to its developmental phase, very low total light emission is becomes a diagnostic issue. Hence, it was necessary to accumulate light over several 10s of formation events before discrete lines in the spectra became evident. With a sufficient number of pulses, an experimental spectrum is generated in the 120+ nm range which was found to be in excellent agreement with theoretical atomic spectra of nitrogen and oxygen, with an assumed electron temperature of 1.6 eV. Using a gas puff system combined with an otherwise evacuated spectrograph enables the observation of extreme UV (EUV) to VUV light (80nm - 120 nm). In the photoionization relevant region of the spectra (below 103 nm), the observable emission consists of atomic neutral and singly ionized oxygen as well as molecular nitrogen. Data suggests that additional ground state transitions are present, but unobservable due to self-absorption and low overall light intensity. Most notably, the presence of photoionization capable emission in developing low temperature plasmas in air is verified.

IEEE Conferences

Authors: J. M. Parson; C. F. Lynn; D. H. Barnett; S. L. Holt; P. M. Kelly; J. C. Dickens; A. A. Neuber; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296909

Abstract: Repetitive operation of high-power microwave (HPM) sources is primarily restricted by thermal properties of anode and cathode materials. Note that even in single shot operation, vircators generally inject undesired plasma from the anode/cathode surfaces under high current densities, leading to excessive heating of electrode materials, vacuum degradation and impedance collapse that may ultimately lead to cut-off of microwave power. Heating of the anode and cathode materials during repetitive operation intensifies these problems and will cause accelerated erosion and/or permanent damage of the anode and cathode surfaces, especially when metallic electrodes are used. Hence, the vircator herein utilizes a carbon fiber cathode and a pyrolytic graphite anode. This paper elucidates the heating of the anode and cathode, and their spectrally resolved black-body radiation properties during repetitive operation.

Conference Paper/Presentation

Authors: K. H. Becker; B. B. Godfrey; E. E. Kunhardt; M. Laroussi; L. D. Ludeking; A. A. Neuber; E. Schamiloglu; A. J. Woods

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7047925

Abstract: This paper summarizes the Dr. Robert J. Barker memorial session at the IEEE International Conference on Plasma Science 2014. Each section summarizes progress in a plasma research area strongly influenced by Dr. Barker's leadership: 1) plasma medicine; 2) atmospheric-pressure plasmas; 3) high-power microwaves; 4) pulsed power; and 5) numerical simulation of plasmas. He had a profound influence on these and other plasma science applications, as well as on numerous individual researchers. He will be missed greatly.

IEEE Journals

Authors: Lin, S; Beeson, S; Liu, C; Dickens, J; Neuber, A

PDF: https://aip.scitation.org/doi/full/10.1063/1.4917471

Abstract: Self-induced gaseous plasma is evaluated as active opening switch medium for pulsed high power microwave radiation. The self-induced plasma switch is investigated for N-2 and Ar environments under pressure conditions ranging from 25 to 700Torr. A multi-pass TE111 resonator is used to significantly reduce the delay time inherently associated with plasma generation. The plasma forms under the pulsed excitation of a 4MW magnetron inside the central dielectric tube of the resonator, which isolates the inner atmospheric gas from the outer vacuum environment. The path from the power source to the load is designed such that the pulse passes through the plasma twice with a 35 ns delay between these two passes. In the first pass, initial plasma density is generated, while the second affects the transition to a highly reflective state with as much as 30 dB attenuation. Experimental data revealed that virtually zero delay time may be achieved for N-2 at 25 Torr. A two-dimensional fluid model was developed to study the plasma formation times for comparison with experimental data. The delay time predicted from this model agrees well with the experimental values in the lower pressure regime (error < 25%), however, due to filamentary plasma formation at higher pressures, simulated delay times may be underestimated by as much as 50%. (C) 2015 AIP Publishing LLC.

Journal

Authors: L. Collier; M. B. Walls; J. Dickens; J. Mankowski; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296884

Abstract: The Marx pulse generator topology has been widely used in pulsed power applications1,2. Another pulse generator topology, the linear transformer driver (LTD), has been developed3 that may serve as a viable alternative to the Marx generator. LTDs utilize inductively added stages to achieve high voltages and currents. Unlike a Marx generator, each stage in an LTD features multiple bricks, all of which are ground referenced and allow current to be distributed amongst an arbitrary number of switches. This allows for LTDs that utilize solid-state switches, potentially resulting in more compact and reliable pulse generators. A solid-state, >10 kA peak current, multiple-stage LTD is developed. The generator's performance will be analyzed for viability as a replacement for driving a high power microwave generator.

IEEE Conferences

Authors: R. S. Garcia; D. V. Reale; J. M. Johnson; W. H. Cravey; A. A. Neuber; J. C. Dickens; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296893

Abstract: This paper describes the system integration of a four element, nonlinear transmission (NLTL) array. The HPM system components include a negative polarity 40 kV capacitor charging supply, a trigatron spark gap, a solid-state trigger generator, four delay lines with biasing coils, four coaxial ferrimagnetic NLTLs with biasing coils, eight dc current supplies, and four TEM horn antennas. A National Instruments cRIO FPGA based controller is used to interface the capacitor charging supply, the biasing coil power supplies and the trigger generator to a laptop based GUI. In order to minimize impact from EMI, all lines were properly shielded and the data acquisition equipment was located a sufficient distance away from the NLTL array. The GUI consists of options for controlling the current output on all 8 power supplies for biasing purposes, as well as pulse options that include repetitive pulsing based on the number of pulses or for a specific length of time. Pulse duration for the capacitor charger, the command charge, and the trigger can also be adjusted. An emergency stop button is included for safe shutdown of the trigger generator and all power supplies. A diagnostics setup is outlined describing the placement of inline coaxial D-Dot probes and a high voltage probe for laboratory testing.

IEEE Conferences

Authors: J. M. Johnson; J. M. Parson; D. V. Reale; A. A. Neuber; J. J. Mankowski; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7287219

Abstract: This paper presents a design and implementation of a 1 kHz, 10 kV, positively pulsed trigger generator with an average risetime of 44 ns for use in a coaxial ferrimagnetic Nonlinear Transmission Line (NLTL) High Power Microwave (HPM) system. The pulse trigger generator is based on two stacked 4 kV current-controlled n-type thyristors in a low inductance package capable of driving inductive loads with risetimes less than 200 ns. However, by implementing a magnetic switch this relatively is slow risetime is sharpened to tens of nanoseconds. The stacked boards allow for voltage addition at the output while maintaining the fast riestime.

IEEE Conferences

Authors: Andrew S. Fierro; James C. Dickens; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6779603

Abstract: The development of a low-temperature plasma in a needle-protrusion to plane gap is investigated utilizing a 3-D particle-in-cell/Monte Carlo collision method implemented to run on single NVIDIA graphics processing unit. In addition to electron collisions, the model includes field detachment, photon tracking, and a drift-diffusion approximation for positive ions. The simulated geometry tracks several million electrons with 15-μm spatial resolution.

Journals

Authors: E. Cordero; S. Holt; J. Dickens; A. Neuber; J. Mankowski; S. Calico; M. Scott

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7287370

Abstract: This paper presents the design of a scalable, high power battery system for pulsed power operations. The battery system is modular in design, with each module containing four Lithium Ion Polymer (LiPo) cells and a custom designed cell management board that actively monitors the voltage and temperature of each cell and also provides cell balancing functionality. The system is designed to be scalable by adding up to 25 modules in a series configuration. While the battery management system should be compatible with any lithium ion cells, this implementation uses 8 Ah capacity dual-core LiPo cells, rated for a 150 C discharge rate; allowing for a peak current output of 1,200 A. With 25 modules (96 LiPo cells) the system would have an open circuit voltage of 385 V and be capable of providing up to 1,200 A at 355 V for a peak output power of 420 kW. Special attention has been placed on safety features including overvoltage, undervoltage and temperature monitoring of every cell in the system. The charging/balancing system is capable of automatically shutting down if any of the voltages or temperatures exceeds established limits. The management circuitry is designed to have a low off-state power draw in order to maximize battery life when the system is not in use.

Conference Paper/Presentation

Authors: Laity, G; Fierro, A; Dickens, J; Frank, K; Neuber, A

PDF: https://aip.scitation.org/doi/pdf/10.1063/1.4869895

Abstract: We demonstrate a method for determining the dissociation degree of atmospheric pressure air discharges by measuring the self-absorption characteristics of vacuum ultraviolet radiation from O and N atoms in the plasma. The atom densities are determined by modeling the amount of radiation trapping present in the discharge, without the use of typical optical absorption diagnostic techniques which require external sources of probing radiation into the experiment. For an 8.0 mm spark discharge between needle electrodes at atmospheric pressure, typical peak O atom densities of 8.5 x 10(17) cm(-3) and peak N atom densities of 9.9 x 10(17) cm(-3) are observed within the first similar to 1.0 mm of plasma near the anode tip by analyzing the OI and NI transitions in the 130.0-132.0 nm band of the vacuum ultraviolet spectrum. (C) 2014 AIP Publishing LLC.

Journal

Authors: Eldridge, K; Fierro, A; Dickens, J; Neuber, A

PDF: https://ieeexplore.ieee.org/document/7287262

Abstract: Wavelet decomposition and reconstruction are utilized to synthesize a high power microwave (HPM) signal at a frequency below the frequency of the employed radiating sources. Employing a larger number (on the order of ten) of smaller sources that produce short radiating pulses combined with appropriate amplitude scaling and shifting of the individual pulses enables the generation of a single waveform of longer duration. We describe the mathematical approach to the wavelet synthesis and give examples. For instance, an array of 10 sources, each producing a 0.5 ns pulse can be adjusted to generate a sinusoidal wave with a period of approximately 2 ns. The results of low power experiments are discussed in detail to demonstrate the practical feasibility of the wavelet approach.

Conference Paper/Presentation

Authors: S. Holt; E. Cordero; J. Mankowski; J. Dickens; A. Neuber; M. Scott; S. Calico

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7287211

Abstract: The design of a battery powered, rapid capacitor charger is discussed. The charger design is capable of an average output power of 80 kVA, for a burst time of roughly 5 seconds. Cool down time is required between bursts. A Lithium Ion Polymer (LiPo) battery pack with a nominal voltage of 355 V and fully charged open circuit DC voltage of 385 V will be used to power the rapid capacitor charger. The general topology of the charger is as follows. An IGBT H-bridge inverter uses pulse width modulation to create an AC waveform that is stepped up to a maximum of 58kV by a 1:170 ratio transformer. This high voltage output of the transformer is rectified and used to charge the capacitor bank.

IEEE Conferences

Authors: P. Gatewood; A. Neuber; J. Dickens; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7287248

Abstract: A metamaterial-inspired, efficient, electrically small antenna (ESA) is designed for the 2 to 20 MHz range, intended for use in a mobile Ionospheric Heating (MIH) system. The ESA consists of two main parts: a small semiloop antenna (SLA), and an outer capacitively loaded loop (CLL). To increase the capacitance of the CLL and reduce the size of the ESA, a high permittivity dielectric is utilized. All designs were simulated in Ansoft HFSS with particular attention on radiation efficiency and bandwidth. The design aims for these ESAs are a high overall efficiency at dimensions much smaller than a wavelength.

IEEE Conferences

Authors: Kaili Eldridge; Andrew Fierro; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7287262

Abstract: Wavelet decomposition and reconstruction are utilized to synthesize a high power microwave (HPM) signal at a frequency below the frequency of the employed radiating sources. Employing a larger number (on the order of ten) of smaller sources that produce short radiating pulses combined with appropriate amplitude scaling and shifting of the individual pulses enables the generation of a single waveform of longer duration. We describe the mathematical approach to the wavelet synthesis and give examples. For instance, an array of 10 sources, each producing a 0.5 ns pulse can be adjusted to generate a sinusoidal wave with a period of approximately 2 ns. The results of low power experiments are discussed in detail to demonstrate the practical feasibility of the wavelet approach.

Conferences

Authors: Reale, DV; Bragg, JWB; Gonsalves, NR; Johnson, JM; Neuber, AA; Dickens, JC; Mankowski, JJ

PDF: https://aip.scitation.org/doi/pdf/10.1063/1.4878339

Abstract: Gyromagnetic Nonlinear Transmission Lines (NLTLs) generate microwaves through the damped gyromagnetic precession of the magnetic moments in ferrimagnetic material, and are thus utilized as compact, solid-state, frequency agile, high power microwave (HPM) sources. The output frequency of a NLTL can be adjusted by control of the externally applied bias field and incident voltage pulse without physical alteration to the structure of the device. This property provides a frequency tuning capability not seen in many conventional e-beam based HPM sources. The NLTLs developed and tested are mesoband sources capable of generating MW power levels in the L, S, and C bands of the microwave spectrum. For an individual NLTL the output power at a given frequency is determined by several factors including the intrinsic properties of the ferrimagnetic material and the transmission line structure. Hence, if higher power levels are to be achieved, it is necessary to combine the outputs of multiple NLTLs. This can be accomplished in free space using antennas or in a transmission line via a power combiner. Using a bias-field controlled delay, a transient, high voltage, coaxial, three port, power combiner was designed and tested. Experimental results are compared with the results of a transient COMSOL simulation to evaluate combiner performance. (C) 2014 AIP Publishing LLC.

Journal

Authors: Thomas, D; Mauch, D; White, C; Neuber, A; Dickens, J

PDF: https://ieeexplore.ieee.org/document/7287329

Abstract: A method of characterizing mid-bandgap defect states in high purity semi-insulating 4H-SiC through leakage current analysis for optimization of SiC photoconductive semiconductor switches, PCSS, is presented. The method utilizes two custom IV curve tracer systems to measure leakage currents through the material under various voltage/current conditions. The first system is used under low current conditions and is capable of measurements from 0 to 45 kV at currents ranging from 0 to 3 mA with pA resolution. While voltage/current measurements in the region >0.1 mA are of primary interest for quantifying defect states near the conduction band, standard IV measurements become difficult due to excessive power dissipation in the PCSS. Hence, a second system operating in transient mode is used for currents higher than 0.1 mA. This system measures the transient discharge of a charged capacitor through the PCSS, allowing for high current measurements while subjecting the material to high power dissipation for only a short period of time (milliseconds). It is the goal to extract from the combined data of these two systems characteristics of the defect states (concentration, energy level).

Conference Paper/Presentation

Authors: J. M. Parson; C. F. Lynn; J. J. Mankowski; M. Kristiansen; A. A. Neuber; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6847740

Abstract: This paper presents a study on outgassing and electrical conditioning for three carbon fiber cathode types in a vacuum-sealed, high-power microwave virtual-cathode-oscillator (vircator) that operates in the low 10-9 torr pressure regime. The three cathode types consist of a bare bimodal fiber structure, a bare unimodal fiber structure, and a cesium-iodide coated bimodal fiber structure with identical fiber coverage of 2% by area with a surface area of ~20 cm2. The electrodes are cleaned by a 1.2 kW, argon/oxygen microwave plasma prior to complete vircator assembly, followed by a 300 °C bake-out for 72 h. Each cathode was pulsed in a single pulse operation by an 80 J, low inductance Marx generator with an approximate pulsewidth of 100 ns full-width at half-maximum for 10000 current pulses. The data presented includes individual gas constituents, high-speed intensified charge coupled device (ICCD) imaging, and voltage and current waveforms. The conditioning process resulted in a gas load reduction of ~80% overall, with the indication that the bare bimodal fiber structure performed the best as diode power increased throughout the experiment, while the power decreased for the other tested cathode types.

IEEE Journals

Authors: D. H. Barnett; J. Parson; C. Lynn; P. Kelly; M. Taylor; J. Dickens; A. Neuber; J. Mankowski; S. Calico; M. Scott

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7287287

Abstract: This paper presents the design and performance characteristics of an optically isolated, trigger generator for spark gap applications that is capable of operating at over 500 Hz rep rate. The pulsed trigger generator is designed to achieve fast risetimes on the order of 1kV/ns to cause breakdown between the trigatron trigger pin and the opposite electrode. The pulsed trigger generator is designed to trigger a 10-stage, 500 kV, 42 J Marx generator. The system is capable of delivering a 4"“25 kV pulse with a 10/90 risetime of 20 ns. A board was fabricated to control a capacitor charger and high voltage thyristor via fiber optic inputs. The capacitor charger, a 60 W constant current dc-dc power supply with variable voltage control, is capable of charging 1 nF in less than 0.3 ms. The system is battery-powered using a 33 V lithium polymer battery pack for isolated operation in noisy environments. For burst mode operation, a larger 250 nF buffer capacitor, which is initially charged, is connected through a 100 kΩ resistor to the trigger capacitor, which is varied between 1 and 40 nF. The value of the trigger capacitor is adjusted to provide optimum performance for a given transformer and magnetic switch. System modeling results along with experimental data are discussed including the reliability and performance of the overall system including jitter at 500 Hz operation.

Conference Paper/Presentation

Authors: J. M. Parson; C. F. Lynn; J. J. Mankowski; A. A. Neuber; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6945869

Abstract: When subjected to high electric fields in vacuum, carbon fiber cathodes produce intense electron beams suitable for high-power microwave (HPM) generation at very high current densities. However, the production mechanisms of these intense electron beams are not fully understood. This paper presents the postmortem examination of three conditioned carbon fiber cathode types. The three cathode types consist of an uncoated, bare unimodal fiber structure, a bare bimodal fiber structure, and a cesium-iodide (CsI)-coated bimodal fiber structure, all with identical fiber coverage of 2% by area. Each cathode was conditioned prior to testing by single pulse operation driven by an 80 J Marx generator for 10 000 pulses. HPM, voltage, and current waveforms of each cathode are presented. The bare bimodal cathode radiated more microwave power than the CsI-coated cathode and bare unimodal cathode. Scanning electron microscopy imagery presents evidence of two emission mechanisms: 1) explosive electron emission and 2) surface flashover, which both were found on the CsI-coated cathode. In addition, no evidence of surface flashover was found on either uncoated cathode.

IEEE Journals

Authors: C. F. Lynn; J. Parson; P. Kelly; M. Taylor; D. Barnett; A. Neuber; J. Dickens; J. Mankowski; S. Calico; M. Scott

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7287245

Abstract: Vircators (virtual cathode oscillators) are easily frequency tunable by simply altering the electron beam current density through either adjusting the accelerating voltage or varying the anode cathode gap (A-K gap). It is noted that vircators typically exhibit frequency hopping, an often undesired feature. Here we address the frequency hopping problem by introducing a rectangular shaped carbon fiber cathode with the width of the cathode less than one quarter of a wavelength in the direction of microwave propagation. Operating the vircator with the rectangular cathode revealed a match with a resonant condition from back wall reflections, which explains the observed dominant microwave frequency selection. That is, the vircator emitted stably during 50 ns operation at 3.89 GHz or 2.07 GHz with a 3 dB bandwidth of 37.5 MHz or 93.7 MHz, at A-K gap spacing of 8 mm, and 12 mm respectively. To further investigate frequency tuning, the A-K gap was swept from 3 mm to 17 mm while keeping a constant 5.1 cm diameter circular cathode. The resulting frequencies of operation were 1.5 GHz, 2.1 GHz, 4.1 GHz, 4.6 GHz, and 5.9 GHz at A-K gap spacing of 17mm, 14 mm, 7 mm, 5.75 mm, and 3 mm respectively. This demonstrates that a tuning range of 4.4 GHz should be achievable for a practical vircator system.

IEEE Conferences

Authors: Sterling R. Beeson; James C. Dickens; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6923463

Abstract: A global model of high-power microwave (HPM) window breakdown is elucidated. The model provides a practical approach for estimating the maximum microwave power and pulse length that can be transmitted for a given window geometry at varying background gas pressure. Based on recent experimental and modeling progress, the formative and statistical breakdown delay time contributions are included in the model. The provided details are intended to give the reader a starting point in designing an HPM system for which surface breakdown along the output window is a major concern. Spanning five orders of magnitude in power, four microwave bands, three orders of magnitude in pulsewidth, three orders of magnitude in pressure, and three different gas types, the model serves to determine the probability of breakdown for a given set of input parameters with the modest computational effort. Examples of how to use the model are given, and the results are compared with actual systems and measured experimental delay times.

Journals

Authors: Fierro, A; Dickens, J; Neuber, A

PDF: https://aip.scitation.org/doi/10.1063/1.4903330

Abstract: A 3-dimensional particle-in-cell/Monte Carlo collision simulation that is fully implemented on a graphics processing unit (GPU) is described and used to determine low-temperature plasma characteristics at high reduced electric field, E/n, in nitrogen gas. Details of implementation on the GPU using the NVIDIA Compute Unified Device Architecture framework are discussed with respect to efficient code execution. The software is capable of tracking around 10 x 10(6) particles with dynamic weighting and a total mesh size larger than 10(8) cells. Verification of the simulation is performed by comparing the electron energy distribution function and plasma transport parameters to known Boltzmann Equation (BE) solvers. Under the assumption of a uniform electric field and neglecting the build-up of positive ion space charge, the simulation agrees well with the BE solvers. The model is utilized to calculate plasma characteristics of a pulsed, parallel plate discharge. A photoionization model provides the simulation with additional electrons after the initial seeded electron density has drifted towards the anode. Comparison of the performance benefits between the GPU-implementation versus a CPU-implementation is considered, and a speed-up factor of 13 for a 3D relaxation Poisson solver is obtained. Furthermore, a factor 60 speed-up is realized for parallelization of the electron processes. (C) 2014 AIP Publishing LLC.

Journal

Authors: Jonathan M. Parson; John J. Mankowski; James C. Dickens; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6905850

Abstract: Cold cathode operation under high current densities leads to explosive electron emission (EEE) that contributes to early A-K gap closure. Hence, inconsistent vacuum conditions and, if utilized in a high power microwave device, degradation of microwave output power are observed. The EEE centers are known to produce localized plasmas on the surface of the cathode that release and ionize the electrode material. Further, low melting point material in the anode is released due to electron bombardment accompanied by a significant surface temperature increase. Postmortem analysis has revealed particles up to 50 μm in diameter embedded in the opposite electrode. High speed ICCD imaging during A-K gap operation enabled resolving the plasma's spatial characteristics in time. Images of cathode and anode plasma during the operation of a virtual cathode oscillator at 250 A/cm2 under ultrahigh vacuum conditions are presented.

Journals

Authors: Stephens, J; Fierro, A; Dickens, J; Neuber, A

PDF: https://iopscience.iop.org/article/10.1088/0022-3727/47/32/325501/pdf

Abstract: A microdischarge (MD) vacuum ultraviolet (VUV) light source is fired onto a N-2-NO (99.92 : 0.08%) target gas. The minor gas constituent, NO, was chosen for its ionization potential (9.23 eV) and photoionization cross-section (1.4 x 10(-18) cm(2)) at the wavelength of interest (121.6 nm, 10.2 eV). The result is a plasma generated entirely by volume photoionization in a N-2-NO background. Using a very low electric field amplitude, charge carriers are drifted though the photoplasma at picoampere levels, serving as a non-invasive diagnostic. Using a simple one-dimensional fluid approximation for the low electric field condition, theoretical predictions of photoplasma current were found to be in meaningful agreement with experimental data. The impact of direct photoionization and pre-ionization on nanosecond timescale high voltage breakdown yielded two primary observations: (1) a significant reduction in the formative delay time necessary for spark formation, and (2) almost complete elimination of the statistical delay time. Again utilizing one-dimensional fluid approximations, reasonable agreement between experimental and simulated breakdown voltage was observed. Utilizing the same VUV source to illuminate a HV spark gap biased to about 95% self-breakdown voltage revealed that direct volume photoionization alone was insufficient to trigger breakdown of the high voltage gap. However, permitting electrode illumination, the same source was found to be capable of triggering breakdown in the undervoltaged gap, albeit with a large temporal jitter.

Journal

Authors: Andrew Fierro; James Dickens; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7287318

Abstract: The development of plasma in a medium pressure (50 torr) environment in nitrogen was studied by simulation and measurement under the influence of non-uniform, pulsed electric fields. A GPU-accelerated, 3-dimensional particle-in-cell (PIC)/Monte Carlo Collision (MCC) simulation code was written utilizing the CUDA platform to simulate pulsed plasma development in a nitrogen environment and uncover the transient plasma characteristics in detail. The simulation provides significant speed-up over the CPU equivalent implementations. Experimentally, a needle-protrusion (1.5 mm in length, 200 μm tip radius) opposite a brass ground plane with the distance between needle-tip and wall held at 1.5 mm provided a non-homogeneous field. Excitation of the needle-plane gap was achieved with a ~100 ns rise-time high-voltage pulser with a peak voltage of 30 kV. Diagnostics included time-resolved nanosecond gated imaging for light intensity measurements and high speed electrical probes for timing. A time series of the plasma formation captured with a 5 ns camera gate revealed a mostly uniform expanding plasma cloud from the needle tip.

Conferences

Authors: J. C. Stephens; A. S. Fierro; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6837539

Abstract: The application of a short, pulsed excitation is known to allow for higher power deposition into microdischarges without the onset of instabilities. Here, a MOSFET-based high voltage pulser is used to drive a 50-torr argon microdischarge with short pulsed currents of ~75 A, with <;100-ns full-width at half-maximum, and a repetition rate of 1 MHz. With this excitation, an average power density of ~1013 W/m3 is achieved, with a peak power density~3.1014 W/m3. A high speed iCCD camera is used to observe ignition processes and confirm the absence of unstable operation. The images were taken using a 5-ns gate time with a spatial resolution of ~2.5 μm.

IEEE Journals

Authors: Stephens, J; Fierro, A; Walls, B; Dickens, J; Neuber, A

PDF: https://aip.scitation.org/doi/10.1063/1.4866040

Abstract: A microdischarge is driven by short pulses (80 ns FWHM) with peak current levels up to 80 A, with a repetition frequency of 1 MHz (1 pulse/mu s) allowing for similar to 550W input power. Experiments in pure argon (Ar-2*, 127 nm) and argon-hydrogen (Lyman-alpha, 121.6 nm) were conducted. Using short pulses, the argon excimer emission was not observed. Alternatively, Ar-H-2 operated at both higher power and efficiency (0.63%) whenever pulsed. Using Ar-H-2, the experiments result in an average generated vacuum ultraviolet power just above 3.4W with a peak power of 42.8 W, entirely at Lyman-alpha. (C) 2014 AIP Publishing LLC.

Journal

Authors: M. B. Taylor; P. M. Kelly; J. M. Parson; C. Lynn; J. C. Dickens; A. A. Neuber; J. J. Mankowski; J. . -W. B. Bragg; S. Calico; M. Scott

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7287288

Abstract: A 42 J, 10-stage pulse forming network (PFN) Marx generator capable of producing a 500 kV, 50 ns full-width-half-max (FWHM), ~5 ns rise time pulse into an open load at a rep-rate of 500 Hz has been designed for use as a pulsed power source for a reflex triode virtual cathode oscillator (vircator). Rayleigh PFNs are used in place of discrete capacitors for each stage of the 10-stage Marx generator. Effort was taken to minimize parasitic inductance such that the quality of the pulse shape is maintained as much as possible. In order to rep-rate the Marx generator, a trigatron-based triggering scheme is used to initiate erection of the Marx generator. A 20 ns risetime, 24 kV solid-state pulse trigger generator capable of operating at high repetition rates is used to drive the trigatron. The required charge rate for a 500 Hz pulse repetition frequency (PRF) for the Marx generator is 24 kW. Repetitive operation requires additional design considerations that would be irrelevant to single pulse firing. Pressurized air is jetted across the spark gaps by means of built-in gas manifolds to remove remaining ionized gas between each pulse and prevent premature erection during the subsequent charging cycle. The built-in gas manifolds were designed using a hydrodynamic simulation to ensure equal flow rate across each of the spark gaps and equal pressure along the length of the tube chamber.

IEEE Conferences

Authors: J. Stephens; D. Ryberg; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7012397

Abstract: Summary form only given. A 400 μF capacitor bank, charged as high as 3.5 kV (2.45 J) is used to drive an exploding wire in air with up to 10 s kA over a 100 s of microsecond timescale. Utilizing a short wire with 286 μm diameter enables generating an intense shockwave with overpressures on the order of 1 psi (6.9 kPa). Investigation of the energy dependence of the overpressure magnitude was completed by both varying the initial capacitor voltage, as well as parasitically introducing additional resistive dampening into the circuit. Additionally, variation in both capacitance and inductance were made in order to observe the effect of varying the driving source timescale on generated overpressure. Optimization of the exploding wire load involved exploring various materials, radii, length, as well as the number of exploding wires. Preliminary theoretical predictions are made using the Sandia National Laboratories ALEGRA-MHD software. Theoretical and experimental results are benchmarked against one another to assess the accuracy of the theoretical predictions.

IEEE Conferences

Authors: D. Mauch; C. White; D. Thomas; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7287198

Abstract: Recent advances and the current state-of-the-art for high speed 4H-SiC photoconductive semiconductor switches (PCSS) developed at Texas Tech University are summarized. A performance comparison of multiple generations of switch designs is also presented. These devices have experimentally demonstrated the capability of blocking DC electric fields up to 705 kV/cm (<;0.1 mA leakage current), rise times of 0.63 ns (20/80), and switching 20 kV at 250 A with a di/dt of 75 kA/us at a burst repetition frequency of 65 MHz. Findings and optimizations pertaining to device geometry, sub-contact doping, contact thickness, triggering wavelength, and electron irradiation are presented. Device modeling and experimental results investigating current issues with device lifetime are presented as well.

IEEE Conferences

Authors: P. Kelly; J. M. Parson; C. Lynn; M. Taylor; J. C. Dickens; A. Neuber; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7012655

Abstract: Summary form only given. This study focuses on the use of St707 non-evaporable getter (NEG) material in a high power microwave (HPM) sealed-tube virtual cathode oscillator (vircator) operated at 500 Hz repetition rate. High-current pulsed operation releases gases trapped within the bulk materials and gas monolayers on material surfaces, leading to localized plasma production in the A-K gap. This can lead to gap closure, shorten the duration of microwave emission, and spoil vacuum. A single current pulse increases the chamber pressure to the low 10-6 Torr range from an initial background pressure in the low 10-9 Torr range, desorbing approximately 1014 particles. At 500 Hz operation, a sufficiently large pumping speed (~2,500 L/s) is necessary to evacuate desorbed particles from the vircator volume during consecutive shots. Previous work has identified hydrogen (H, H2) as the main outgassing species during vircator operation, with contributions from CH4, N2, CO, CO2, and Ar as the other primary gas constituents[1]. The St707 NEG pumps shows an affinity for pumping hydrogen, making it a suitable choice to adsorb vircator outgassing species, of which hydrogen is an order of magnitude greater than any other gas species. Previously, without the presence of NEG material, degradation of microwave output power from the vircator has been observed during subsequent shots at 1 Hz, 10 second burst mode operation. Subsequently, an increase in chamber pressure from 1.25×10-6 Torr on the first shot to 10-5 Torr on the tenth shot has been experimentally observed. This paper details the performance of the St707 NEG material (70% Zr, 24.6% V, 5.4% Fe) for maintaining UHV conditions during rep-rated vircator operation. Pumping characteristics of multiple St707 NEG pumps in the presence of rep-rated high-current pulses are presented. Diagnostic results obtained with a residual gas analyzer to observe individual gas constituents and two inverted magnetron cold cathode gauges for absolute pressure are utilized to analyze vircator and getter performance in detail.

IEEE Conferences

Authors: M. Walls; J. Dickens; J. Mankowsi; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7012301

Abstract: Summary form only given. An eleven stage flat top type output Marx utilizing sparkgap switches is operated to produce a desired 500 kV output at 10 Hz rep-rate with > 500 J pulse energy to drive a ~ 20 Ohm load. While this Marx generator utilizes custom-designed spark gaps, the significant progress in semi-conductor pulsed switching technology is evaluated through PSICE modeling of the semiconductor switches integrated into the Marx electrical model. The switches are experimentally evaluated at conditions that a single switch (stack) would experience in a full Marx generator. Alternatively to the Marx topology, the potential benefits of choosing a linear transformer driver (LTD) as a HPM pulsed power driver are evaluated. We note that the main advantage for the LTD is the savings in insulation material and size; the same semiconductor switches are utilized. The potential benefits of such a generator include significant reduction of electromagnetic noise, increased lifetime, improved reliability, and reduced maintenance.

IEEE Conferences

Authors: J. Stephens; A. Fierro; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7012344

Abstract: Summary form only given. A micro hollow cathode discharge VUV source, with time averaged power above 3 watts, and peak power above 40 watts is presented1. Such operation is achieved by driving the microdischarge (MD) in the arc mode where high electron densities and low discharge impedances exist. Improved theoretical estimations regarding the timescales of arc formation are presented. With typical electrical sources, the arc regime is associated with a collapse of the potential and poor coupling of energy. However, the pulsed driver utilized in this study features a low output impedance, thus sustaining a reasonable voltage across the MD while simultaneously delivering 10s of amperes when the arc mode is achieved. This allows for time averaged input power on the order of 550 watts. Gas mixtures such as Ar-H2 (99.7%/0.3% by volume) have demonstrated strong VUV emission at Lyman-α (121.6 nm), with measured power levels exceeding 40 watts peak.This study details the influence of the MD performance under various drive modes. Utilizing a pulse forming line based driver the behavior of the MD under 10 ns excitation and various repetition frequencies, with and without DC bias are presented. The MOSFET based low impedance driver, enables investigating the operation of the MD under high average input power. We will demonstrate the feasibility of utilizing the developed MD to achieve volume pre-ionization in appropriate target gases.

IEEE Conferences

Authors: J. M. Parson; J. -W. B. Bragg; M. Taylor; D. Barnett; P. Kelly; C. F. Lynn; S. Holt; J. C. Dickens; A. A. Neuber; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7012435

Abstract: Thermal limitations of anode and cathode materials have shown to negatively impact operation of cold-cathode high-power microwave (HPM) sources. High pulse-repetition-frequency (PRF) operation of these devices exacerbates the problems already experienced in single shot mode where cold-cathode devices, specifically carbon fiber cathodes, are plagued by plasma formation on the anode and cathode followed by plasma/gas expansion that causes impedance collapse of the anode-cathode (A-K) gap. Hence, for frequency stable, repetitive operation, cold-cathode HPM devices require the use of thermally robust electrode materials and ultra-clean surfaces, leading to repeatable tube operation. This study focuses on burst-mode operation of an HPM sealed tube reflex-triode virtual-cathode-oscillator (vircator) for PRFs greater than 100 Hz. The vircator is driven by a 54 J, ∼200 kV Marx generator with an approximate pulse width of 50 ns FWHM, and the vircator chamber has an empty volume of approximately 5 L with background pressures in the low 10−9 Torr. The anode materials studied include grade-1 titanium (TiG1), nickel 201L (Ni201L), and stainless steel 316L (SS316L); all in combination with a carbon fiber cathode. Empirically observed outgassing characteristics in conjunction with anode thermal modeling are presented under single-shot and rep-rate conditions. In addition, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) techniques were employed to investigate anode and cathode surface integrities before and after vircator operation.

IEEE Conferences

Authors: D. V. Reale; D. Mauch; J. M. Johnson; A. A. Neuber; J. C. Dickens; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7287223

Abstract: An all solid-state high power microwave (HPM) source is constructed using a photoconductive semiconductor switch (PCSS) based HV pulse generator to drive a sulfur hexaflouride (SF6) insulated coaxial ferrimagnetic nonlinear transmission line (NLTL) which feeds a TEM horn antenna. The PCSS was fabricated from high purity semi-insulating (HPSI) 4H-SiC and is illuminated with 2 mJ from a frequency tripled Nd:YAG laser at 355 nm with a 7 ns FWHM. Fixed fiber optic delay lines are utilized to generate a burst of four optical pulses from a single solid-state laser source for rep-rate operation. The input to the NLTL is an adjustable pulse from 3 kV to 6 kV with sub-ns rise time and the resulting output of the NLTL is radiation in the L-band to S-band regime with RF power from 100–200 kW depending on charge voltage. A Rexolite® insulated zipper transition, in which the inner conductor of the coax is gradually exposed, is used at the output of the NLTL to transition from a coaxial structure to parallel plate structure in order to feed a free space TEM horn. The zipper transition and feed section of the TEM horn are potted using Sylgard® 184 silicone elastomer to prevent breakdown at the antenna feed and maintain a homogenous dielectric constant for the transition region. Radiated waveforms are presented for several charge voltages and bias conditions.

IEEE Conferences

Authors: S. Beeson; J. Dickens; A. Neuber; S. Lin

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7012670

Abstract: Summary form only given. The delay time between the rising edge of a high power microwave (HPM) pulse and plasma formation in a gas at close to atmospheric pressure is studied for a multi-pass geometry. A fundamental lower limit of the breakdown delay time is the formative delay time, defined as the time it takes for the exponentially rising electron density to reach the critical electron density for which the EM wave begins to reflect and attenuate; which is on the order of 10's to 100's of nanoseconds for the conditions of interest. The goal of this research was to surpass this limit by making two passes through the generated plasma with the HPM pulse. That is, an external structure is utilized to time delay the HPM pulse on itself for a second pass through a gas cell.

IEEE Conferences

Authors: J. M. Parson; J. -W. B. Bragg; M. Taylor; D. Barnett; P. Kelly; C. F. Lynn; S. Holt; J. C. Dickens; A. A. Neuber; J. J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7012435

Abstract: Thermal limitations of anode and cathode materials have shown to negatively impact operation of cold-cathode high-power microwave (HPM) sources. High pulse-repetition-frequency (PRF) operation of these devices exacerbates the problems already experienced in single shot mode where cold-cathode devices, specifically carbon fiber cathodes, are plagued by plasma formation on the anode and cathode followed by plasma/gas expansion that causes impedance collapse of the anode-cathode (A-K) gap. Hence, for frequency stable, repetitive operation, cold-cathode HPM devices require the use of thermally robust electrode materials and ultra-clean surfaces, leading to repeatable tube operation. This study focuses on burst-mode operation of an HPM sealed tube reflex-triode virtual-cathode-oscillator (vircator) for PRFs greater than 100 Hz. The vircator is driven by a 54 J, ~200 kV Marx generator with an approximate pulse width of 50 ns FWHM, and the vircator chamber has an empty volume of approximately 5 L with background pressures in the low 10^-9 Torr. The anode materials studied include grade-1 titanium (TiG1), nickel 201L (Ni201L), and stainless steel 316L (SS316L); all in combination with a carbon fiber cathode. Empirically observed outgassing characteristics in conjunction with anode thermal modeling are presented under single-shot and rep-rate conditions. In addition, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) techniques were employed to investigate anode and cathode surface integrities before and after vircator operation.

Conference Paper/Presentation

Authors: J. M. Parson; C. F. Lynn; J. -W. B. Bragg; P. Kelly; M. Taylor; D. Barnett; S. Holt; J. C. Dickens; A. A. Neuber; J. J. Mankowski; M. C. Scott; S. E. Calico

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7287274

Abstract: Thermal limitations of anode materials are known to impose limits on rep-rate operation of cold-cathode high-power microwave (HPM) sources. This study focuses on performance of pyrolytic graphite (PG) anodes at a 500 Hz burst-mode operation in a reflex-triode virtual-cathode-oscillator (vircator). In most experiments, a 42 J, 300 kV pulse forming network (PFN) based Marx generator with an approximate pulse width of 50 ns full-width-half-max (FWHM), was utilized to drive the vircator. Rep-rated operation of the vircator exacerbates the problems already experienced in single-pulsed mode where vircators are plagued by plasma formation on the anode and cathode followed by plasma/gas expansion that causes degradation of anode materials. Hence, for frequency-stable, repetitive operation, vircators require the use of thermally robust electrode materials and ultra-clean vacuum surfaces, leading to repeatable diode operation. This contribution presents thermal modeling of anode heating and experimental electrical behavior of vircator rep-rate operation.

IEEE Conferences

Authors: A. Neuber; J. Dickens; J. Mankowski; L. Hatfield; H. Krompholz; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7012489

Abstract: We review the past 20 years of progress in pulsed power science and technology at Texas Tech University, a US academic institution, specifically focusing on efforts that came to fruition with the support of Dr. Robert Barker, AFOSR.

IEEE Conferences

Authors: Stephens, J; Dickens, J; Neuber, A

PDF: https://pubmed.ncbi.nlm.nih.gov/25353899/

Abstract: Based on well-established physical relationships, a semiempirical set of equations dictating the electrical conductivity of dense, strongly coupled, partially ionized copper is presented. With the empirical coefficients, the model is tuned to experimental conductivity data obtained from exploding wire experiments [A. W. DeSilva and J. D. Katsouros, Phys. Rev. E 57, 5945 (1998)]. The result is a wide-range conductivity model, with demonstrated accuracy from room temperature-density conditions to 0.01 g/cm(3) and 30 kK. Using magnetohydrodynamic simulation the ability to utilize the conductivity model for predictive simulations is demonstrated. A complete electrical conductivity dataset for copper has been made available to the public.

Journal

Authors: Ryberg, D; Fierro, A; Dickens, J; Neuber, A

PDF: https://aip.scitation.org/doi/pdf/10.1063/1.4897295

Abstract: A system for time-discretized spectroscopic measurements of the vacuum ultraviolet (VUV) emission from spark discharges in the 60-160 nm range has been developed for the study of early plasma-forming phenomena. The system induces a spark discharge in an environment close to atmospheric conditions created using a high speed puff value, but is otherwise kept at high vacuum to allow for the propagation of VUV light. Using a vertical slit placed 1.5 mm from the discharge the emission from a small cross section of the discharge is allowed to pass into the selection chamber consisting of a spherical grating, with 1200 grooves/mm, and an exit slit set to 100 mu m. Following the exit slit is a photomultiplier tube with a sodium salicylate scintillator that is used for the time discretized measurement of the VUV signal with a temporal resolution limit of 10 ns. Results from discharges studied in dry air, Nitrogen, SF6, and Argon indicate the emission of light with wavelengths shorter than 120 nm where the photon energy begins to approach the regime of direct photoionization. (C) 2014 AIP Publishing LLC.

Journal

Authors: Stephens, J; Fierro, A; Dickens, J; Neuber, A

PDF: https://iopscience.iop.org/article/10.1088/0022-3727/47/46/465205

Abstract: Using high speed spectroscopic diagnostics, temporally resolved optical emission spectroscopy is performed on a nanosecond, repetitively pulsed microdischarge. The microdischarge operates in an argon-hydrogen gas mixture (99%/1%) to provide a Lyman-alpha vacuum ultraviolet emission. Based on the Stark broadening of the 486.1 nm, Balmer-beta hydrogen transition, the temporally resolved electron density was determined. Experimental electron density data are compared with the results of a 0D rate equation model. Peak electron density is estimated to be 5.6 . 10(15) cm(-3), corresponding to a similar to 0.25% degree of ionization. Using the approximate experimental ionization rate, the electron temperature is estimated to be similar to 3.5 eV.

Journal

Authors: Sullivan, WW; Mauch, D; Bullick, A; Hettler, C; Neuber, A; Dickens, J

PDF: https://aip.scitation.org/doi/10.1063/1.4794734

Abstract: This paper discusses a compact high voltage curve tracer for high voltage semiconductor device characterization. The system sources up to 3 mA at up to 45 kV in dc conditions. It measures from 328 V to 60 kV with 15 V resolution and from 9.4 pA to 4 mA with 100 fA minimum resolution. Control software for the system is written in Microsoft Visual C# and features real-time measurement control and IV plotting, arc-protection and detection, an electrically isolated universal serial bus interface, and easy data exporting capabilities. The system has survived numerous catastrophic high voltage device-under-test arcing failures with no loss of measurement capability or system damage. Overall sweep times are typically under 2 min, and the curve tracer system was used to characterize the blocking performance of high voltage ceramic capacitors, high voltage silicon carbide photoconductive semiconductor switches, and high voltage coaxial cable. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794734]

Journal

Authors: N. R. Gonsalves; D. V. Reale; J.-W. B. Bragg; W. W. Sullivan; A. A. Neuber; S. L. Holt; J. J. Mankowski; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6627665

Abstract: Recent work on Coaxial Ferrimagnetic Nonlinear Transmissions Lines (NLTL) has been focused on developing an array of NLTLs for use as a solid-state High Power Microwave (HPM) source. The pulsed output of an NLTL requires a combiner that can combine transient signals at voltage levels up to 50 kV. Existing combiner designs found in literature require resonant structures to achieve efficient power combination. The presented coaxial combiner is an in-plane structure designed to combine two 50 Ω transmission lines into a single 25 Ω coaxial line output which then uses a logarithmic taper to transition back to 50 Ω. The combiner design was simulated using a transient Finite Element Method (FEM) model in COMSOL Multiphysics® and experimental results are compared with simulation. A 4-way combiner based on an in-plane 2-way design is simulated and the field stresses are examined to determine maximum electric field levels encountered in the structure. This enabled estimating the associated maximum voltage level that the structure is able to support.

Conferences

Authors: G. Laity; A. Fierro; L. Hatfield; J. Dickens; A. Neuber; K. Frank

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6635196

Abstract: Summary form only given. There has been a continued interest in utilizing streamer and spark discharges for new technologies that require low temperature plasma generation at atmospheric pressure, but diagnostics of these plasmas typically require external sources of probing radiation such as UV lamps or laser systems. Specifically, it is desired to measure the dissociated gas density from pulsed surface flashover plasmas, without the use of potentially invasive optical techniques such as two-photon absorption laser induced fluorescence (TALIF) spectroscopy, which may artificially increase the dissociation degree. We demonstrate a method for determining the dissociated gas density of N and H atoms in an N2/H2 surface flashover plasma by passively monitoring the self-absorption of intrinsic radiation produced by the 2s2 2p2 3s→2s2 2p3 NI transition(s) at 120.0 nm, and the 2p→ls HI Lyman-α transition at 121.57 nm. This radiation is partially trapped by the spark plasma, assumed to be of Gaussian cylindrical shape with 500 μm diameter. The resulting emission line shapes can be calculated by inferring the plasma temperature, gas mixture, and the estimated dissociated atom density of each species in the plasma volume of measurement. For example, 80%/20% N2/H2 discharges with a measured electron temperature of ~3.0 eV produce peak dissociated concentrations of 2% and 9% for atomic N and H, respectively, during the spark phase ~100 ns after voltage collapse. By assuming the quasi-contiguous approximation of the Holtsmark micro-field due to local electron perturbation of the HI radiators, the Stark line width of Lyman-α radiation yields electron densities on the order of 1018 cm3 during the spark phase. This self-absorption method has been extended to provide density information of surface flashover plasmas in air environments by passively monitoring the 2s2 2p3 3s→2s2 2p4 OI transitions) at 130.2 nm / 130.5 nm / 130.6 nm, which yield peak dissociated concentrations of 20% and 7% for atomic O and N, respectively.

IEEE Conferences

Authors: Bragg, JWB; Sullivan, WW; Mauch, D; Neuber, AA; Dickens, JC

PDF: https://aip.scitation.org/doi/10.1063/1.4804196

Abstract: An all solid-state, megawatt-class high power microwave system featuring a silicon carbide (SiC) photoconductive semiconductor switch (PCSS) and a ferrimagnetic-based, coaxial nonlinear transmission line (NLTL) is presented. A 1.62 cm(2), 50 kV 4H-SiC PCSS is hard-switched to produce electrical pulses with 7 ns full width-half max (FWHM) pulse widths at 2 ns risetimes in single shot and burst-mode operation. The PCSS resistance drops to sub-ohm when illuminated with approximately 3 mJ of laser energy at 355 nm (tripled Nd:YAG) in a single pulse. Utilizing a fiber optic based optical delivery system, a laser pulse train of four 7 ns (FWHM) signals was generated at 65 MHz repetition frequency. The resulting electrical pulse train from the PCSS closely follows the optical input and is utilized to feed the NLTL generating microwave pulses with a base microwave-frequency of about 2.1 GHz at 65 MHz pulse repetition frequency (prf). Under typical experimental conditions, the NLTL produces sharpened output risetimes of 120 ps and microwave oscillations at 2-4 GHz that are generated due to damped gyromagnetic precession of the ferrimagnetic material's axially pre-biased magnetic moments. The complete system is discussed in detail with its output matched into 50 Omega, and results covering MHz-prf in burst-mode operation as well as frequency agility in single shot operation are discussed. (C) 2013 AIP Publishing LLC.

Journal

Authors: D. Ryberg; G. Laity; A. Fierro; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6635150

Abstract: Summary form only given. Limited experimental literature exists for measuring radiation with wavelengths shorter than 115 nm primarily due to the difficulty associated with measuring VUV radiation due to the high absorption in molecular oxygen and optical materials. Furthermore, there is an increasing interest in quantifying the role of this radiation in the generation of low temperature plasmas. This paper describes a system for studying the early mechanisms of vacuum UV generation below 115 nm by inducing breakdown in an air environment near atmospheric pressure. However to avoid the attenuating effects of O2, the experiment is set up such that the pressure drops to deep vacuum (i.e. ~10-6 torr) within a distance of several millimeters from the location of breakdown without the use of a solid barrier. In order to perform this measurement, the experiment consists of an enclosure at high vacuum, a pair of electrodes, and a micro puff valve. The puff value releases an above atmospheric pressure wave of air in the vicinity of the electrodes, whose differential electrical potential is then raised to about 20kV with a rise-time on the order of 100 nanoseconds. With application of the pulsed electric field, the radiation emitted from the excited species escapes the very localized puff of air and propagates through the high vacuum largely unattenuated where it is resolved by vacuum UV-sensitive optics and detectors. With this setup, the generation of vacuum UV radiation below 115 nm can be studied in detail and in the future the data recorded with this system will be used in our modeling approaches for further understanding of low temperature plasma generation.

IEEE Conferences

Authors: David Reale; J.-W. Braxton Bragg; Shad Holt; Andreas Neuber; John Mankowski; James Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6633209

Abstract: Coaxial Ferrimagnetic Nonlinear Transmission Lines (NLTL) are utilized as all solid state High Power Microwave (HPM) sources with virtually fixed phase relationship between input voltage pulse and output rf wave. Since the output power of a single NLTL at a given frequency, and of a given size, is limited by the effect of line dimensions as well as charge voltage on operating frequency, we employ an NLTL array to achieve higher power levels. To operate in microwave generation mode the ferrite domains are aligned along the axial direction of the NLTL using a biasing magnetic field generated by a dc current driven solenoid. This biasing method also provides a means of controlling the line delay by adjusting the magnetization of the ferrites using an auxiliary delay coil. By utilizing the delay coil and the primary biasing coil together on an NLTL, the operating frequency can be locked at a given frequency while the delay of the line can be independently adjusted. The relationship between the delay coil field and the corresponding temporal delay is investigated, and two NLTLs fed by identical voltage pulse shapes are combined using the phase control method described.

Conferences

Authors: D. V. Reale; J.-W. B. Bragg; S. L. Holt; A. A. Neuber; J. J. Mankowski; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6627412

Abstract: Coaxial Ferrimagnetic Nonlinear Transmission Lines (NLTL) can be utilized as High Power Microwave (HPM) sources. The output power of a single NLTL at a given frequency, and of a given size, is limited by the effect of line dimensions and charge voltage on operating frequency. Therefore, to achieve higher power levels, an array is required. To operate in microwave generation mode the ferrite domains are aligned along the axial direction of the NLTL using a biasing magnetic field generated by a dc current driven solenoid. This biasing method also provides a means of controlling the line delay by adjusting the magnetization of the ferrites using an auxiliary delay coil. By utilizing the delay coil and the primary biasing coil together on an NLTL, the operating frequency can be locked at a given frequency while the delay of the line can be independently adjusted. The relationship between the delay coil field and the corresponding temporal delay is investigated and two NLTLs are combined using the phase control method described.

Conferences

Authors: C. F. Lynn; J. Parson; P. Kelly; M. Taylor; J. Mankowski; J. Dickens; A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6635085

Abstract: Summary form only given. Vircators are high power microwave sources which operate at frequencies approximately proportional to the plasma frequency of the electron beam as it passes through the anode. As a result, stable frequency operation requires flat top voltage pulses, and diodes which exhibit little variation in impedance over a given pulse duration. However, plasma formation on the anode and cathode followed by plasma expansion causes the effective A-K gap and thus the impedance of the diode to collapse with time. As the impedance collapses current density increases, possibly causing a shift to higher frequency operation. The vircator presently discussed exhibits frequency chirping from 4 to 5.5 GHz during a pulse length of 700 ns when operated with an 8 stage, 1.5 kJ Marx generator that has a pulse shape of an overdamped RLC discharge (assuming arc formation does not occur). However, stable frequency operation is observed with an 8 stage 80 J Marx, each stage constructed from pulse forming networks rather than discrete capacitors. The PFN Marx generator produces a flat top voltage for a 75 ns FWHM pulse; however the energy radiated from the vircator is much lower due to the reduced energy storage of the PFN Marx. Burst mode operation has the potential to improve frequency stability and energy radiated from the vircator. This contribution reports the experimental results of a compact sealed tube vircator operated in burst mode with a repetition rate of up to 100 Hz. The data show voltage and current waveforms during burst cycles as well as microwave waveforms with temporally resolved frequency spectra.

IEEE Conferences

Authors: Patrick Kelly; Curtis Lynn; Jonathan Parson; Mark Taylor; John Mankowski; Andreas Neuber; James Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6634857

Abstract: The electrical characteristics and current capabilities of a low inductance Marx generator used to drive a high power microwave (HPM) source are described. The operating frequency of many HPM sources is dependent upon various characteristics of the input voltage pulse from the pulsed power generator. Texas Tech University has developed an 8 stage, 80 J Marx generator used to drive HPM sources at repetition rates up to 500 Hz. The Marx generator is constructed using pulse forming networks rather than discrete capacitors. Experimental efforts have shown stable frequency operation of a virtual cathode oscillator (Vircator) source using a low energy, PFN Marx. Efforts were taken to reduce parasitic inductance of the PFN Marx resulting in a voltage pulse with a risetime less than 25ns with 50ns FWHM. Additionally, efforts were taken to improve impedance matching between the PFN Marx and the vircator load. This article presents experimental results of an 80 J Marx at voltages ranging from 150 kV to 250 kV operating in burst mode. Voltage and current waveforms from the Marx are shown as well as some typical results of compact sealed tube vircator operation.

Conferences

Authors: J. Stephens; B. Loya; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6634980

Abstract: Summary form only given. A high side, high voltage pulser is used to excite a micro hollow cathode discharge (MHCD) geometry with short pulsed voltages adjustable on the order of 100-500 ns, at repetition rates from 10-200 kHz. The goal of this research is to develop an intense, efficient UV/VUV radiation source capable of generating photons of sufficient energy to affect direct photo-ionization in a gaseous volume rather than photoemission from surfaces. The pulser used in this study has experimentally demonstrated to be capable of efficiencies on the order of 80% when delivering power to a 1 KΩ resistive load at 200 kHz repetition rate. The final design is to be compact, and robust, capable of long term rep-rated operation with minimal electrode erosion and dielectric degradation. Present designs involve laser machining a direct bonded copper (DBC) substrate consisting of Cu-Al2O3-Cu. Transient ignition of the MHCD, voltage and current waveforms, and spectral emission are all experimentally studied. A transient, 2-dimensional, cylindrical coordinate (r-z) fluid model is used for the theoretical treatment of the microdischarge. Using known rate coefficients for argon, time and space resolved behavior of discharge under pulsed conditions is presented.

IEEE Conferences

Authors: J. -. B. Bragg; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6359866

Abstract: Ferrimagnetic nonlinear transmission lines (NLTLs) have the potential to fill a high-power microwave niche where compact cost-effective sources are lacking. NLTLs utilize nonlinear ferrimagnetic properties and magnetization dynamics to provide ultrafast pulse rise times (100 ps or less) and microwave signals with peak power ranging from kilowatts to hundreds of megawatts. The frequency of operation has been shown to range from 900 MHz up to 5 GHz depending on geometry and external magnetic fields. NLTLs, theoretically, can be pulsed to tens of kilohertz with little to no variance in microwave signal between shots. This paper covers recent advances in ferrimagnetic-based NLTLs, specifically effects of applied and bias magnetic fields on peak power and frequency, as well as temperature dependence.

Journal

Authors: Lynn, CF; Dickens, JC; Neuber, AA

PDF: https://aip.scitation.org/doi/10.1063/1.4826209

Abstract: Virtual cathode oscillators, or vircators, are a type of high power microwave device which operates based on the instability of a virtual cathode, or cloud of electrons, which forms when electron current injected into the drift tube exceeds the space charge limited current within the drift tube. Anode heating by the electron beam during vircator operation ultimately limits achievable pulse lengths, repetition rates, and the duration of burst mode operation. This article discusses a novel cathode design that focuses electrons through holes in the anode, thus significantly reducing anode heating by the electrons emitted from the cathode during the first transit through the A-K gap. Reflexing electrons continue to deposit energy on the anode; however, the discussed minimization of anode heating by main beam electrons has the potential to enable higher repetition rates as well as efficiency and longer diode lifetime. A simulation study of this type of cathode design illustrates possible advantages. (C) 2013 AIP Publishing LLC.

Journal

Authors: Mark Taylor; Patrick Kelly; Curtis Lynn; Jonathan Parson; John Mankowski; Andreas Neuber; James Dickens; Magne Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6635087

Abstract: Summary form only given. A virtual cathode oscillator (vircator) is a high power microwave device that exhibits frequency tunability which is inherent to its principle of operation. Two types of electron motion within the vircator generate microwaves (virtual cathode oscillation and reflexing electron motion). Although it is difficult to precisely predict the dominant microwave frequency of a vircator design prior to experimental observation, the oscillation frequency of the virtual cathode is approximately proportional to the plasma frequency of the electron beam as it enters the virtual cathode. Additionally, the reflexing electrons oscillate at a frequency which is inversely proportional to approximately four times the transit time of an electron through the anode-cathode gap (A-K gap). As a result, assuming space charge limited diode operation, the virtual cathode and reflexing electron oscillation frequencies, though different, are proportional to V1/2/d, where d is the gap separation of the A-K gap and V is the accelerating voltage applied to the gap. Thus vircators are tunable via adjusting the A-K gap and varying the applied accelerating voltage. Texas Tech University has developed a sealed tube vircator which radiates approximately 100 MW with an operational frequency of 4GHz. Operating at 4GHz the diode has an A-K gap of 8mm, an accelerating voltage of ~200 k V, and electron beam current of ~3.5kA. Here we present an experimental investigation of the tunability of a reflex triode vircator by varying the A-K gap from 4 mm to 10 mm and accelerating voltages ranging from 150 kV to 250 kV.

Conferences

Authors: David Reale; J.-W. Braxton Bragg; Richard Solarski; Andreas Neuber; Shad Holt; John Mankowski; James Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6633210

Abstract: Previously developed coaxial ferromagnetic Nonlinear Transmission Lines (NLTL) relied solely on pressurized Sulfur Hexaflouride (SF6) as high-voltage insulating dielectric medium [1]. While the use of SF6 provides the necessary electric insulation, there are drawbacks including gas storage and pressure fittings that increase system size and add to the design complexity of the NLTLs themselves. We evaluate solid dielectric materials as an alternative. Initial attempts used a standard high voltage (HV) epoxy to pot the NLTL assembly. This method was effective at producing magnetic precession in the NLTL; however, there was a reduction in output power due to the high loss tangent of the epoxy. Sylgard 184, commonly used in solar cells, has been widely used as an HV potting material and has an order of magnitude lower loss tangent than standard HV epoxy at 1 kHz. Samples of HV epoxy and Sylgard 184 are tested in a microwave cavity resonator at 3 GHz to determine their loss tangents. The performance of an NLTL potted with Sylgard 184 is compared to that of the HV epoxy NLTL and the current SF6 insulated NLTL.

Conferences

Authors: R. C. Solarski; D. V. Reale; J.-W. B. Bragg; A. A. Neuber; S. L. Holt; J. J. Mankowski; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6627413

Abstract: At the Center for Pulsed Power and Power Electronics, previous coaxial ferrimagnetic Nonlinear Transmission Lines (NLTL) relied solely on pressurized Sulfur Hexafluoride (SF6) as high-voltage insulating dielectric medium [1]. While the use of SF6 provides the necessary electric insulation, there are drawbacks including gas storage and pressure fittings that increase system size and add to the design complexity of the NLTLs themselves. Hence it was deemed necessary to evaluate solid dielectric materials as an alternative. Initial attempts utilized a standard high voltage (HV) epoxy to pot the NLTL assembly. This method was effective at producing magnetic precession in the NLTL; however, there was a reduction in output power due to the high loss tangent of the epoxy. Sylgard 184, commonly used in solar cells, has also found use as an HV potting material. Per datasheet, its loss tangent is an order of magnitude lower compared to standard HV epoxy at 1 kHz. Samples of HV epoxy and Sylgard 184 were tested in a microwave cavity resonator at 3GHz, which yielded their respective loss tangents. The performance of an NLTL potted with Sylgard 184 is compared to that of the HV epoxy NLTL and the earlier SF6 insulated NLTL.

Conferences

Authors: J. Stephens; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6635217

Abstract: Exploding wires (EWs) are subjected to high current densities of the order of 107 A/cm2 inducing metal vaporization and plasma formation on a microsecond timescale. Single strand EWs, silver and copper, are tested in gaseous media from atmospheric pressure to 790 kPa. To theoretically predict EW behavior, one-dimensional, radially directed cylindrical, Lagrangian coordinate hydrodynamic (HD) and magnetohydrodynamic (MHD) models are applied. Such models require accurate material equation-of-state (EOS) and electrical conductivity data throughout the temperature density range experimentally achieved (ρ=0.1-10 gm/cm3 and T=300-20,000 K). In this study, the Lee-More-Desjarlais (LMD) conductivity, and its quantum molecular dynamic modification (QLMD) are used. The Los Alamos National Laboratory SESAME database is used to provide the EOS parameters. When utilized as an opening switch the metal plasma is exposed to higher electric fields, atypical to traditional exploding wire experiments. Recent studies have shown that the behavior of the strongly coupled plasma in such conditions is reasonably well modeled assuming a semi-empirical electron impact ionization process. The HD and MHD based models are benchmarked against experimental data to confirm their accuracy for predicting the behavior of EWs in an opening switch type operation.

IEEE Conferences

Authors: Bragg, JWB; Dickens, JC; Neuber, AA

PDF: https://aip.scitation.org/doi/abs/10.1063/1.4792214?journalCode=jap

Abstract: The growing need for solid-state high power microwave sources has renewed interest in nonlinear transmission lines (NLTLs). This article focuses specifically on ferrimagnetic-based NLTLs in a coaxial geometry. Achieved peak powers exceed 30 MW at 30 kV incident voltage with rf power reaching 4.8 MW peak and pulse lengths ranging from 1-5 ns. The presented NLTL operates in S-band with the capability to tune the center frequency of oscillation over the entire 2-4 GHz band and bandwidths of approximately 30%, placing the NLTL into the ultra-wideband-mesoband category of microwave sources. Several nonlinear materials were tested and the relationship between NLTL performance and material parameters is discussed. In particular, the importance of the material's ferromagnetic resonance linewidth and its relationship to microwave generation is highlighted. For a specific nonlinear material, it is shown that an optimum relation between incident pulse magnitude and static bias magnitude exists. By varying the nonlinear material's bias magnetic field, active delay control was demonstrated. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4792214]

Journal

Authors: J.-W. Braxton Bragg; William W. Sullivan; David V. Reale; Daniel L. Mauch; Shad Holt; Andreas A. Neuber; John Mankowski; James C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6633211

Abstract: An all solid-state high power microwave system comprised of a photoconductive semiconductor switch (PCSS) and nonlinear transmission line (NLTL) is presented. A single 50 kV 4H-Silicon Carbide (SiC) PCSS switches 7 ns (FWHM) pulses with 2 ns risetime into the gyromagnetic NLTL. The PCSS achieves sub-ohm resistance when illuminated by approximately 3 mJ of laser energy emitted from a tripled Nd:YAG laser (355 nm). Utilizing a fiber optic based optical triggering system enabled generating a train of laser pulses, and burst-mode operation with 65 MHz pulse repetition frequency was achieved. The NLTL sharpens the fast rising pulse from the PCSS and produces MW-class rf power levels at 2-4 GHz. Microwave generation is achieved through stimulation of damped gyromagnetic precession when the static and pulsed external magnetic fields interact with the magnetic moments of the nonlinear ferromagnetic material. The acting nonlinear material is a NiZn composite with dimensions 3 mm × 6 mm (ID × OD). The system is terminated with a wideband antenna consisting of a coaxial to parallel-plate conversion balun which is then immediately flared to provide a linearly-flared TEM horn antenna. The entire length of the compact HPM system measures 1.15 m. Radiation results for single shot and burst-mode operation are presented as well as demonstration of operational frequency control.

Conferences

Authors: D. L. Mauch; W. W. Sullivan; A. B. Bullick; A. A. Neuber; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6635034

Abstract: A 20 kV, lateral geometry, contact face illuminated, silicon carbide (SiC) photoconductive semiconductor switch (PCSS) is presented. The SiC PCSS was fabricated from high purity semi-insulating, bulk 4H-SiC (12.7 mm × 12.7 mm × 0.35 mm), in a lateral geometry, (0.6 mm gap) with both the anode and cathode contacts located on the same face of the device. The device was illuminated with light from a tripled Nd:YAG laser (355 nm-7 ns FWHM) entering from the contact face. The device demonstrated sub-ohm on-state resistance for laser pulse energies in the mJ range, and micro-amp leakage currents at 20 kVdc in the off-state. Voltage hold-off and low leakage currents in the off state were achieved through high energy electron beam irradiation of the bulk material. The switch's geometry and packaging are discussed, along with experimental switching and blocking characteristics.

IEEE Conferences

Authors: P. Kelly; J. M. Parson; C. F. Lynn; J. J. Mankowski; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6635086

Abstract: Summary form only given. This study focuses on the use of non-evaporable getter (NEG) materials in a high power microwave (HPM) virtual-cathode oscillator (vircator). The vircator has a background pressure in the low 10-9 torr range with a vacuum volume of 5 L backed by a small 20 L/s ion pump. Presently, the source is driven by an 80 J, 300 kV Marx generator with a current density of 225 A/cm2 and pulse width of 100 ns. Initial current pulses increases the chamber pressure to the low 10-6 torr range desorbing approximately 1014 particles1,2. Reduction in the number of particles desorbed decreases an order of magnitude after approximately 2,000 current pulses, with no further noticeable decrease after 10,000 pulses3. The HPM system will eventually be modified to operate at 500 Hz with a pulse width between 10-20 ns. In first order approximations, a pumping speed of 2500 L/s will be needed to pump 1014 particles within 2 ms from a 5 L volume. This corresponds to a surface area of approximately 2400 cm2 for a zirconium-vanadium-iron (ZrVFe) getter material. An introduction of NEG materials into the vacuum chamber will aid in achieving repetitive operation; and preliminary results on the use of the NEG material in the vacuum environment will be presented. A residual gas analyzer (RGA) and an inverted-magnetron cold cathode vacuum sensor are used for the gas diagnostics and are rated to survive the 300°C, 72 hour vircator bake out process.

IEEE Conferences

Authors: Beeson, S; Dickens, J; Neuber, A

PDF: https://aip.scitation.org/doi/abs/10.1063/1.4822343?journalCode=php

Abstract: Microwave transmission and reflection characteristics of pulsed radio frequency field generated plasmas are elucidated for air, N-2, and He environments under pressure conditions ranging from 10 to 600 torr. The pulsed, low temperature plasma is generated along the atmospheric side of the dielectric boundary between the source (under vacuum) and the radiating environment with a thickness on the order of 5 mm and a cross sectional area just smaller than that of the waveguide. Utilizing custom multi-standard waveguide couplers and a continuous low power probing source, the scattering parameters were measured before, during, and after the high power microwave pulse with emphasis on the latter. From these scattering parameters, temporal electron density estimations (specifically the longitudinal integral of the density) were calculated using a 1D plane wave-excited model for analysis of the relaxation processes associated. These relaxation characteristics ultimately determine the maximum repetition rate for many pulsed electric field applications and thus are applicable to a much larger scope in the plasma community than just those related to high power microwaves. This manuscript discusses the diagnostic setup for acquiring the power measurements along with a detailed description of the kinematic and chemical behavior of the plasma as it decays down to its undisturbed state under various gas type and pressure conditions. (C) 2013 AIP Publishing LLC.

Journal

Authors: Sterling R. Beeson; James C. Dickens; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6635055

Abstract: Summary form only given. We study the recovery of the low temperature plasma generated by a microsecond high power microwave, HPM, pulse in different gases at pressures approaching one atmosphere. The experiment mimics the electrical breakdown at the interface between the vacuum HPM-source environment and the radiating environment (generally held at atmospheric pressure). This low temperature plasma is generated along the surface on the atmospheric side where the high Eeff/p, or reduced effective electric field with pressure, causes high levels of ionization leading to the formation of a highly conductive plasma within the duration of the microsecond pulse. With the maximum HPM pulse repetition rate being highly dependent on the relaxation of this plasma, it is the goal of this study to determine the kinematics and chemistry of this low temperature plasma as it pertains to ion, electron, and excited species densities. For this study, a 2.85 GHz magnetron operating in the TE10 mode of a WR-284 standard waveguide is used to generate a 3 MW, 3 microsecond pulse with a rise time of less than 50 ns that is incident on the dielectric window separating the two environments. The atmospheric side is enclosed in a structure that mimics an open radiation pattern similar to that of current HPM systems. To understand the post-pulse features, two custom multi-standard waveguide couplers were designed to implement and extract a low power 10 GHz CW source into the main waveguide structure while keeping a low insertion loss for the HPM pulse. The results of this power signal (max attenuation values range -10 to -40 dB) along with a 1D plane wave excited plasma model is used to infer the temporal average electron density (specifically the longitudinal integral of the surface plasma density) at a range of pressures and different gases, typically 10-400 torr for air, N2, Ar, and He. The peak electron density and loss rates are then correlated with diffusion lengths, recombination and attachment rates given in literature to ascertain the dominant plasma relaxation path and species along with a means to extrapolate the time required to relax to a nominal background electron density. For instance, the dominant electron loss process in 90 torr air is attachment with a frequency of 121 kHz and peak electron density of ~1013 cm-3 resulting in relaxation times of a few hundred microseconds while in N2, the dominant process soon after the pulse is determined to be 2-body dissociative recombination.

Conferences

Authors: Laity, G; Fierro, A; Dickens, J; Neuber, A; Frank, K

PDF: https://aip.scitation.org/doi/10.1063/1.4804369

Abstract: We demonstrate a method for determining the dissociation density of N and H atoms present in a developing low temperature plasma, based on the emission and self-absorption of vacuum ultraviolet radiation produced from the plasma. Spark plasmas are produced via pulsed discharge in N-2/H-2 mixtures at atmospheric pressure, where information on the dissociated densities of the constituent gas molecules is desired without employing invasive diagnostic techniques. By analyzing the self-absorption line profile of 121.5 nm Lyman-alpha H radiation emitted within the first similar to 1.0 mm of plasma near the anode tip, a peak dissociated H atom concentration of 5.6 x 10(17) cm(-3) was observed similar to 100 ns into spark formation, with an estimated electron density of 2.65 x 10(18) cm(-3) determined from Stark broadening. Similarly, simultaneous line fitting of the N 120.0/124.3 nm emission profiles revealed a peak dissociated N atom concentration of 3.8 x 10(17) cm(-3) during the same discharge period. (C) 2013 AIP Publishing LLC.

Journal

Authors: Andrew S. Fierro; George R. Laity; Sterling R. Beeson; James C. Dickens; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6634918

Abstract: Summary form only given. A GPU-accelerated 3-dimensional PIC/MCC simulation code was developed using the CUDA environment to study the physical processes involved in the development of a low-temperature plasma. The simulation results aid in quantifying transient plasma development as it is often inaccessible experimentally in detail even with modern noninvasive techniques such as non-linear laser spectroscopy or high-speed electrical diagnostics. Hence, computational methods, such as Particle-in-Cell (PIC) and Monte Carlo Collision (MCC), provide a complementary approach to determining the mechanisms leading to plasma development. However, fully modeling the physics of the plasma development is made difficult by the number of plasma processes that must be tracked simultaneously, and only recently have computing resources provided the capability to track tens of millions of particle interactions. Furthermore, the introduction of graphics processing unit (GPU) computing provides an attractive means for economical and efficient parallelization of scientific codes through a framework such as NVIDIA CUDA. As such, a GPU-accelerated 3-dimensional PIC/MCC simulation was developed using the CUDA environment to provide characteristics during the initial stage of plasma development in atmospheric pressure nitrogen. The simulation was run on a NVIDIA GTX 580 with 3 GB of memory and 512 CUDA cores. The simulated geometry consists of two paraboloid electrodes with a gap distance of 5 millimeters with Dirichlet boundary conditions, and 22 unique electron interactions with molecular nitrogen are considered. The electrodes are excited with a step voltage pulse of several thousand volts also assuming a uniformly distributed initial electron density of 104 cm-3 in the volume. For instance, results from a 5 nanosecond simulation reveal the development of positive ion space charge channels near the anode and cathode regions. These channels appear consistent with high-speed streamer photographs captured during plasma formation. The electron energy distribution function (EEDF) indicates a non-Maxwellian velocity distribution during the application of the high electric field. Furthermore, a typical electron density near the cathode is on the order of 7 × 108 cm-3. The results from numerical simulation will be compared in detail to experimentally accessible parameters such as electron temperature and dissociation degree.

Conferences

Authors: P. J. Ford; J. Krile; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6518732

Abstract: Surface flashover induced by a High Power Microwave fast rise-time pulse causes a significant drop in transmitted power, along with reflections that can damage the source. Momentum transfer collision rates in the range of 100s of GHz (for pressures exceeding 5 kPa) lead to low plasma conductivity, corresponding to absorption levels of up to 60 % of the incident power. A simulation algorithm was developed using the finite-difference time-domain (FDTD) method in order to model the growth and transport of the electron density near a dielectric surface, and the resulting interaction with the microwave pulse. The time-dependent plasma parameters are governed by empirical and simulated scaling laws for ionization and collision rates, along with diffusion coefficients; the resulting frequency-dependent plasma permittivity is transformed to a discrete algorithm to describe the spatially resolved plasma in the FDTD algorithm. A plasma thickness of up to 2 mm is simulated that compares with side-on ICCD imaging of surface flashover. Breakdown parameters, such as delay times and breakdown electric fields, in nitrogen, air and argon, are compared with experimental data on surface flashover across a polycarbonate window at atmospheric pressures; the simulated results correlate well with measured, and the model exhibits low computational complexity when simulating a pulse on the order of microseconds, making it a good alternative to standard particle-in-cell codes. The source is a S-band magnetron that produces a 2.5 MW peak power, 50 ns rise-time pulse with 3 μs duration at 2.85 GHz center frequency.

IEEE Conferences

Authors: Elsayed, MA; Neuber, AA; Dickens, JC; Walter, JW; Kristiansen, M; Altgilbers, LL

PDF: https://aip.scitation.org/doi/10.1063/1.3681443

Abstract: The increased popularity of high power microwave systems and the various sources to drive them is the motivation behind the work to be presented. A stand-alone, self-contained explosively driven high power microwave pulsed power system has been designed, built, and tested at Texas Tech University's Center for Pulsed Power and Power Electronics. The system integrates four different sub-units that are composed of a battery driven prime power source utilizing capacitive energy storage, a dual stage helical flux compression generator as the main energy amplification device, an integrated power conditioning system with inductive energy storage including a fast opening electro-explosive switch, and a triode reflex geometry virtual cathode oscillator as the microwave radiating source. This system has displayed a measured electrical source power level of over 5 GW and peak radiated microwaves of about 200 MW. It is contained within a 15 cm diameter housing and measures 2 m in length, giving a housing volume of slightly less than 39 1. The system and its sub-components have been extensively studied, both as integrated and individual units, to further expand on components behavior and operation physics. This report will serve as a detailed design overview of each of the four subcomponents and provide detailed analysis of the overall system performance and benchmarks. (C) 2012 American Institute of Physics. [doi:10.1063/1.3681443]

Journal

Authors: J. M. Parson; J. C. Dickens; A. Neuber; J. Walter; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6383332

Abstract: Summary form only given. This presentation describes a study on gas evolution of plasma expansion in a reflex-triode virtual cathode oscillator (vircator) at ultra-high vacuum (UHV). Research has shown that explosive electron emission (EEE) processes at the cathode and ion / electron bombardment at the anode cause material erosion that produces regions of localized plasma. This plasma expansion has shown to lower gap impedance, cut off microwave performance and spoil low vacuum levels over time1. The goal of the study is to identify the gas species and their sources to better understand and limit the negative effects of plasma expansion in sealed tubes. The vacuum tube under investigation is a triode-geometry vircator with 20 cm2 cathode surface, driven by a 80 J Marx Generator with an approximate peak voltage and current, and pulse width of 200 kV and 5 kA, and 200 ns, respectively.

IEEE Conferences

Authors: Stephens, J; Beeson, S; Dickens, J; Neuber, A

PDF: https://aip.scitation.org/doi/10.1063/1.4767649

Abstract: A quasi-permanent charged electret is embedded into the radiation window of a high power microwave system. It was experimentally observed that the additional electrostatic field introduced by the electret alters the delay times associated with the development of plasma at the window surface, resulting from high power microwave excitation. The magnitudes of both the statistical and formative delay times are investigated in detail for different pressures. Experimental observations are related to calculated discharge parameters using known E/p dependent properties. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767649]

Journal

Authors: C. Lynn; J. Walter; A. Neuber; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6518859

Abstract: Many high power microwave devices use explosive or flashover electron emission cathodes in order to generate the electron beam and thus drive the device. These diodes are simple to operate, requiring no heater or other external systems, and are capable of generating beam currents of several kA at accelerating voltages on the order of 100s of kV. However, they generally suffer from non-uniform emission, anode heating, out-gassing, and pulse shortening due to anode and cathode plasma expansion. The ability to rep-rate such a diode is generally limited by anode heating and out-gassing which are both affected by beam uniformity. Two diodes are compared in this work. One uses a machined aluminum cathode, which is made from solid aluminum with grooves milled onto the surface. The other diode utilizes a carbon velvet cathode which is CVD coated with CsI. Time integrated scintillator images of the electron beam at the anode were taken for both the carbon velvet and aluminum cathodes. Additionally, time resolved images of the emission centers were taken for the carbon velvet cathodes. Data sets of over 1000 shots were taken with each cathode and shot to shot variation in the peak “turn-on” electric field are compared. The lifetime of the aluminum cathode was exceeded before 1000 shots, whereas the carbon velvet cathode showed no degradation in operation.

IEEE Conferences

Authors: S. Beeson; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6383529

Abstract: Plasma generation along the dielectric interface between the vacuum medium of the source and the atmospheric environment of the antenna is one of the limiting factors in power thresholds of high power microwave, HPM, systems. The maximum repetition rate for HPM platforms are ultimately determined by the relaxation times of this low-temperature surface plasma. While the microwave scattering parameters can be determined during the HPM pulse from the transmitted and reflected pulse itself, additional diagnostics effort has to be expended to capture post-pulse plasma relaxation and microwave scattering parameters. For this, a Bethe hole-type multi-standard waveguide coupler was designed and fabricated to inject a continuous low power signal into the main waveguide structure that carries the main 3 MW, 3 µs pulse at 2.85 GHz in an S-band waveguide structure. To facilitate flashover the waveguide is terminated by a Lexan window into a controlled atmospheric chamber with absorbing walls that mimic radiation into free space. The coupler injects 1W of continuous power at 10 GHz to measure the scattering parameters many microseconds after the pulse extinguishes. Using a model developed from a 1D plane wave excitation, the plasma's electron density can be inferred from these power measurements. Then, from the temporal analysis of the electron density, the kinematic and chemical behavior of the plasma in relation to the recombination and attachment properties of the gas are inferred. Tested conditions include three gas types: N2, air, Ar, ranging in pressure from 10 to 155 torr. The transmission coefficients range from −40 to −10 dB corresponding to an electron density of ∼1014 to ∼1011 cm−3 for these pressures, respectively. The decay of this plasma has been experimentally determined to be on the order of tens of microseconds with the density falling off proportional to t−1, t−2, exponential, or a linear combination thereof, depending on the dominant electron loss path. Reported here, are the design parameters of the waveguide coupler along with the post-pulse evolution of the electron density and recombination physics that it entails.

IEEE Conferences

Authors: Sterling Beeson; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6518710

Abstract: The relaxation time of pulsed rf-generated plasma is investigated. A 3 MW, 3 μs width, 50 ns risetime HPM pulse is transmitted through a dielectric window that terminates a WR-284 waveguide filled with insulating gas. The investigated plasma is formed across the dielectric window on the atmospheric side. This produces electron densities on the order of 1013 to 1012 cm-3 for 60 to 145 torr in air, respectively. In the same pressure range, initial attenuation (~ 0.5 dB) of the microwaves is observed after tens to hundreds of nanoseconds with final attenuation values approaching -40 to -10 dB, respectively. To determine plasma relaxation times after the HPM pulse terminates a multi-standard waveguide coupler (X/S-band) was designed to inject a low power 10 GHz signal used for probing the surface plasma. The coupler was designed to have high coupling coefficients (> -5 dB) for the specific narrowband around 10 GHz (BW ~ 10 MHz) along with negligible insertion loss of the HPM propagation. From the measured attenuation and reflection of the 10 GHz probe signal, the evolution of the electron density is inferred. Furthermore, in an air environment, the density falls exponentially implying attachment as the dominate electron loss mechanism.

Conferences

Authors: Stephens, J; Neuber, A; Kristiansen, M

PDF: https://aip.scitation.org/doi/10.1063/1.3689855

Abstract: This paper discusses the effect of surface coatings on exploding wire behavior. Three different surface coatings of different thicknesses and materials have been studied, each with a 99.99% pure silver conducting core. Experimentally, the wires are subjected to peak current densities in excess of 10(7) A/cm(2) on a microsecond time scale. High Speed intensified Charge-Coupled Device (iCCD) images. A theoretical one-dimensional finite difference model has been developed to predict wire behavior and determine the mechanism(s) responsible for the deviations in behavior induced by the presence of a surface coating. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3689855]

Journal

Authors: J. C. Stephens; A. A. Neuber; J. C. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6518739

Abstract: The primary objective of the research discussed in this paper is to develop a compact electro-explosive fuse (EEF) for a flux compression generator (FCG) power conditioning system, capable of rapidly obtaining and maintaining high impedance. It was observed that significant gains in EEF performance are introduced with the application of an insulating coating to the surface of the EEF wire. A 2 kA small scale test bed has been designed to provide a single wire EEF with similar current density (~107 A/cm2), voltage gradient (~7 kV/cm), and timescale (~8 μs) as to what is seen by and EEF utilized in a HPM generating FCG system. With the small scale test bed EEF performance data was rapidly obtained at a significantly lower cost than equivalent full scale FCG experiments. A one-dimensional finite difference model coupled with the Los Alamos National Laboratory SESAME Equation-of-State database was utilized to simulate the resistive behavior of the single wire EEFs. Further, a large scale test bed, designed to provide a similar current action as to what is provided by a FCG is used to test 18 wire EEF arrays at the 40 kA level.

IEEE Conferences

Authors: G. Laity; A. Fierro; L. Hatfield; A. Neuber; K. Frank

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6383426

Abstract: There has been a continued interest in utilizing streamer and spark discharges for new technologies which require low temperature plasma generation at atmospheric pressure. One key area of study is the role of photon dominated processes, such as ionization and dissociation, as a result of emission and re-absorption of vacuum UV radiation (e.g. photons with energies greater than 8 eV). This experiment consists of a triggered surface flashover event along a dielectric surface between two point-point electrodes, where spectral measurements are performed in the vacuum UV regime (i.e. 115 – 175 nm). Previous studies of air breakdown alluded to a cross-species photon absorption process between N2 and O2 molecules, and a later study revealed an impurity in the form of HI Lyman-α radiation in spark discharges in air. In an effort to understand the self-absorption of this radiation at atmospheric pressure, a basic study was performed in various N2/H2 mixtures in a controlled environment. Spectral measurements were taken via intensified CCD devices in the VUV range, photomultiplier electronics with accompanying electrical diagnostics, and external imaging with nano-second resolution. It was concluded from the experiments that significant self-absorption of HI radiation is occurring during plasma formation at electron temperatures of ∼10 eV, and detailed spectral line-fitting showed that the parameters which define absorption vary as a function of distance from the anode (i.e. where the streamers originate due to the geometry in this experiment). Therefore, rough estimates can be made of parameters such as H2 dissociation percentage (as much as 10−3 in the plasma channel) and electron density (upwards of 1019 cm−3 in the spark phase) as a function of position between the electrodes. These studies elucidate the plasma kinetics (specifically absorption) which occurs during the transition from streamer to spark discharge at atmospheric pressure.

Conferences

Authors: Christopher Simmons; J.-W. Braxton Bragg; James Dickens; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6518853

Abstract: A nonlinear transmission line (NLTL) provides a solidstate means of generating high power, microwave pulses. The NLTLs in this study are coaxial transmission lines whose center conductor is encapsulated by ferrite beads. Operational frequency can be controlled by varying the dimensions of the ferromagnetic material, which affects azimuthal magnetic fields and material losses, or by varying the biasing field strength. This research demonstrates frequency agility of an NLTL by documenting the design, construction, and testing of three NLTLs. The NLTL will be one meter long, and to test for frequency agility, three different sizes of ferrites will be loaded onto NLTL with various biasing fields applied. Azimuthal field strengths due to an incident high voltage pulse range from 10-36 kA/m with magnetic biasing fields between 0 kA/m and 50 kA/m.

Conferences

Authors: J. Parson; J. Dickens; J. Walter; A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6518724

Abstract: This paper presents a study on gas evolution of three different anode materials in vacuum sealed tubes under UHV conditions. The experimental apparatus consists of a high-power microwave (HPM) virtual-cathode oscillator (vircator) driven by a 200 ns, 80 J, 225 kV low-impedance Marx Generator. Plasma expansion due to explosive electron field emission has shown to lower gap impedance, spoil consistent low vacuum levels, and cut-off microwave radiation. The anode materials compared are nickel 201L (Ni201L), stainless steel 316L (SS316L) and grade-1 titanium (TiG1); with the cathode material being aluminum. The anodes were cleaned by the following method: rough polishing followed by electro-polishing, a ten minute microwave argon / 10% oxygen plasma cleaning process (ArO₂) and finally, a 72 hour in situ bake-out at 300°C. Outgassing characteristics of each anode material are presented and compared.

Conference Paper/Presentation

Authors: J. -. B. Bragg; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6148287

Abstract: In pulsed power systems, coaxial based nonlinear transmission lines (NLTLs) loaded with ferrimagnetic materials act as pulse sharpeners or high power microwave sources. Microwave generation comes by way of nonlinearities present in the ferrimagnetic material as well as excitation of damped gyromagnetic precession at large incident power levels. Ferrimagnetic properties highly depend on operating temperature; therefore, there exists a need to understand operational performance of ferrite loaded NLTLs under different temperature environments. Ferrite samples are chilled or heated to temperatures between -20°C to 150 °C, providing a wide range of possible operating temperatures. The Curie temperature of the tested samples is approximately 120 °C; therefore, this study allows observation of precession performance in possible operating temperatures as well as a brief look at the consequences of exceeding the Curie temperature. The design, testing, and results for an NLTL measuring 0.3 m in length with ferrite inner and outer diameters of 3 mm and 6 mm, respectively, are detailed. Results reveal precessional performance, both peak power and frequency of oscillations, versus temperature.

IEEE Journals

Authors: G. Laity; A. Neuber; A. Fierro; L. Hatfield; J. Dickens; K. Frank

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6518678

Abstract: This paper describes recent advances in the study of self-generated emission of vacuum ultraviolet (VUV) radiation which is produced during the early time period leading to high voltage breakdown at atmospheric pressure. Previous studies of air breakdown showed the presence of 121.5 nm radiation which is spontaneously emitted by excited hydrogen atoms, HI. Since this Lyman-α line is self-absorbed, it enabled inferring various plasma parameters from recording emission spectra from 115-135 nm for species of HI and NI. For instance, measurements in H2/N2 mixtures have revealed that the highest amount of absorption via HI atoms occurs in the high field region near the anode, implying that significant H2 dissociation for radiation-trapping is occurring in this zone. Selective spatial measurements further showed that the apparent VUV emission centers (i.e. streamer heads) move away from the anode and the mechanisms leading to line broadening (i.e. Stark effect from space charge) are a function of streamer position. The presented self-absorption mechanisms are essential in quantitatively understanding the role of VUV radiation transport via absorption for photo-ionization during streamer breakdown, in which re-absorption of high energy photons is inherently a requirement.

IEEE Conferences

Authors: Curtis F. Lynn; Andreas A. Neuber; John W. Walter; James C. Dickens; Magne Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6329448

Abstract: Many high-power electron devices utilize cold-cathode diodes to generate intense electron beams. These cold cathodes have the advantage of being capable of supplying several kiloamperes of current spread over a large cross section without the need for auxiliary components such as a heater supply. However, they suffer from many known problems such as nonuniform emission that can result in small areas of high current density on the anode and, thus, excessive anode heating. As a consequence, outgassing and vaporization of bulk material frequently leads to premature impedance collapse. Hence, minimizing nonuniform anode heating due to beam nonuniformity is paramount. As previously demonstrated, the use of a CsI-coated carbon velvet cathode improved beam uniformity, reduced outgassing, and mitigated early impedance collapse. To quantify the uniformity, temporal and spatially resolved images of the cathode plasma were taken for a CsI-coated carbon fiber cathode, operated at an average current density of ~150 A/cm2 under various conditions, i.e., without a field shaping ring, before and after discharge cleaning, and with a field shaping ring. All cathodes were operated in a sealed tube with a small integrated sputter ion pump to restore vacuum levels to 10-9 torr levels between subsequent shots.

Journals

Authors: A. Fierro; G. Laity; A. Neuber; L. Hatfield

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6383314

Abstract: High energy photons released during the early stages of pulsed surface flashover are believed to be a contributing factor to the overall plasma formation. It is clear that only radiation with wavelength in the Ultraviolet (UV) and Vacuum Ultraviolet (VUV) regimes is capable of ionization and excitation processes of atmospheric gases, thus potentially providing seed electrons for secondary electron avalanches. To investigate the role of VUV photons in the breakdown process, an experiment was designed enabling detection of photons with energies greater than 7 eV with high temporal and spatial resolution. A transient low temperature plasma is generated on an MgF2 surface between two point-to-point stainless steel electrodes using a 200 ns rise-time high voltage pulser. Light emitted during the plasma formation is transmitted through the VUV transmissive MgF2 window into vacuum avoiding the strong absorption of VUV radiation in the atmosphere. High resolution measurements of the flashover event are acquired using a VUV-sensitive ICCD camera and photomultiplier tube (PMT) mounted to the exit ports of the VUV spectrograph. Detailed analysis of the two electrode regions clearly shows the presence of VUV emission lines prior to voltage collapse in atmospheric gases. Although a symmetric field geometry was chosen, it was seen in all cases that initial VUV emission originates from the anode. Furthermore, measurements of the 120 nm nitrogen ground transition taken at locations away from the anode demonstrated peak emission that coincided with streamer head locations. The observed behavior is consistent with a build-up of positive ion space charge in the gap attracting electrons from the surrounding medium causing excitations and ionizations in this high field region. Further time-resolved spectroscopy of the developing plasma in a high purity nitrogen environment captured the presence of the second positive system from molecular nitrogen as well as atomic emission lines. It was seen that the lifetime of the second positive system is much shorter (nanoseconds) than that of the atomic nitrogen emission lines (microseconds). The experimentally observed behavior will be discussed is it relates to the transient formation of a low temperature plasma along a dielectric surface.

Conferences

Authors: C. Hettler; W. W. Sullivan; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6518682

Abstract: A 50 kV silicon carbide photoconductive semiconductor switch (PCSS) is presented. The SiC PCSS device is fabricated from semi-insulating 4H-SiC in a newly-proposed rear-illuminated, radial switch structure. The improved structure reduces the peak electric field within the switch, extending the blocking voltage to over 50 kVdc. Electrostatic field simulations of the PCSS are presented along with experimental blocking curves. The PCSS demonstrated low on-state resistance, delivering over 27 MW of peak power into a 31 Ω load. Device modeling was performed to further optimize the switch for peak efficiency when illuminated with 355 nm light, a common laser wavelength. The switch structure was modified for peak operation at 355 nm and the experimental and theoretical results are compared.

Conference Paper/Presentation

Authors: J.-W. Braxton Bragg; Christopher Simmons; James C. Dickens; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6518721

Abstract: Nonlinear transmission lines (NLTLs) utilizing ferrimagnetic materials for microwave generation have been realized as a possible solid-state replacement to traditional high power microwave (HPM) sources. The nonlinearities present in the material, along with interaction between pulsed, azimuthal magnetic fields and static, axial-biasing magnetic fields provide microwave (mesoband) generation with peak powers exceeding 30 MW at 2-4 GHz center frequency with 25 kV incident pulse magnitude. Additionally, an incident pulse of several nanoseconds is sharpened to hundreds of picoseconds. This study focuses on a serial arrangement of two NLTLs with 5 ns electrical length separation. Tests with 25 kV incident voltage are performed with varying bias schemes for each NLTL structure. The lines are terminated into a 50 Ω matched load. Measurements taken before and after each NLTL provide insight to the behavior of the traveling pulse. Results regarding peak power, frequency of operation, and system delay are discussed.

Conferences

Authors: J. C. Stephens; A. A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6781418

Abstract: An exploding wire model that accounts for the electric field enhanced conductivity of dense metal plasma is applied to simulate an exploding wire opening switch. In contrast to many z-pinch experiments, operated in vacuum, the experiments here discuss wires vaporized in a high pressure gas environment. In addition to this, these experiments are primarily concerned with sub-eV temperatures, with a specific emphasis on the liquid-vapor phase transition, where significant decreases in conductivity provide the opening switch behavior. It is common that fuses operating within this regime are analyzed using 0-dimensional models, where the resistance is taken to be an experimentally determined function of energy or action. A more accurate 1-dimensional model with added field enhanced conductivity has been developed to better model the fuse dynamics throughout a significantly larger parameter range. The model applies the LANL SESAME database for the equation-of-state, and the conductivity data developed with the Lee-More-Desjarlais (LMD) algorithm. Using conductivity based on conditions of thermal equilibrium accurately predicts fuse opening as well as current re-emergence after a few microseconds dwell time for the case of small electric fields, however, this simple approach fails to capture early fuse restrike if the differential voltage across the wire becomes too large (~few kV/cm for the investigated conditions). It is demonstrated that adding an electric field driven conductivity term to the model will accurately capture the fuse dynamics for the low field as well as the high field case.

IEEE Conferences

Authors: Andrew Fierro; George Laity; Andreas Neuber; Lynn Hatfield; James Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6518677

Abstract: The development of streamers during the initial stage of a pulsed atmospheric discharge, which carries a high content of vacuum UV (VUV) emission, is investigated. Due to the high spectral absorptivity of atmospheric air in the VUV regime, few experiments have been conducted that observe the wavelength range shorter than 180 nm. However, direct photoionization is believed to play an important role in streamer formation during this phase of breakdown. VUV radiation (hν > 7 eV) is energetic enough to promote step-ionization and directly ionize background gas mixtures. Utilizing a VUV-sensitive experimental apparatus, spatially-resolved photomultiplier tube (PMT) measurements were recorded showing that initial VUV emission is dependent on the inhomogeneous field distribution near the electrodes. It was revealed that further into the developing streamer the position of instantaneous VUV emission is propagating from anode to cathode away from the initial VUV emissions. From these observations, the low temperature plasma is capable of producing high energy photons during the streamer phase as well as after voltage collapse.

Conferences

Authors: A. Neuber; J. Stephens; C. Lynn; J. Walter; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6383930

Abstract: Summary form only given. The challenges in developing and designing a 5 GW stand-alone pulsed power generator for driving a >; 100 MW high power microwave, HPM, source are discussed. Both, applied physics aspects of operating principles and limits of the individual sub-systems will be addressed. The energy, initially provided through a set of lithium-ion batteries internal to the generator, is boosted by an explosively driven flux compressor, FC, and inductively stored at a several kJ level. While the energy is stored on a microsecond timescale, it is, however, released into the load on a nanosecond timescale via an electro-explosive fuse, EEF, thus delivering GWs of electrical power to the HPM source for a duration of about 100 ns.

IEEE Conferences

Authors: Jacob Stephens; William Mischke; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6189083

Abstract: The environment surrounding an exploding wire is known be a controlling factor in electro-explosive fuse performance. Recent experiments have shown that the application of an insulating surface coating to the fuse wire can significantly increase the rate of impedance transition and impedance magnitude of the exploding wire. This paper discusses the performance of surface coated fuses tested in commonly used solid and gaseous media. For comparison, these experiments are compared to bare wire fuse experiments in identical environments. Previously developed exploding wire models are utilized to aid in the interpretation of the experimental fuse behavior. Differential wire voltage, voltage pulse length, and degree of post vaporization conduction (i.e., restrike) are discussed for each experiment.

Journals

Authors: P. Ford; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6191415

Abstract: A Finite Difference (FD) algorithm was developed to calculate the formative delay time between the application of an RF field to a dielectric surface and the formation of a field-induced plasma interrupting the RF power flow. The analysis is focused on the surface being exposed to a background gas pressure above 50 torr. The FD-algorithm is chosen over particle-in-cell methods due to its higher computational speed and its ease of being ported to commercial electromagnetics solvers. The dynamic frequency-dependent permittivity of the plasma is mapped to the time domain of the FD algorithm using the Z transform. Therefore, together with the electron density, the effect of the developing plasma on the instantaneous microwave field is calculated. The high observed value of absorption, up to 60 %, is a result of the momentum transfer collision frequencies in the developing plasma being much larger than the microwave frequency. As a result, the electron density increases to values well beyond the density calculated from setting a plasma frequency equal to the microwave frequency. In the experiment, flashover is induced across a Lucite window by a 4 MW S-band magnetron operating at 2.85 GHz with ~50 ns rise time. The results of the FD simulation are compared with experimental data obtained from flashover with background gases such as nitrogen, air, and argon all at pressures exceeding 50 Torr.

Conferences

Authors: A. Fierro; G. Laity; L. Hatfield; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6191414

Abstract: Vacuum Ultraviolet (VUV) radiation is commonly thought to enhance streamer formation, as it is energetic enough to cause photoionization in the gaseous volume. Light with wavelengths below 180 nm, i.e. VUV, is highly absorbed in the atmosphere which increases the difficulty of measuring any VUV emission from gaseous breakdown at atmospheric pressure. Nevertheless, VUV emission from pulsed surface flashover at atmospheric conditions was previously recorded at Texas Tech. A second generation system was designed to image VUV and visible emission directly while also preserving the spatial profile. The visible emission is imaged through an air-side focused ICCD, while VUV emission is imaged through a vacuum spectrograph. The variable length gap was excited with a pulser designed for a 100 ns rise time and 50 kV peak output. Captured images of visible light emission from streamers produced in oxygen are diffuse whereas nitrogen produces streamers that are segmented. VUV spatial images taken in oxygen reveal stronger emission closer to the cathode region, while nitrogen produces a more distributed intensity profile across the gap. While MgF2 enabled transmission and measurement of VUV, streamer characteristics recorded in the visible light spectrum of surface flashover on BK7 dielectric windows were also investigated. In this paper, the observed streamer images in both visible and VUV wavelength range will be discussed as it relates to surface flashover at atmospheric pressure.

IEEE Conferences

Authors: Mohamed A. Elsayed; Andreas A. Neuber; John W. Walter; Andrew J. Young; Charlie S. Anderson; Shad L. Holt; James Dickens; Magne Kristiansen; Larry L. Altgilbers

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6191480

Abstract: Continued efforts at the Center for Pulsed Power and Power Electronics at Texas Tech University have led to improvements to the design, testing, and performance of a high power microwave (HPM) system, which is sourced by Lithium-ion Polymer batteries, a polypropylene capacitor, and high energetics. An indirectly seeded two-stage helical flux compression generator (HFCG) produces electrical energies in the kilo-Joule regime into a low impedance inductive load, varying from 2 μH to 3 μH. This high current output of the explosively driven generator is conditioned with a pure silver-wire-based electro-explosive opening switch, which reaches voltage levels in excess of 300 kV into a 18 Ohm load. Upon reaching levels high enough to close an integrated peaking switch, this high voltage is sufficient to drive a reflex triode virtual cathode oscillator, also known as a Vircator, into radiation. The Vircator employed in the system has reached microwave radiation levels well over 100 Megawatts from a cavity volume of less than 5 Liters at a microwave frequency of a few GHz. The complete system is governed through a microcontroller that regulates seed and detonator charging levels as well as discharge times using built-in feedback diagnostics. The complete system aforementioned is constrained to 15 centimeter diameter and measures 183 centimeter in length with an overall volume of less than 34 Liters. No external power or vacuum pumping for the HPM tube is required. This report will discuss recent design advancements and improvements on the HPM system and its sub-components that include the compact seed source, HFCG, and the power conditioning system. Improved safety features implemented as well as novel diagnostic integration will be discussed as well.

Conferences

Authors: J. C. Stephens; A. A. Neuber; J. C. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6191477

Abstract: This paper presents the optimization of a compact electro-explosive fuse designed for the power conditioning system to be driven by a helical flux compression generator (HFCG). An electro-explosive fuse interrupts the current flow from the HFCG through a storage inductor on a 50 to 100 ns timescale inducing a voltage large enough to close a peaking gap, which commutates the energy in the storage inductor into a 20 Ohm load at voltage levels above 200 kV. Experimental data has revealed that electro-explosive fuses with wires in closer proximity to one another have consistently produced lower pulsed voltages than fuses with larger wire spacings. This paper addresses possible factors that might contribute to this drop in performance. An electro-magnetic field solver is used to model the current redistribution in the fuse wires. The experimentally observed performance of compact fuses with varying wire spacings is presented.

IEEE Conferences

Authors: J. Foster; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6191487

Abstract: Breakdown phenomena in a high power microwave (HPM) system present unique obstacles for the further development of HPM technology. The non-uniformity of a high frequency electric field and the statistics associated with breakdown in general along with the stochastic nature of naturally occurring electron generating mechanisms introduce significant challenges for predicting and preventing breakdown occurrences within a HPM system. An experiment consisting of an S-band multi-megawatt HPM pulse is used for observing an alternating field induced plasma sheath across a dielectric surface. In order to minimize experimental deviations, a continuous UV lamp is used to provide a constant source of initiatory electrons through the process of photoemission. This reduces the waiting time for flashover initiating electrons to appear, however, primarily due to avalanche statistics, variations are still observed. A statistical model that uses an exponential distribution sampling procedure was developed to predict the surface flashover delay times for a variety of conditions. A supporting experiment that uses a continuous UV lamp and a DC electric field is used for measuring low current due to photoemission from the dielectric window. An explanation of the model describing these phenomena is presented along with a comparison of current measurements from the supporting experiment.

Conferences

Authors: A. Young; A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6191479

Abstract: An investigation aimed at optimizing the integration between a capacitor based prime power source and flux-trapping helical flux compression generator (FT-HFCG) is presented. An FT-HFCG simulation code, previously benchmarked with single and multi-pitch generators, was employed to study the kilo-joule class explosive system for this purpose. The details of this effort, which include the optimization of the field coil and stator coupling, as well as an examination of the effect of field coil parameters on the system performance, will be described in this document. For the simulated parameter space, the choice of field coil configuration caused the system energy gain to vary by 300%, and the optimum field coil configuration was found to be a single Litz wire conductor that had an axial length which was approximately 60% of the stator axial length.

Conferences

Authors: J. Parsony; J. Dickens; A. Neuber; J. Walter; J. Krile; J. Vara

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6191565

Abstract: This study focuses on the design of an experimental apparatus for optical and vacuum ultraviolet (VUV) spectroscopy of explosive emission processes at the cathode in a high power vacuum sealed tube. The system consists of a high power pulsed Nd:YAG laser with a highly tunable optical parametric oscillator (OPO, 200 to 2600 nm), a 300 kV, 80 J Marx Generator, and a triode-geometry vircator that is 6 inches in diameter and 11 inches in length. It has been observed that the explosive emission occurring at the cathode forms a plasma front propagating across the A-K gap negatively affecting the impedance of the gap thus shortening pulse length and spoiling the desired consistent low vacuum within the sealed tube. The scope of this project is to probe the plasma formed between the A-K gap to determine the species emitted off the cathode. The timing integration of each sub-system is very critical as the window for measurement is approximately 200 ns. A compact, fiber optically coupled, battery-operated, low jitter (500 ps), fast risetime (20 ns) pulse trigger generator has been designed and incorporated as the trigger source in a trigatron triggering scheme for the Marx generator. Preliminary jitter measurements of 20 to 30 ns have been seen on the fully erected Marx Generator. The intent of this paper is to discuss the details of the various sub-systems and the timing between them, enabling optical / VUV spectroscopic measurements of the explosive emission process.

IEEE Conferences

Authors: C. Lynnx; J. Walter; A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6191570

Abstract: Vacuum diodes utilizing explosive emission cathodes generally suffer from non-uniform emission, gap closure by anode and cathode plasma expansion, and outgassing. Also, the Child-Langmuir relation does not apply at the edge of the emission area. This results in a high current density sheath at the edge of the emission area. Each of these phenomena presents its own technical challenge in HPM source design and optimization. Diagnostic techniques and particle in cell simulations for a vacuum diode, to be operated as a greater than 100 MW class vircator, have been used in order to compare the characteristics of the electron beam produced by various cathode materials and geometries. Uniform current density plays a key role in vircator performance and efficiency, as well as in the lifetime of the diode. The diode under investigation has an emission area of 20.3 cm2, and operates at a current density on the order of 300 A/cm2 at 200 kV. The background vacuum level and the associated adsorbed and absorbed gases at the surface play a major role in the behavior of an explosive emission cathode. This vircator source is operated as a sealed tube, requiring no vacuum pumping until the device is repetitively operated. The small amount of gas generated during operation is pumped down in between shots, if needed, utilizing an integrated small sputter-ion pump. The background pressure in the diode is in the ultra-high vacuum (UHV) range, on the order of 10−9 Torr to 10−8 Torr. Particle in cell simulations were performed to investigate current density distributions at the surface of both the anode and cathode due to various field shaping profiles. Additionally, scintillator images were taken to compare the beam profile of a machined aluminum cathode with a CsI coated carbon fiber cathode.

Conferences

Authors: S. R. Beeson; P. J. Ford; J. Foster; H. G. Krompholz; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5772013

Abstract: Open-shutter and intensified charge-coupled device images of high-power microwave breakdown were taken in an effort to characterize the pressure dependence of plasma development. These images were taken with a Nikon D200 and Andor iStar DH734-25U-03, respectively. With the pressures increasing from 200 mtorr to 155 torr, the plasma changes from a diffuse discharge encompassing a large volume to a multichannel structure following the electric field lines.

IEEE Journals

Authors: D. Bolyard; A. Neuber; J. Krile; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6191538

Abstract: A linear transformer driver (LTD) is being constructed at Texas Tech University's Center for Pulsed Power and Power Electronics with the goal to achieve roughly 100 ns pulse width into an 18 Ohm load and energy densities similar to or greater than a previously designed and built 500 J compact Marx generator. A single experimental LTD stage, previously constructed, tested, and reported on by TTU, has undergone several improvements as well as the inclusion of additional diagnostics. Testing at a charging voltage of 14 kV into a 1.7 Ω resistive load has experimentally achieved a 10% to 90% risetime of ~60 ns, peak voltage of 9.2 kV, peak current of 5.5 kA, and instantaneous power of 50.6 MW. Simulations of the LTD stage correlate closely with the experimental results. Extrapolating the simulations to the final desired charging voltage indicate peak voltages and peak currents exceeding 15 kV and 10 kA respectively, with instantaneous power greater than 150 MW from a single LTD stage. The design for a multi-stage LTD using magnetic cores with a flux swing of 6 kV-μs will be presented. Improvements in diagnostics and modeling of the LTD will also be discussed as well.

Conferences

Authors: J. Walter; J. Vara; C. Lynn; J. Dickens; A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6191517

Abstract: During the development and optimization of a compact sealed tube virtual cathode oscillator (vircator) at Texas Tech University, it has become apparent that processes at the anode have a significant impact on tube performance. The impact of the high energy, high current density (100-200 A/cm2 or higher) beam on the anode will cause outgassing, plasma production, and anode melting and material ejection. The emitted material expands, eventually impacting the anode transparency and (combined with the plasma formed at the cathode) shorting out the anode-cathode gap. This expansion limits the maximum radiated pulse width, and can also limit the peak output power. The residual evolved gas also negatively impacts the maximum repetition rate of the tube. An effort is underway to study the thermal behavior, gases evolved, and transparency versus time for different vircator anode materials and material treatments. Several different anode materials are under investigation, including stainless steel, copper tungsten, tantalum, nickel, and molybdenum. The effect of different treatments on the anodes before tube assembly is also being studied. The gases that are evolved during operation have been characterized utilizing pressure and residual gas analyzer measurements. The pre-shot background pressure in the tube is in the ultra-high vacuum range (10-8 to 10-9 Torr), and the vircator is not pumped on during firing. The data collected for the different materials is presented.

IEEE Conferences

Authors: S. Beeson; J. Foster; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6191488

Abstract: When dealing with the propagation of High Power Microwaves (HPM), special precautions must be used to prevent the onset of plasma generation. In this paper, we investigate the plasma located on the high pressure side of the dielectric boundary separating the vacuum environment of the microwave source from the high pressure environment of the transmitting medium, e.g., atmosphere. Because the collisional ionization rates are a monotonously increasing function of Eeff/p in the range of interest, the effective electric field normalized with pressure, implementation of HPM in high altitude (low pressure) environments are subject to dielectric breakdown due to this generated plasma, more than at sea-level altitudes. Dielectric breakdown causes the interruption in transmission of electromagnetic radiation due to the reflection and absorption properties of the plasma generated on the dielectric surface. In this paper, transmission, reflection, and absorption data is presented for plasma generated under various pressures ranging from 5 to 155 torr in N2 and air environments. In addition, seed electrons from UV illumination of the dielectric surface and physical vapor deposited metallic points are implemented and their implications to the overall transmission properties are discussed.

Conferences

Authors: G. Laity; A. Neuber; A. Fierro; J. Dickens; L. Hatfield

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6191416

Abstract: This paper describes a 2nd-generation system for directly studying the emission of vacuum ultraviolet (VUV) light from pulsed dielectric surface flashover at atmospheric pressure. The role of self-produced VUV emission (i.e. energies greater than 7 eV) on photo-ionization processes during the early nanoseconds of pulsed discharges is virtually unexplored, and yet could be a significant factor in the physics of fast breakdown of directed energy systems (such as MW-class high power microwave devices) in the aerospace community. First generation experiments at Texas Tech University have shown that VUV emission corresponding to nitrogen and oxygen excitation in the energy range 8-10 eV is easily produced, but the use of MgF2 optics inhibited future work with existing hardware due to the transmission cutoff of this dielectric material and chromatic aberration if used as a lensing medium. In an effort to enhance the detection capabilities of our hardware in the wavelength range from 115-135 nm, the current system utilizes a custom designed set of off-axis parabolic MgF2-Aluminium coated mirrors as the primary focusing element. High resolution spectroscopy with the upgraded system resulted in the observation of the nitrogen doublet at 149.5 nm, leading to a better fit for the appropriate line broadening parameters for an approximate 10 eV Boltzmann electronic temperature. Evidence of self-absorption for HI (121.5 nm) provides new insight into the generation of space charge in these plasma structures, which has been investigated quantitatively in both SF6-H2 and N2-H2 mixtures.

IEEE Conferences

Authors: G. Laity; A. Neuber; A. Fierro; J. Dickens; L. Hatfield

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5976080

Abstract: There is a growing demand for understanding the physics of surface flashover, as it relates to the breakdown of electric fields on high power systems in the aerospace community. Specifically, the quantitative role of vacuum ultraviolet (VUV) radiation which is self-produced during the initial nanoseconds of surface flashover is virtually unknown. An experiment was constructed which allows detailed electrical and optical measurements of VUV emission during the timescales in which streamers are propagating before the transition into spark discharge. Repeated surface flashover events are generated using a solid-state high voltage pulser, with breakdown recorded in a number of gases at atmospheric pressure. Streamers are photographed using fast optical imaging with 3 ns resolution. Fast voltage and current diagnostics revealed a number of distinct stages of streamer development ranging from the onset of cathode directed streamers to the sharp current rise during final voltage collapse. The emission of VUV radiation is discussed in context to the observed streamer and electrical characteristics.

IEEE Journals

Authors: G. Laity; A. Fierro; A. Neuber; L. Hatfield; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5993029

Abstract: Summary form only given. The role of self-produced VUV emission (i.e. energies greater than 7 eV) on photo-ionization processes during the early nanoseconds of pulsed discharges is a new area of interest in the aerospace community, and could play a significant role in the theoretical understanding of plasma generation at short (nanosecond) timescales. Our previously reported experiments have shown VUV emission from atomic species of oxygen and nitrogen, which were excited during breakdown, but detailed analysis of emission in the range 115 - 135 nm was difficult due to chromatic aberration of VUV transparent optics in this regime. These limitations were recently alleviated by fitting the spectral apparatus with a custom set of MgF2-Aluminum coated off-axis parabolic mirrors used in conjunction with a high resolution vacuum monochromator. VUV emission is observed by either VUV sensitive intensified CCD or photomultiplier sensors, with additional current/voltage monitors and externally focused fast-frame (nanosecond capable) imaging complimenting the diagnostic setup. High resolution spectroscopy has been achieved in the excitation range of interest (8 - 10 eV), where species of atmospheric gases and electrode metal have been identified during the early nanoseconds of plasma generation. Temporal studies have shown that most VUV emission occurs during the time before voltage collapse and subsequent power flow, while the majority of visible emission is released after breakdown when the electron energy distribution has shifted to lower energies. This experiment also allowed for direct emission imaging of VUV radiation, where the spatial profile relative to plasma position is still intact. While the observed metal species are only emitted in the regions close to the electrodes as expected, significant differences were observed for species of NI (which is released throughout the plasma volume) and OI (which is released strongly in regions of high electric field) atoms during breakdown. This is most likely due to the accumulation of ionic space charge from photo-ionization in the gas volume, coupled with the high absorption cross section of molecular oxygen in the VUV range. These measurements will be discussed in context to current physics models of electric breakdown and plasma generation at atmospheric pressure.

IEEE Conferences

Authors: J. Foster; S. Beeson; M. Thomas; J. Krile; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5976082

Abstract: High power microwave (HPM) dielectric surface flashover can be rapidly induced by providing breakdown initiating electrons in the high field region. An experimental setup utilizing a 2.85 GHz HPM source to produce a 4.5 MW, 3 μs pulse is used for studying HPM surface flashover in various atmospheric conditions. If flashover is to occur rapidly in an HPM system, it is desirable to provide a readily available source of electrons while keeping insertion loss at a minimum. The experimental results presented in this paper utilize a continuous UV source (up to 0.3 mW/cm2) to provide photo-emitted seed electrons from the dielectric surface. Similarly, electrons were provided through the process of field emission by using metallic points deposited on the surface. Initial experiments utilizing 0.2 mm2 aluminum points with a spatial density of 25/cm2 have increased the apparent effective electric field by a factor of ~1.5 while keeping the insertion loss low (<;0.01 dB). The field enhancements have sharply reduced the delay time for surface flashover. For an environment consisting of air at 2.07×104 Pa (155 Torr), for instance, the delay time is reduced from 455 ns to 101 ns. Two radioactive sources were also used in an attempt to provide seed electrons in the high field regions. Presented in this paper is a comparison of various field-enhancing geometries and how they relate to flashover development along with an analysis of time resolved imaging and an explanation of experimental results with radioactive materials.

Journals

Authors: G. R. Laity; A. S. Fierro; L. L. Hatfield; J. C. Dickens; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5776698

Abstract: The quantitative role of self-produced vacuum-ultraviolet (VUV) light on photoionization-dominated gas discharges is currently an area of interest in the aerospace community. In this paper, we present the images of the VUV spectroscopic analysis of a pulsed atmospheric flashover, where the spatial content of emission relative to electrode geometry has been preserved. The observed spatial profile of emission is dependent on radiating species in the range of 120-125 nm and is discussed in relation to the physics of nanosecond discharges.

IEEE Journals

Authors: A. A. Neuber; B. Novac

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5976078

Abstract: It is our pleasure to present to the professional reader this "third" Special Issue on Power Modulators and Repetitive Pulsed Power of the IEEE Transactions on Dielectrics and Electrical Insulation (previous issues were published in 2007 and 2009), and the fifth such Special Issue published by an IEEE Transactions (1991-IEEE TED; 2005-IEEE TPS). Most of the papers that appear in this issue are based on contributions to the 2010 International Power Modulator and High Voltage Conference Paper/Presentation, which was held in Atlanta, GA, from May 23rd to 27th, 2011. Others heeded the "Call for Papers" that attracted many contributions in the general field of interest to this Special Issue.

Journal

Authors: J. Foster; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5993097

Abstract: Summary form only given. The development of high power microwave (HPM) systems and technologies has been inhibited by breakdown phenomena which limit their transmitting capabilities. For a system in which there is no obvious source of breakdown initiating electrons, HPM breakdown can be even less predictable. A system in which microwaves are generated in a vacuum environment for the purpose of radiating into atmosphere typically uses a dielectric window to separate the vacuum and atmospheric sides of the system. At sufficient field levels, surface flashover can occur across this dielectric window resulting in a severe drop in transmitted power. The time between the application of the HPM and the sharp drop in transmitted power is described as the delay time, which consists of a statistical waiting time for initiatory electrons combined with an electron amplification time, or formative time. The experimental setup for this project consists of a 4 MW HPM source operating a 2.85 GHz attached to a traveling wave structure and a dielectric window mounted on the output side of the system. Dielectric surface flashover has been observed in air and nitrogen with pressures ranging from 60 to 155 torr. To provide a constant source of seed electrons, a UV lamp is used to illuminate the window resulting in photo-emitted electrons appearing at the surface. Another way to provide seed electrons is the inclusion of metallic points on the window which provide a source of field emitted electrons, and also results in a field enhancement at the dielectric surface. In the absence of a constant source of seed electrons, it is expected that field detachment from ion clusters is the primary mechanism for providing the high field region with flashover initiating electrons. A statistical model has been developed for predicting surface flashover that takes into account relevant parameters such as field level, ionization rate, gas type, and pressure. This model has shown good agreement with experimental data in nitrogen with UV illumination providing a constant electron seed rate. Presented here is an adaptation of this statistical model to an environment consisting of field enhancing metallic points as well as a comparison of results for UV illumination and stochastic seeding through field detachment from ion clusters.

Conferences

Authors: J. -. Braggy; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6191411

Abstract: Ferrite loaded nonlinear transmission lines (NLTLs) are able to act as high power microwave sources, utilizing the nonlinearities present in ferrimagnetic materials and the excitation of damped gyromagnetic precession at high incident power levels. Ferrimagnetic properties depend greatly on operating temperatures; therefore, there exists a need to know the ideal temperature at which to operate ferrite loaded NLTLs. Ferrites are chilled or heated to a certain temperature for a time suitable to allow internal ferrite temperature uniformity. Experimental temperatures ranged from approximately -20 °C up to 150 °C, which is slightly above the Curie temperature of the loaded ferrites. This temperature range allows observation of precession dependence on temperature while maintaining ferrimagnetic properties and a single look at the behavior outside the ferrimagnetic regime. Above the Curie temperature the loaded ferrites become paramagnetic and lose ferrimagnetic properties. The design, testing, and results are detailed for an NLTL measuring 0.3 m length and ferrite inner and outer diameters of 3 mm and 6 mm respectively. Figures comparing output waveforms at different temperatures, output power versus temperature, and output frequency versus temperature are shown.

IEEE Conferences

Authors: C. S. Anderson; A. A. Neuber; A. J. Young; J. T. Krile; M. A. Elsayed; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6191476

Abstract: Helical Flux Compression Generators, HFCGs, are powerful high current sources for pulsed power applications. Due to the single shot nature of HFCGs, electrical output reproducibility is of great importance. One factor known to contribute to unpredictable performance is mechanical inconsistencies introduced during manufacturing of the stator. In an attempt to minimize these deviations during productions, two different winding forms for stator coils, designed to ensure repeatable generator dimensions, turn and coil pitch, were investigated. The differences between the methods were quantified by comparison of measurements made of the physical parameters of the coil (i.e. radius, inductance, etc.), as well as analysis of experiments conducted with the HFCGs fired into a 3 μH load inductor. With any particular fabrication method, the stator insulation material has a distinct impact on generator operation. Quad-built Polyimide coated magnet wire as stator insulation material and Teflon Fluorinated Ethylene Propylene (FEP) as field coil insulation material were investigate to improve HFCG performance. Insulation testing was carried out by firing HFCGs into the inductive load mentioned above. Experimental data and analysis, as well as conclusions on insulation material, will be presented along with a brief discussion of the optimum fabrication method.

IEEE Conferences

Authors: C. Lynn; A. Neuber; E. Matthews; J. Walter; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5958383

Abstract: A low impedance Marx generator was developed as part of a test bed for vacuum diodes of various electrode materials and geometries. The generator supplies sufficient energy to initiate and sustain the typically unwanted plasma formation within the diode; which facilitates the observation of the plasma, current uniformity, and electron current densities of various diode structures. The generator consists of ten stages; each stage utilizes a 220 nF 50 kV capacitor, with a series inductance of ~20 nH. When charged to the rated voltage of the capacitors the energy density of the complete generator with case, spark gaps, insulation, etc., is 19.2 mJ/cm3; this is roughly the energy density of a typical ceramic doorknob capacitor without any supporting structure or isolation. The energy density of the capacitors utilized in the Marx generator by themselves is 104 mJ/cm3. Fired into a low inductance short, the ringing frequency was measured to be 1.4 MHz resulting in an output impedance of 5.2 Ω. Erection of the Marx required adding forward feeding capacitors as the stray capacitance to ground is smaller than the capacitance (~60 pF) of the low inductance, low profile spark gap switches. The design and construction of this generator are discussed as well as selected experimental results obtained with the generator.

Conferences

Authors: J. Foster; M. Thomas; J. Krile; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5534225

Abstract: The production of high power microwaves (HPM) in a vacuum environment for the purpose of radiating into atmosphere requires the use of a dielectric interface to separate the vacuum and atmospheric sides of the radiating structure. For high power to pressure ratios the interface will exhibit surface flashover on the atmospheric side, thus limiting the transmission of microwave power. An experimental setup that utilizes a magnetron operating at 2.85 GHz to produce a 4.5 MW, 3μs pulse propagating in the TE10 mode along with an atmospheric test chamber enables investigating HPM surface flashover phenomena in the presence of various atmospheric conditions. One of the principle parameters measured is the delay time between application of the microwave pulse (50 ns rise time) and the sharp drop in transmitted power due to the flashover plasma formation. Several methods of delay time reduction have been employed to gain a better understanding of the source of breakdown initiatory electrons. For an environment composed of air at, for instance, 155 torr a delay time of 600 ns is observed. Illuminating the dielectric surface with continuous UV radiation reduces the average delay to about 380 ns. An even more distinct reduction in delay time was observed when electric field enhancement was introduced to the window surface via vapor deposition of sub-mm metallic points on the dielectric. These metallic points have proven to reduce the delay time to ~150 ns while increasing the global effective electric field by a factor of ~1.5. This presentation will include an overview of a variety of methods for investigating flashover initiation, including UV radiation and the application of an external DC electric field, as well as the introduction of field enhancing metallic points on the dielectric surface. An analysis of flashover behavior at atmospheric pressures (60-155 torr) in air, argon, and nitrogen will also be given along with an estimation of field enhancement factors for various geometries.

IEEE Conferences

Authors: G. Rogers; A. Neuber; L. Hatfield; G. Laity; K. Frank; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5958293

Abstract: In an attempt to identify the mechanisms leading to pulsed dielectric surface flashover in atmospheric conditions, surface flashover across a magnesium fluoride (MgF2) window was studied. The electrode configuration and the applied pulsed voltage level were chosen such that the generated electric field was symmetric with respect to the centerline between the electrodes. That is, neither electrode was favored with respect to flashover/breakdown initiation. A semiconductor-switched 32 kV pulse with 140 ns rise time was applied to the 8 mm wide flashover gap in air, nitrogen, and oxygen at atmospheric pressure. Fast voltage and current measurements along with nanosecond imaging revealed four stages of flashover development: (1) Onset of a cathode directed streamer with a charge on the order of 100 pC and traveling with a speed of ~1 mm/ns at a macroscopic field level of ~10 kV/cm associated with a slow current rise (on the order of 10-3 A/ns) temporarily augmented by (2) a 5 ns wide current spike at the moment when the streamer reaches the cathode followed by (3) a cathode directed streamer focused toward the center of the flashover gap with a slow rising current leading to (4) a sharp current rise (on the order of 10 A/ns) reaching roughly a circuit limited 45 A about 20 ns after the return strike meets an anode directed streamer. Although present in all tested gases, the current spike at the end of stage (2) is most different for all three gases and having the greatest impact in air.

IEEE Conferences

Authors: A. Young; A. Neuber; M. Elsayed; J. Korn; J. Walter; S. Holt; J. Dickens; M. Kristiansen; L. L. Altgilbers

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5958309

Abstract: Recent progress in the development of a compact, portable, explosively-driven high power microwave source is presented. The envelope to which the system must fit has a 15 cm diameter, which means each sub-system fits within this dimension, with an optimized overall length. The system includes an autonomous prime energy source, which provides the initial energy for a two-stage, flux-trapping helical flux compression generator (FCG). Typical output from the FCG is several kilojoules into a 3 μH inductor. The amplified energy from the generator, after pulse conditioning, is used to drive a virtual cathode oscillator (vircator). Recorded voltages at the vircator with this arrangement were greater than 200 kV in experiments, where radiated output powers of greater than 100 MW have been measured. Voltages of at least 300 kV, with an electrical output power of 4 GW or greater, were generated by the FCG driven pulsed power source into a water resistor load with an impedance similar to the operating impedance of the vircator. A description of each component of the compact microwave source will be given, along with waveforms from tests performed with the components independent of the rest of the system. Data from experiments with the fully integrated microwave system will be shown, and analysis will be offered to detail the performance of the system in its present state.

IEEE Conferences

Authors: E. J. Matthews; M. Kristiansen; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5395624

Abstract: An evaluation of the energy density and efficiency of high-voltage capacitors, from various manufacturers, at voltages above their rated level is presented. Characteristics such as decreasing capacitance, decreasing efficiency, and increasing energy density are described. Data are taken from eight capacitors; six of which are composed of varying nonlinear ceramic materials as a dielectric, and the remaining two are wound capacitors, which were chosen to exhibit their linear characteristics. Rapid (1-100 ms) charging and discharging, similar to that of a generator operation at a repetition rate of more than 10 Hz with an ~100-ns pulse, were the conditions simulated in this particular test setup.

IEEE Journals

Authors: C. Hettler; C. James; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5958289

Abstract: A contactless microwave photoconductivity decay (MPCD) method has been used to measure recombination lifetime and relative conductivity of semi-insulating (SI) silicon carbide (SiC) wafers. A pulsed laser, tunable from 210 nm to 2 μm, has been used to probe above and below band gap photoconductive responses of four SI SiC wafers. The carrier lifetimes were calculated by comparing the reflected microwave signal to the photo response of a fast (<; 300 ps) photodiode. Three vanadium-doped 6H-SiC wafers, with bulk resistivities ranging from 105 Ω-cm to 1011 Ω-cm, and one high purity semi-insulating (HPSI) 4H-SiC wafer (>; 109 Ω-cm) were studied. The photoconductive response of each wafer set is presented. The HPSI wafer demonstrated longer carrier lifetime and improved above band gap photoconductivity compared to the vanadium-doped wafers. The difference in carrier lifetimes are attributed to higher densities of recombination centers (vanadium acceptors) in the 6H-SiC substrates.

IEEE Conferences

Authors: David Matia; Hermann Krompholz; Travis Vollmer; Andreas Neuber; Michael Giesselmann; Magne Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5958372

Abstract: A detailed characterization of a 50 J linear transformer driver (LTD) stage is presented. The specific goal of the design is to achieve energy densities superior to typical Marx generators, such as a 500 J compact Marx generator previously designed and built at Texas Tech's Pulsed Power lab. Experimental and analytical techniques for determining circuit elements and especially parasitic elements were used, yielding the magnetizing, primary and secondary leakage inductances associated with the transformer, core saturation effects, parasitic capacitances, the inductance of the pulse discharge circuit, and losses in both copper and the deltamax core. The investigations into these characteristics were carried out using both sinusoidal excitation from 1 kHz to 20 Mhz, and pulsed excitation with rise times down to 5 ns. Pulse amplitudes were varied to cover both the linear and saturation regimes of the core. Distributed parasitic capacitances and the inductance of the pulse discharge circuit were estimated analytically and compared with experimental results. This work was carried out to seek an ideal arrangement of the capacitors and switches on the LTD stage and gain a better basic understanding of fast rise time pulse transformers. Adjustments to the 50 J stage are proposed based on this characterization in order to optimize a future ten stage, 500 J assembly.

Conferences

Authors: J. Foster; M. Thomas; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5958313

Abstract: High power microwave (HPM) surface flashover can be rapidly induced by introducing metallic points on to the dielectric surface with negligible effect on the transmission properties. An experimental setup comprised of a magnetron operating at 2.85 GHz to produce a 4.5 MW, 3 μs pulse is used for observing surface flashover in various atmospheric conditions. An active pulse sharpening mechanism is used to reduce the pulse rise time in order to apply the electric field in tens of nanoseconds. For a system in which HPM transmission must be quickly suppressed, field enhancing geometries can provide a way for flashover to develop rapidly while keeping insertion loss at a minimum (<;0.01 dB). Initial experiments utilizing 0.2 mm2 aluminum points with a spatial density of 25/cm2 have increased the global effective electric field by a factor of ~1.5. This increase in electric field has sharply reduced delay times for surface flashover (i.e. the time between the application of the HPM pulse and a sharp drop in transmitted power). For an environment consisting of air at 155 torr, for instance, the delay time is reduced from 455 ns to 101 ns. Presented in this paper is a comparison of various field enhancing geometries and how they relate to flashover development. Also, an analysis of time resolved images will be given along with an estimation of field enhancement factors.

Conferences

Authors: M. Thomas; J. Foster; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5958314

Abstract: Surface flashover imposes a fundamental limitation to the magnitude of high power microwaves which can be radiated from the vacuum environment of the source into atmospheric conditions. Providing seed electrons through various methods allows for initiatory conditions to be more closely controlled and the delay time variations to be reduced so that developmental mechanisms can be more closely examined. The experiment uses a coaxial magnetron capable of producing a ~4.5 MW, 3 μs pulse, at 2.85 GHz propagating in the TE10 mode. The pulse rise time measured at the window is reduced using a spark gap pulse steepening technique. The fast rise time pulse propagates through the dielectric into an atmospheric test chamber where various conditions such as gas pressure, type of gas, UV illumination, and charged particle creation by radioactive sources can be controlled. Previous research has shown the significant impacts of UV radiation on the delay time averages and statistical distributions. Surface distributed seeding sources and volume distributed sources will be discussed while the primary focus of this paper will address use of alpha radiation as an ionizing agent. Thus far, a reduction in average delay time by as much as 60% has been achieved at sub-microsecond time scales, which also significantly affected the width of the statistical distributions of the delay time. Alpha particles have a short penetration distance in air which makes them a good candidate for study since the number of electron-ion pars created along the path is large. Analysis of the alpha particles influences will be discussed along with a statistical analysis of breakdown delay in the presence of ionization.

Conferences

Authors: J. Bragg; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5958430

Abstract: Ferrite filled coaxial lines have been shown to sharpen input pulses due to their nonlinear magnetic properties. This effect can be amplified by applying an axial magnetic biasing field. In addition to a steeper leading edge, oscillations in the microwave bands have been observed. These oscillations arise from damped gyromagnetic precession occurring in the ferrite. The magnetic bias necessary to generate damped oscillations and steepened pulse sharpening effects is produced by two separate means; a current carrying solenoid or, the less investigated case, permanent magnets. Multiple NLTLs are tested at lengths of 0.3 meters and 1 meter with an outer diameter of 13 mm. Permanent magnets with varying strengths were placed in different locations for multiple tests. Experiments and setups for these designs are detailed and their results are discussed.

IEEE Conferences

Authors: A. Young; A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5471092

Abstract: A novel modeling and simulation method for flux-trapping flux-compression generators (FT-FCGs) is presented, which utilizes PSpice circuit-simulation software to solve complex differential equations derived from circuit analysis. The primary motivation for the model development is the desire for a technique to rapidly design and prototype FT-FCGs for use as drivers in high-power microwave sources. The derivation of FT-FCG equations will be given, both in the ideal (lossless) and nonideal cases. For the nonideal case, three flux conservation coefficients are added to the equations to account for intrinsic flux loss in the circuit. Time-varying inductance curves are calculated using zero-dimensional models found in literature and adapted to fit this model. A simple FT-FCG design is used as an example to show the steps taken to complete a simulation. The same design was also fabricated and tested for comparison with predicted results from the model. A comparison of the waveforms acquired through simulation and experiment was found to result in good agreement for a given set of values for the flux conservation coefficients. A discussion of the derived equations, both lossless and nonideal, is given, as well as a discussion on the investigation of the impact of the three flux constants on the circuit. Analysis is offered on the results of this investigation, and conclusions are given on the effectiveness of this model to predict FT-FCG behavior.

IEEE Journals

Authors: D. Bolyard; A. Neuber; J. Krile; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5958310

Abstract: Hydrodynamic pressure simulations combined with an empirical algorithm are used to model the open-circuit voltage output of several explosively compressed ferroelectric materials. The empirical algorithm was initially developed using detonating cord containing PETN and a metal driver element to compress the ferroelectric materials while the open-circuit voltage is recorded. A hydrodynamic code suite, CTH from Sandia National Labs, enables calculating Shockwave propagation and localized pressures. The resulting pressure profile in the ferroelectric material is then used as input for an empirically derived algorithm to calculate the predicted open-circuit voltage of the ferroelectric material. This previously developed empirical algorithm exhibited reasonable correlation between experimental and calculated open-circuit output voltages, but began to deviate when more powerful explosives were used. Hence, the amount of explosive material and geometry of the metal drive was varied to produce a wide range of peak pressures, including pressures higher then the maximum of 3.1 GPa previously modeled by the empirical algorithm. This data serves as the base to further develop the empirical algorithm for various ferroelectric materials and to more accurately model the open-circuit output voltage (experimentally observed range, normalized for thickness, of 1.3 to 3.8 kV/mm) over the wide range of applied pressures.

Conferences

Authors: J. Krile; J. Foster; M. Thomas; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5958294

Abstract: High Power Microwave (HPM) induced surface flashover is currently being investigated in order to gain a better understanding of the underlying processes involved and reduce the limitations it places on transmittable pulse lengths. The present experimental setup is designed to produce a flashover on the high pressure side of a transmission window without the influence of a triple point. A 2.5 MW magnetron produces a 900 ns pulse at 2.85 GHz with a 50 ns rise time. The experimental setup allows for the control of several parameters including gas pressure, gas composition, and external UV illumination of the window. Diagnostic equipment enables the analysis of incident, reflected, and transmitted power levels with sub-nanosecond resolution. A previously developed Monte Carlo simulation is used to model the processes involved in the flashover discharge formation. This Monte Carlo code is upgraded to account for the occurrence of field induced electron detachment from negative ion clusters within the high field region, >; 10 kV/cm, near the window. The code has also been expanded to include the occurrence of photoelectrons, emitted from the window while under UV illumination. Such illumination of the transmission window was experimentally shown to reduce the time to flashover by over 100 ns in air at 155 torr, and thereby the total pulse energy that can be transmitted. In addition, UV illumination also reduces the variation in flashover delay times from shot to shot, up to 67% in air at 155 torr. The simulation will determine if the observed reductions in delay time and variation can be explained by the addition of initiatory electrons via UV illumination of the surface.

Conferences

Authors: J. Korn; A. Young; A. Neuber; C. Davis; M. Elsayed; M. Kristiansen; L. L. Altgilbers

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5958378

Abstract: The results from an investigation into the performance of a fuse opening switch, to be used as a component of a compact power conditioning unit, PCU, in an explosively driven high power microwave system, is presented. A pulse forming network designed to mimic the current action of a flux compression generator is utilized for these experiments. The investigation focused on the effect of altering fuse parameters, such as the diameter of fuse conductors and conductor winding geometry, on voltage and power levels delivered to a resistive load. Also, experiments were conducted with a thin layer of Semicosil, a commercial silicon material used in slower fusing opening switches, applied to fuse conductors, to investigate the possible advantages of using such a coating. Experiments showed that an increased number of conductor wires (with an approximately constant total cross-sectional area) resulted in similar voltages delivered to the resistive load, and that coating fuse conductors with Semicosil had a negligible effect on power delivery. A detailed description of these experiments will be given, along with data and waveforms illustrating the effects of these parameter variations on power levels delivered to the load.

Conferences

Authors: C. Davis; A. Neuber; J. Stephens; A. Young; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5958416

Abstract: Exploding wire arrays as fast switches are of interest for explosive pulsed power applications utilizing magnetic flux compression. This type of opening switch has proven effective in producing pulses of several hundred kilovolts into vacuum diode loads. The research presented here discusses an exploding wire array capable of producing single digit kilo-joules, 100 nanosecond pulses when driven by a 45 kilo-Amp current with a waveform closely resembling typical magnetic flux compression output. For this specific parameter range, the optimal fuse design was developed based on the experimental behavior of the fuse under variation of parameters such as wire spacing, shielding, and quenching medium. Each fuse is composed of several silver wires arranged in a straight wire cylindrical array and is typically pressurized in a chamber filled with about 0.6 MPa of SF6. The tradeoff between wire spacing and voltage output was addressed by designing four different fuse termination pairs each with a diameter that increased wire spacing from 5 to 20 mm in 5 mm increments. A wire shield test was also conducted as an extension to the wire spacing experiment to uncover any mutual radiative effects between wires on fuse opening behavior. The optimum fuse design, including the optimum fuse wire diameter, will be discussed with a 20 Ohm resistive load as well as a vacuum diode load with similar impedance.

IEEE Conferences

Authors: M. Elsayed; A. Neuber; C. Lynn; J. Korn; C. Anderson; A. Young; J. Dickens; M. Kristiansen; L. Altgilbers

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5958300

Abstract: Recent efforts at the Center for Pulsed Power and Power Electronics at Texas Tech University have been focused on the development of a compact and explosively driven High Power Microwave, HPM, system. The primary energy source (other than the seed energy source) driving the microwave load in this system is a mid-sized, dual-stage helical flux compression generator, HFCG. The HFCG has a constant stator inner diameter of 7.6 cm, a length of 26 cm, with a working volume of 890 cm3. Testing at the Center has revealed energy gains in the 30's and 40's with output energy levels in the kilo-joules regime into loads of several micro-Henries. Over the last few years, close to one hundred shots have been taken with these generators into various loads consisting of dummy inductive loads, power conditioning systems, and HPM sources. Throughout these tests, the working volume of the HFCG, i.e. the volume in between the wire stator and the explosive-filled aluminum armature, was filled with SF6 at atmospheric pressure. This was primarily done do avoid electrical breakdown in the generator volume during operation, resulting in flux loss. Recent design updates enable pressurizing the generator volume to pressures up to 0.5 MPa, which is needed, for instance, to replace the SF6 with other gases such as air or nitrogen. The performance of the dual-stage HFCG with pressurized working volume (SF6 and N2) is presented in this paper along with an analysis of the maximum electric field amplitude held off in the volume during operation. The design technique to seal the HFCG will also be briefly discussed.

IEEE Conferences

Authors: Parson, J; Dickens, J; Walter, J; Neuber, AA

PDF: https://aip.scitation.org/doi/10.1063/1.3499245

Abstract: This paper presents a study on energy deposition and electromagnetic compatibility of match-type electroexplosive devices (EEDs), which recently have found more usage in pulsed power environments with high electromagnetic interference (EMI) background. The sensitivity of these devices makes them dangerous to intended and unintended radiation produced by devices commonly used in pulsed power environments. Match-type EEDs have been found to be susceptible to such low levels of energy (7-8 mJ) that safe operation of these EEDs is vital when in use near devices that produce high levels of pulsed EMI. The scope of this paper is to provide an investigation that incorporates results of similar studies to provide detonation characteristics of these EEDs. The three topics included in this study are sensitivity testing, modeling of the thermodynamic heat propagation, and electromagnetic compatibility from pulsed electromagnetic radiation. The thermodynamic joule heating of the primary explosive has been modeled by a solution to the 1D heat equation. A simple pulsed generator, Marx generator with an inductive load, was used for the electromagnetic compatibility assessment of the coupled field between the pulse generator and shorted EED. The results of the electromagnetic compatibility assessment relate the resistive, inductive, and capacitive components of the pulse generator to the area of the shorted EED. (C) 2010 American Institute of Physics. [doi:10.1063/1.3499245]

Journal

Authors: S. L. Holt; M. A. Elsayed; B. Gaston; J. C. Dickens; A. A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5958312

Abstract: The design and testing of an integrated front-end power and control system for helical flux compression generators (HFCG) is presented. A current up to 12 kiloamps needs to be pushed into the 5.8 microhenry field coil of the HFCG to establish the necessary seed flux for generator operation. This current is created with the discharge of a 5 kilovolt, 50 microfarad metalized polypropylene film capacitor using a single-use semiconductor closing switch. Once peak current/flux is obtained in the seed coil an exploding bridge wire (EBW) detonator is initiated with a discharge from a 1 kilovolt, 500 millijoule capacitor array contained in the compact fire set. Both capacitances, seed and fire set, are charged using a rapid capacitor charger system. The rapid capacitor charger is a solid state step up converter supplied by lithium-ion polymer (LiPo) batteries. It provides the 5 kilovolts and 1 kilovolt dual output voltages required for the compact seed source and compact fire set, respectively. The rapid capacitor charger operates at an average output power of 3 kilowatts and charges both capacitances simultaneously in under 250 milliseconds. The rapid capacitor charger is reusable if protected from the explosive detonation.

IEEE Conferences

Authors: P. Hawkins; A. Neuber; K. Vepa

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5551471

Abstract: Applied Materials is working with customers to assess new ways to identify opportunities to reduce resources usage (energy, process chemicals/gasses) by existing semiconductor process tools. Through six customer pilot projects we have identified potential average savings of $22.5K per year, for the CMP and CVD process tool examined, by using a consultative service approach to business and financial analysis that pinpoints opportunities for process resource reduction without diverting critical engineering personnel from their core responsibilities.

IEEE Conferences

Authors: D. W. Bolyard; A. A. Neuber; J. T. Krile; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5415669

Abstract: Pressure simulations have been performed for several experimental explosively driven ferroelectric generators, with 2.54-cm-diameter PZT EC-64 discs as the ferroelectric material, using the hydrodynamic code system CTH, developed by Sandia National Laboratories. An empirical relationship was found between the results of the pressure simulations and the output voltages of the experimental generators, and an algorithm was generated to convert the simulated pressure into open-circuit voltage. This empirical algorithm has been applied to simulations of different experimental ferroelectric generators, and the results show a good correlation when compared to the corresponding experimental open-circuit output voltages. The experimentally achieved output voltages normalized for a thickness range from 14 to 34 kV/cm.

Journal

Authors: David W. Bolyard; Andreas A. Neuber; John T. Krile; Magne Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5415669

Abstract: Pressure simulations have been performed for several experimental explosively driven ferroelectric generators, with 2.54-cm-diameter PZT EC-64 discs as the ferroelectric material, using the hydrodynamic code system CTH, developed by Sandia National Laboratories. An empirical relationship was found between the results of the pressure simulations and the output voltages of the experimental generators, and an algorithm was generated to convert the simulated pressure into open-circuit voltage. This empirical algorithm has been applied to simulations of different experimental ferroelectric generators, and the results show a good correlation when compared to the corresponding experimental open-circuit output voltages. The experimentally achieved output voltages normalized for a thickness range from 14 to 34 kV/cm.

Journals

Authors: G. Laity; A. Neuber; G. Rogers; K. Frank; L. Hatfield; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5534416

Abstract: Summary form only given. It is commonly accepted that vacuum ultraviolet (VUV) radiation, corresponding to emission from 180 nm to 115 nm and below, is responsible for photoionization contributing to streamer propagation during the initial stages of atmospheric discharges. An experimental setup was constructed to observe the VUV emission of pulsed surface flashover along a dielectric surface between atmosphere and vacuum. However, VUV radiation is highly attenuated in the atmosphere, which makes observation of detailed spectra difficult. For VUV transmission down to 115 nm the light emitted by surface flashover across an MgF2 window (front side of window in air, backside in vacuum) was focused by an MgF2 lens onto the entrance slit of the spectrograph. The high speed detection scheme consists of a VUV sensitive ICCD camera and a photomultiplier, both with nanosecond temporal resolution. Spectra were measured in various gas mixtures at atmospheric pressure with a flashover spark length of about 8 mm with a 35 kV pulsed excitation, and spectral calibration was done utilizing a VUV calibration lamp with a known emission spectrum. Virtually all lines from 115 to 180 nm can be identified as atomic oxygen and nitrogen transitions during flashover in dry air, with most VUV emission occurring during the initial breakdown stage (current rise). The extremely fast decay of VUV emission intensity following this initial stage is evidence of radiationless quenching of the excited energy levels associated with the observed spectral lines. Flashover studies were also performed in pure oxygen and nitrogen environments to reinforce the observed emission trends. Spectroscopy must be carefully detailed, for instance, the Oxygen-I line at 130.2 nm (which corresponds to a ground level transition) is shown to be strongly self absorbed in the atmospheric spark when compared to a similar oxygen emission line at 130.4 nm. Full spectra were simulated using SpectraPlot, a temperature dependent spectral software suite developed at Texas Tech. It has been concluded from the comparison of simulated and measured Nitrogen spectra between 140 and 150 nm that the electronic temperature is about 4.5 eV, assuming that the electronic nitrogen energy level population density is Boltzmann distributed. The measured spectra will be discussed in relation to the physics of surface flashover and volume breakdown at atmospheric pressure.

IEEE Conferences

Authors: Truman G. Rogers; Andreas A. Neuber; Klaus Frank; George R. Laity; James C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5559489

Abstract: There is a growing interest in the physics of surface flashover between the interface of atmosphere and vacuum in some high-power systems. More specifically, the quantitative role of vacuum ultraviolet (VUV) radiation for the photoionization leading to a streamer development during the initial stages of a breakdown is unknown. This paper describes an experimental setup used to measure the VUV radiation emitted from atmospheric flashover as well as time-resolved imaging of the flashover event. A pulser providing the voltage to the gap was designed with special considerations in mind, including long lifetime, low noise, and high reproducibility. This enabled the study of the flashover in various background gases with an emphasis on spectroscopic measurements. The calculated spectra are compared with the measured spectra, and it is found that atomic oxygen and nitrogen are responsible for most of the VUV production in an air breakdown at atmospheric pressure in the wavelength range of 115-180 nm. Time-resolved spectroscopy reveals that the VUV radiation is emitted during the initial stages while the streamers are developing.

Journals

Authors: D. Matia; H. Krompholz; M. Giesselmann; A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5386263

Abstract: The design of a 15 kA linear transformer driver (LTD) is presented. The specific goal of this LTD was improved energy density over the 500 J compact Marx generator previously designed and built at Texas Tech's Pulsed Power lab. The design of an individual 50 joule, 30 kV stage is discussed. For successful operation of the LTD, multiple spark gaps have to be fired with low jitter. Possible approaches for the design of a compact, low jitter triggering circuit will be presented as well.

IEEE Conferences

Authors: J. Korn; A. Neuber; A. Young; C. Davis; M. Kristiansen; L.L. Altgilbers

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5386099

Abstract: The generation of high power microwaves using explosively driven pulsed power is of particular interest to the defense community. The high energy density of explosives provides the opportunity to design pulsed power systems which occupy significantly less volume, yet provide the same output power, as traditional methods of high power microwave (HPM) production. Utilizing a flux compression generator (FCG) as explosive driver necessitates introducing an intermediate power conditioning system (PCS) that addresses the typical impedance mismatch between FCG and HPM source. The presented PCS is composed of an energy storage inductor, an opening fuse switch and a self-break peaking gap all of which needed to fit within an envelope of 15 cm diameter. Currents in the tens of kilo-amperes and voltages in the hundreds of kilo-volts have to be handled by the PCS. The design of the system, which takes up less than 11 liters of volume, as well its performance into a 20 ¿ resistive load (used to approximate the operating impedance of certain HPM sources) is presented. Approximately 6 GW of electrical peak power was delivered to the load.

Conferences

Authors: C. Lynn; A. Neuber; J. Krile; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5386199

Abstract: It is known that polymers become conductive under shock loading, which can be critical to the operation of explosive driven high current/voltage devices. Hence, the propensity of several select polymers to conduct under shock loading was investigated. Four polymers, Nylon, Teflon, Polypropylene, and High Density Polyethylene, were tested under shock pressures up to ~22 GPa. Shock waves were generated with high explosives, and CTH, a hydrodynamic code developed at Sandia National Laboratories, was utilized to calculate pressure and temporal resolution of the shock waves. Time of arrival measurements of the shock waves were taken to correlate the hydrodynamic calculations with experimental results. A notable delay between shock front arrival and the onset of conduction is exhibited by each polymer. The delay tends to decrease with increasing pressure down to approximately 500 ns for HDPE at ~22 GPa under electric field strength of ~6.3 kV/cm. The data shows that some polymers exhibit more delay than others, thereby indicating better insulating properties under shock loading. Additionally, experiments revealed that the polymers conducted for a finite time on the microsecond time scale before recovering back to an insulating state. This recovery from a shock wave induced conducting state back to insulating state was investigated for a possible opening switch application.

Conferences

Authors: J. Parson; J. Dickens; J. Walter; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5386262

Abstract: This paper assesses critical activation limits of electroexplosive devices (EED), such as blasting caps, which have recently found more usage in pulsed power environments with high EMI background. These devices, EEDs, can be very sensitive to low levels of energy (7-8 mJ) which make them dangerous to unintended radiation produced by compact pulsed generators. Safe operation and use of these devices are paramount when in use near devices that produce pulsed electromagnetic interference. The scope of this paper is to provide an evaluation of activation characteristics for EEDs that include energy sensitivity tests, thermodynamic modeling, and electromagnetic compatibility from pulsed electromagnetic interference. Two methods of energy deposition into the bridgewire of the EED are used in the sensitivity tests. These methods include single and periodic pulses of current that represent the adiabatic and non-adiabatic heating of the bridgewire. The heating of the bridgewire is modeled by a solution to the heat equation using COMSOL¿ with physical geometries of the EED provided by the manufacturer.

Conferences

Authors: M.A. Thomas; J. Foster; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5227632

Abstract: Summary form only given. The introduction of a high voltage DC electrode into an experimental setup used for observing high power microwave surface flashover has shown to significantly vary the total delay time for this type of breakdown. The experiment utilizes an S-band magnetron operating at 2.85 GHz to produce a 4 MW, 3 mus pulse. A plasma switch mounted in a WR-284 waveguide reduces the 10-90% rise time of the pulse to ~50 ns, and it reflects the pulse towards a dielectric window to induce surface flashover. A wire electrode charged to plusmn20 kV is inserted into the center of the dielectric window and oriented perpendicularly to the major electric field component of the TE10 microwave mode. The DC field from the electrode influences charge carriers in the flashover region, forcing potential breakdown initiating charged particles away from or towards the surface, dependent on polarity and particle charge sign. Initial tests were conducted in pure N2 at a pressure of 125 torr. The low probability of negative ions (stable negative N2 ions do not exist) appearing in the volume simplifies the interpretation of the experimental results by allowing for the existence of primarily electrons and positively charged ions. A significant increase (~50%) in the average total delay time for the case of a positively charged electrode has been observed. An increase in the average statistical delay was also observed as well as a decrease in the presumed formative delay for both voltage polarities. The apparent electron production rate in N2 was estimated to be 2 e/mus and 8 e/mus under HPM pulse application in the case of positive and negative DC voltages, respectively. Results of further tests conducted in Argon and Krypton-85 in Argon balance are presented along with a statistical analysis of measured delay times.

Conferences

Authors: M. A. Elsayed; A. A. Neuber; M. Kristiansen; A. Stults; L. Altgilbers

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5386300

Abstract: Two integral components that accompany an FCG in an explosively driven system is the prime power source and the trigger set. The objective of the prime power source or seed source is to provide the initial seed current/energy into the primary stage of an FCG. Another integral component in an FCG based pulsed power system is the trigger set. The trigger set is used to detonate an exploding bridge wire (EBW) which triggers the high explosives (HE's) in an FCG. This paper will discuss a recent design of a stand-alone apparatus that implements a self-contained (battery powered with full charge time less than 40 sec), single-use Compact Seed Source (CSS) using solid state components for the switching scheme along with a single-use Compact Trigger Set (CTS) that also implements a similar switching technique. The CSS and CTS stand-alone apparatus developed is a system (0.005-m3 volume and weighing 3.9 kg) capable of delivering over 360-J (~12 kA) into a 5.20-¿H FCG load and approximately 2-mJ (~600 A) into the EBW. Both the CSS and CTS have trigger energies of micro-Joules at the TTL triggering level.

IEEE Conferences

Authors: C. B. Davis; A. A. Neuber; A. Young; J. Walter; J. C. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5313897

Abstract: This paper presents a nonexplosive pulsed-power system that replicates the output current waveform of a flux compression generator (FCG). The primary purpose of this system is to efficiently test the power conditioning components of an explosively driven HPM system, while drastically reducing the time between tests which are inherent with explosive experiments. The power conditioning system (PCS) of the HPM system includes an energy-storage inductor, an electroexplosive opening switch (fuse), and a peaking gap and serves to match the FCG output characteristics with the HPM diode load requirements. A secondary purpose of the nonexplosive test bed is to provide data points which could be directly compared with those from explosively driven experiments. For this reason, a reflex-triode virtual cathode oscillator (vircator) was connected to the output of the nonexplosive system, and the results of which were compared with similar testing done with an FCG and a compact Marx generator. Since the behavior of the fuse is known to play a critical role in the performance of the PCS, a study was performed on the effect of different fuse designs on the overall performance of the PCS. Specifically, the quality of the electrical connection between the fuse wire array and the rest of the system was tested. Fuse design experiments were conducted with the nonexplosive test bed firing into a water resistor dummy load, which showed a 13% increase in peak load voltage and more than an 11% increase in energy transfer for fuses with improved wire-electrode connection strength. Some basic rules about fuse design, as well as conclusions on the performance of the PCS when driving an HPM load, are given.

IEEE Journals

Authors: C. Davis; A. Neuber; A. Young; J. Walter; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5386110

Abstract: This paper discusses a non-explosive pulsed power device used to reproduce the output waveforms of a Flux Compression Generator (FCG) driving a High Power Microwave (HPM) source. This system optimizes the power conditioning components of a HPM source while reducing the time and resources inherent to explosively driven FCG schemes. An energy storage inductor, fuse opening switch, and a peaking gap make up the power conditioning system. This system couples large voltage pulses (~300 kV), suited for HPM sources, to the load by disrupting the energy storage inductor current (~40 kA). This paper will show that an optimal fuse length was experimentally searched for by varying the calculated fuse wire base length by ±5, 10, and 15%. Various geometric fuse designs were examined to achieve a 45% reduction in the physical fuse length at constant wire length with acceptable performance losses. This paper will also show that the distance between the electrodes of a peaking gap can be optimized to more efficiently switch in the load. Finally results will be shown that depict the amount of microwave power produced by a vircator before fuse and peaking gap optimization.

Conferences

Authors: D. Bolyard; A. Neuber; J. Krile; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5386196

Abstract: Explosively-driven ferroelectric generators are attractive as potential prime energy sources for one-time use pulsed power systems. While the output voltages of small ferroelectric discs have been shown to be on the order of the theoretical maximum values, scaling the ferroelectric to larger thicknesses has proven less successful. The primary limiting factor is how much of the ferroelectric material is compressed simultaneously. This is difficult to control for thicker ferroelectric discs or stacks of discs due to pressure pulse attenuation in the material and rarefaction waves shortening the pressure pulse. A hydrodynamic code system is utilized to calculate the temporally and spatially resolved pressure. The calculated pressure values are converted into voltage produced by the ferroelectric through an algorithm based on an empirical polarization-pressure hysteresis curve. The validity of the algorithm has been verified for PZT EC-64 with experimental data from a flyer-plate experiment reported in literature and our own experiments with the shock wave from the explosives more directly applied to the ferroelectric. Both calculations and experiments produced normalized output voltages, ranging from 1.4 to 3.4 kV/mm for 2.54 cm diameter discs. We will discuss how this pressure to voltage algorithm along with pressure simulations aided in the scaling of the amount of ferroelectric material in a generator, as well as in the design of new driver elements with the goal to increase the peak output voltage of a generator while keeping the generator compact. The calculated voltage output results are compared with experimental data of explosively-driven ferroelectric generators.

IEEE Conferences

Authors: John T. Krile; Luke McQuage; Gregory F. Edmiston; John Walter; Andreas A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5291765

Abstract: High-power microwave (HPM)-induced surface flashover is investigated in order to gain a better understanding of this phenomenon and reduce the limitations it imposes on transmitted power levels. This paper builds on previous testing using a magnetron producing 5 MW for 4 mus at 2.85 GHz. Both the previous and current experimental setups are designed to produce a flashover on the high-pressure side of a transmission window without the influence of a triple point. The limitations of the previous experiment included a maximum power of 5 MW and a pulse rise time of 50 ns. The current HPM source is an experimental virtual cathode oscillator (vircator), the output of which has been extensively characterized. The vircator is capable of producing 50-MW peak for 100 ns with an adjustable frequency from 3 to 5 GHz and a rise time of < 4 ns. The dominant modes of the vircator and magnetron are the circular TE11 and rectangular TE10 modes, respectively, with the major electric field component in both setups normal to the direction of propagation, yielding comparable field geometries at the transmission window. The experimental setup permits the study of factors, including gas pressure, composition, temperature, and air speed. Diagnostic equipment allows the analysis of power levels and flashover luminosity with subnanosecond resolution. Additional experimental results, including a detailed analysis of the flashover delay times under various conditions, are compared with data from literature and previous testing. A trend of increasing delay time with pressure is clearly observable, and Eeff/p versus p * r data fall within what has been previously observed in literature primarily for HPM volume breakdown.

Journals

Authors: A. Young; A. Neuber; M. Elsayed; J. Walter; J. Dickens; M. Kristiansen; L.L. Altgilbers

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5386301

Abstract: An explosively driven High Power Microwave (HPM) source has been developed that is based on the use of a Flux Compression Generator (FCG) as the primary driver. Four main components comprise the HPM system, and include a capacitor-based seed energy source, a dual-staged FCG, a power conditioning unit and an HPM diode (reflex-triode vircator). Volume constraints dictate that the entire system must fit within a tube having a 15 cm diameter, and a length no longer than 1.5 m. Additional design restrictions call for the entire system to be stand-alone (free from any external power sources). Presented here are the details of HPM system, with a description of each subcomponent and its role in the generation of HPM Waveforms will be shown which illustrate the development of power as it commutates through each stage of the system, as well as power radiated from the diode. Analysis and comparisons will be offered that will demonstrate the advantages of an explosively driven HPM system over more conventional pulsed power devices.

Conferences

Authors: J. Foster; M. Thomas; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5386323

Abstract: Observed delay times for high power microwave surface flashover are influenced significantly by the presence of a DC electric field. The experimental setup to investigate theses influences is comprised of an S-band magnetron operating at 2.85 GHz with a pulse rise time shortening switch assembly that produces a 50 ns rise time at a ~ 2.5 MW power level. A wire electrode charged to ±20 kV is inserted into the dielectric interface perpendicular to the electric field of the TE10 mode to provide a DC electric field in the flashover region. Tests have been conducted in pure N2 at 125 torr in order to provide an environment composed of primarily electrons and positive ions. The average measured delay of window flashover with a DC field pointing into the dielectric has been observed to increase by ~50%. Additionally, effective emission rates of seed electrons initiating breakdown have shown a decrease from 14 e/¿s to 2 e/¿s, indicating the removal of charged species from the high microwave field region due to charge drift in the applied DC field. An overview of the experimental setup is given along with a statistical analysis of delay times measured in Air as well as N2. The open question of where seed electrons originate from and the quantification of the primary processes involved will be addressed.

Conferences

Authors: M. Thomas; J. Foster; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5386180

Abstract: Delay times of high power microwave surface flashover are affected by radiation illuminating the dielectric. A controlled environment of pure Argon at a range of low pressures as compared to normal atmospheric pressure was used with 2 mW/cm2 UV-radiation illuminating the test window. Argon was chosen due to its relatively small number of processes involved such as inelastic electron collisions and due to the well-known cross-sections for these processes. Delay times in the presence of UV are significantly shorter than without UV illumination. The initial electron density contribution due the UV source is very roughly estimated to be ~106 cm-3. A small admixture of radioactive krypton-85 showed only marginal changes in the observed delay times, likely due to an insufficient concentration of Kr-85 producing ionization events only every few microseconds and the high energy distribution associated with the emitted electrons. A detailed discussion of experimental breakdown delay data, along with theoretical expectations and discussion of the statistically dependent mechanisms and analysis, will be given. The ultimate goal is to develop a model for HPM window breakdown in a UV environment, to describe the role of discharge initiating electrons, and to quantify breakdown at high altitudes.

Conferences

Authors: G. Rogers; A. Neuber; G. Laity; J. Dickens; K. Frank; T. Schramm

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5386373

Abstract: Spectroscopic measurements in the vacuum ultraviolet (VUV) regime are difficult to make due to extremely large absorption of VUV radiation in most materials. This paper describes an experimental setup designed for studying the optical emission during pulsed surface flashover for the wavelength range between 115 nm to 180 nm at atmospheric pressures with a focus on the scheme used to excite the spark gap. The surface flashover of interest occurred on an MgF2 window (front side of window in air, backside in vacuum) imaged onto the entrance slit of a 1 m vacuum spectrograph. Emission spectra were recorded with an Andor DH740 series ICCD camera. All data was taken at atmospheric pressure with a flashover spark length of about 8 mm created by a pulser designed for a 500 ns pulse, max 50 kV output. The centerpiece of this pulser is the CCSTA14N40 thyristor by Solidtron/Silicon Power which features a rate of change current of maximum 30 kA/¿s and a hold-off voltage of up to 4 kV. A pulse transformer with Metglas® core was used to elevate the voltage to 50 kV with a rise time of 180 ns and a peak current of 500 A. The pulser was designed for a repetition rate of 10 Hz and is triggered by TTL pulses. Discussed in this paper, along with the measured spectra and their relation to the physics of surface flashover at atmospheric pressure, will be the design of the pulser.

Conferences

Authors: G. Laity; K. Frank; G. Rogers; M. Kristiansen; J. Dickens; A. Neuber; T. Schramm

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5227565

Abstract: Summary form only given. Spectroscopic measurements in the vacuum ultraviolet (VUV) regime are difficult to make due to extremely large absorption of VUV radiation in most materials. This paper describes an experimental setup designed for studying the optical emission during pulsed surface flashover for the wavelength range between 115 nm to 300 nm at atmospheric pressures. A vacuum monochromator VM 505 from Acton Research Corporation was used as the spectrograph. For VUV transmission down to 115 nm the light emitted by surface flashover across an MgF2 window (front side of window in air, backside in vacuum) was focused by an MgF2 lens onto the entrance slit of the spectrograph. A quartz window with sodium salicylate coating exposed to the spectrograph's vacuum was placed in the exit focal plane of the collimating mirror of the spectrograph. This fluorescent coating down-converts the VUV light to longer wavelengths that were recorded with an Andor DH520 series ICCD camera in combination with a Nikon 105 mm lens. Spectra were measured at atmospheric pressure with a flashover spark length of about 9 mm and DC excitation with a capacitance of 4.1 nF. Emission spectra were measured from 300 nm down to 130 nm. In parallel, theoretical spectra were calculated primarily for the identification of radiating species and their temperature. Utilizing the NIST Atomic Spectra Database (ASD) data a library of temperature dependent optical emission spectra was generated with SpectraPlot, a spectral software suite developed at TTU. VUV spectral lines of nitrogen, carbon, magnesium and silicon were identified. In pure nitrogen, for instance, the nitrogen I double line at 174.3 nm and 174.5 nm is clearly visible in the spectrum along with a strong double line at 279.6 nm and 280.4 nm, which is emitted by Magnesium II, eroded from the surface exposed to flashover. Spectra were measured in ambient air, pure nitrogen, and argon. An experiment upgrade is currently underway, increasing the VUV sensitivity of the setup. The measured spectra will be discussed in relation to the physics of surface flashover and volume breakdown at atmospheric pressure.

IEEE Conferences

Authors: A. Young; T. Holt; M. Elsayed; J. Walter; J. Dickens; A. Neuber; M. Kristiansen; L.L Altgilbers; A.H. Stults

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4743600

Abstract: Single-shot high power microwave (HPM) systems are of particular interest in the defense industry for applications such as electronic warfare. Virtual cathode oscillators (vircators) are manufactured from relatively simple and inexpensive components, which make them ideal candidates in single-shot systems. The flux compression generator (FCG) is an attractive driver for these systems due to its potential for high energy amplification and inherent single-shot nature. A self-contained (battery operated prime power), compact (0.038 m3), FCG-based power delivery system has been developed that is capable of delivering gigawatts of power to a vircator. Experiments were conducted with the delivery system connected to a resistive dummy load and then to a reflex-triode vircator. In order to optimize the performance of the vircator when driven by the power delivery system, a second experimental setup was constructed using a Marx-generator based system operating at similar voltages and rise-times. Performance measures of the delivery system when discharged into a resistive load will be presented, as well as vircator output power levels and waveforms from both experimental setups.

Conferences

Authors: J. E. Chaparro; W. Justis; H. G. Krompholz; L. L. Hatfield; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4663141

Abstract: With a RADAN 303-A pulser (a rise time of 150 ps and a maximum voltage of 150 kV into matched load), fast breakdown in argon and air is investigated. An oil-filled coaxial transmission line is coupled with a lens to a biconical section and a radial millimeter-size gap operated at subatmospheric pressure. Diagnostics include capacitive voltage dividers which allow the determination of voltage across and current through the gap with a temporal resolution defined by the digitizer (20 Gs/s, 6 GHz) used. A scintillator-photomultiplier combination with different metal absorber foils and a temporal resolution of 2 ns is used as X-ray detector to obtain a rough energy spectrum of the X-rays and electrons in the range of 10-150 keV. Discharges are characterized by runaway electrons over much of the pressure range, with a strong excitation and ionization layer at the cathode surface, and ldquofree-fallrdquo conditions with negligible gaseous ionization for the rest of the gap. High-energy electrons (> 60 keV) are observed up to atmospheric pressure. Time-to-breakdown curves versus pressure have been measured for different applied voltage rise times. They resemble Paschen curves with a steep increase toward low pressure and a slow increase toward high pressure. The major experimental findings and particularly the time-to-breakdown curves are confirmed using simple force-equation modeling. Monte Carlo calculations simulating collisional ionizations and developing electron avalanches in three dimensions have been used to verify and explain the experimental results.

IEEE Journals

Authors: E. J. Matthews; A. A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4743637

Abstract: The size of compact pulsed power generators capable of producing pulses with ~100 ns duration at Gigawatt power levels is primarily determined by the specific energy density of the utilized energy storage medium. Capacitors capable of delivering large pulsed currents at several 10 kV voltage levels have been most frequently used as the energy storage medium for portable pulse generators. To increase the specific energy density of the pulsed power generator, the capacitors are often voltage overstressed at the cost of capacitor life. However, rapid charging (milliseconds) of the capacitor immediately followed by discharging alleviates somewhat of the lifetime problem. For repetitive operation of the pulsed power generator, the charging/discharging energy loss is the more important parameter. The energy, WC, needed to charge a capacitor to a set voltage is measured along with the energy released, WR, by the capacitor under conditions corresponding to a compact Marx generator operating with ~10 Hz rep-rate into a ~20 Ohm load. For the tested capacitors with Mica as dielectric, the capacitor efficiency, eta, i.e. the ratio between WR and WC, is roughly equal to 97% and largely independent of the charging time. Also tested ceramic capacitors revealed an efficiency of ~90% for fast charging and an efficiency of ~94% for slower charging (from ~2 to 35 mus time constant).

Conferences

Authors: D. R. McCauley; D. W. Belt; J. J. Mankowski; J. C. Dickens; A. A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4663131

Abstract: Compact electroexplosive fuses (EEFs) as part of an explosively driven system are of interest for the one-time single-shot generation of high-power pulses. For instance, the transition from a very large driving current produced by an explosively driven flux compression generator (FCG), i.e., low impedance, to a large voltage spike delivered to the load, i.e., high impedance, can be done using an inductive storage system and an EEF. Typically, the EEF can be as large as, if not larger than, the current driver attached to it, thus making it one of the largest components in the system. Reduction in the size of the fuse will allow for size reductions of the entire high-power microwave (HPM) system. The goal of optimizing an EEF as an opening switch is to produce the greatest voltage multiplication possible to drive a load under physical size constraints. To optimize the fuse, several parameters are taken into account, including, but not limited to, fuse material, fuse length, fuse shape, and quenching medium. Individual optimization of these parameters will lead to complete optimization of an EEF, therefore resulting in a compact fuse capable of consistently producing maximum voltage multiplication for HPM systems.

IEEE Journals

Authors: C. James; C. Hettler; J. Dickens; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4743565

Abstract: Vanadium compensated, 6H silicon carbide (SiC) is investigated as a compact, high-power, linear-mode photoconductive semiconductor switch (PCSS) material. SiC is an attractive material due to its high resistivity, high electrical breakdown strength, and long recombination times compared to other photoconductive materials. The PCSS is designed for fast-rise time, low-jitter (sub-nanosecond) operation in a matched 50 mu test bed. Ohmic contacts were applied by physical vapor deposition and initial test utilized an external Nd:YAG laser trigger source. Analysis of the optical properties of Va-compensated SiC and of switch conduction resistance are presented and performance of contact material is discussed.

IEEE Conferences

Authors: John T. Krile; Andreas A. Neuber; Hermann G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4463641

Abstract: Undesirable surface flashover of high voltage support structures can severely limit the compactness of open air high voltage systems. Only recently, increased effort has been invested in characterizing and quantifying the physical processes involved in surface flashover occurring under atmospheric conditions and under the influence of UV illumination. In this paper, a UV flash lamp and a solid-state UV source, with its much faster turn-off time, were utilized in conjunction with a high temporal resolution testing apparatus. The UV pulse, excitation voltage, discharge current, and flashover self-luminosity were measured with high temporal precision. We relate recent experiments to our experimental findings of surface flashover under atmospheric conditions gained over the past five years. A simple model that describes the observed behavior will be presented. In addition, a more advanced Monte Carlo-type code for electron collision dynamics will be utilized to further analyze the role of UV in surface flashover under atmospheric conditions.

Journals

Authors: J. Parson; J. Dickens; J. Walter; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4743684

Abstract: This paper assesses the critical activation energy required to set off electroexplosive devices (EED) at constant joule heating and rates of joule heating. Safe operation and use of these devices are of great concern in and around pulsed electromagnetic interference. Sensitivity characterization of EEDs include firing sensitivity plots, thermodynamic modeling and electromagnetic interference. Activation energy evaluation of single and periodic rectangular pulses are included to represent adiabatic and non-adiabatic bridge wire heating of the EED. The scope of this paper is to provide a short overview of sensitivity, thermodynamic, and electromagnetic compatibility of EEDs. The results provide crucial information in evaluating energy induced by pulsed electromagnetic fields of compact pulse generators.

Conferences

Authors: Elsayed, M; Kristiansen, M; Neuber, A

PDF: https://aip.scitation.org/doi/10.1063/1.3046280

Abstract: Flux compression generators (FCGs) are some of the most attractive sources of single-use compact pulsed power available today due to their high energy density output and mobility. Driving FCGs requires some seed energy, which is typically provided by applying a high seed current, usually in the kiloampere range for midsized helical FCGs. This initial current is supplied by a high-current seed source that is capable of driving an inductive load. High-current seed sources have typically been comprised of discharging large capacitors using spark gaps and overvoltage triggering mechanisms to provide the prime power for FCGs. This paper will discuss a recent design of a self-contained (battery powered with full charge time less than 40 s), single-use compact seed source (CSS) using solid-state components for the switching scheme. The CSS developed is a system (0.005 m(3) volume and weighing 3.9 kg) capable of delivering over 360 J (similar to 12 kA) into a 5.20 mu H load with a trigger energy of microjoules at the TTL triggering level. The newly designed solid-state switching scheme of the CSS incorporates off-the-shelf high-voltage semiconductor components that minimize system cost and size as necessary for a single-use application. A detailed evaluation of the CSS is presented primarily focusing on the switching mechanics and experimental characterization of the solid-state components used in the system.

Journal

Authors: M. Elsayed; T. Holt; A. Young; A. Neuber; J. Dickens; M. Kristiansen; L. L. Altgilbers; A. H. Stults

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4743567

Abstract: Flux Compression Generators (FCGs) are some of the most attractive sources of single-use compact pulsed power available today due to their high energy density output and mobility. Driving FCGs requires some seed energy, which is typically provided by applying a high seed current, usually in the kilo-Ampere range for mid-sized helical FCGs. This initial current is supplied by a high-current seed source that is capable of driving an inductive load. High-current seed sources have typically been comprised of discharging large capacitors using spark-gaps and over-voltage triggering mechanisms to provide the prime power for FCGs. This paper will discuss a recent design of a self-contained (battery powered with full charge time less than 35 sec), single-use Compact Seed Source (CSS) using solid state components for the switching scheme developed at the Center for Pulsed Power and Power Electronics at Texas Tech University. The CSS developed is a system (0.007-m3 volume and weighing 13 lbs) capable of delivering over 250-J (~10 kA) into a 6-muH load with a trigger energy of micro-Joules at the TTL triggering level. The newly designed solid-state switching scheme of the CSS incorporates off-the-shelf high-voltage semiconductor components that minimize system cost and size as necessary for a single-use application. An in-depth and detailed evaluation of the CSS is presented primarily focusing on the switching mechanics and experimental characterization of the solid state components used in the system.

IEEE Conferences

Authors: J. Krile; A. Neuber; R. Vela

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4512494

Abstract: A recent upgrade of Sandia National Laboratories' Z-machine has exposed a possible failure mode in the 5 MV laser-triggered gas switches (LTGS). During the closure of the cascade section of the switch, the surface flashover (SF) inside the dielectric switch housing occurred sporadically, affecting subsequent closing timing and damaging the switch housing. A small-scale experiment has been constructed to mimic conditions within the LTGS and to examine the survivability of various materials exposed to high-current SFs.

IEEE Journals

Authors: T. A. Holt; A. J. Young; M. A. Elsayed; J. W. Walter; A. A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4599011

Abstract: A recently developed self-contained compact single-shot high-power microwave (HPM) system was tested and characterized. The explosive-driven system utilizes a reflex triode virtual cathode oscillator (vircator) as the HPM source. An open-shutter image acquired with a digital single-lens reflex camera during operation was used to show plasma development extending beyond the anode-cathode gap of the vircator. The plasma's self-emission is due to ionized material eroded and desorbed from both the cathode and the anode.

IEEE Journals

Authors: G. F. Edmiston; J. T. Krile; A. A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4598990

Abstract: Dielectric window flashover is a severe pulse-shortening phenomenon limiting the power levels radiated in high power microwave (HPM) systems. This type of flashover develops in regions under high field stress coinciding with the dielectric interfaces separating the vacuum and atmospheric pressure sections of a microwave system. The formation of plasma at the exit aperture of a transmitting system can have several detrimental effects, including premature termination of the radiated pulse and/or the reflection of potentially damaging levels of radiation back toward the microwave source. Experimental studies of HPM surface flashover have been conducted under a variety of conditions in the S-band at power levels up to 5 MW with the aim of quantifying the relative impact of parameters such as gas pressure, type, and window geometry. One particular geometry variant designed with grooves perpendicular to the major electric field component at the window surface exhibited superior flashover suppression characteristics when compared with smooth window geometries. Images of HPM surface flashover evolution on this corrugated dielectric window geometry are presented.

IEEE Journals

Authors: C. Lynn; J. Krile; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4743696

Abstract: It is known that polymethylmethacrylate, PMMA, becomes conductive under shock loading. To develop an opening switch utilizing shock induced conduction, the reversibility of this process must be studied. It is suggested in literature that changes in electrical properties begin at pressures as low a ~2 GPa. Applying the minimum pressure necessary for conduction is desirable in order to maximize the reversibility by limiting compression heating of the material. CTH, a hydrodynamic code written at Sandia National Laboratory, was used to design various drivers that deliver pressures in the range of ~2 GPa to ~6 GPa to the PMMA. By utilizing the switch to trigger an RC discharge, the resistance and on-time of the switch was characterized. Experiments have shown conduction durations on the order of ~4 mus. The switch was then placed into a capacitive driven inductive energy storage circuit, IES, to determine the polymer's ability to recover. This paper will present experimental data, CTH simulation results, and discuss the attained switching characteristics under varying shock pressure profiles.

IEEE Conferences

Authors: J. Chaparro; H. Krompholz; A. Neuber; L. Hatfield

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4743700

Abstract: Previous research at Texas Tech University has been conducted on the physics governing highly over-voltaged gas breakdown resulting from ultrafast applied voltage pulses with risetimes less than 200 ps and durations less than 400 ps. Experimental results have shown that the breakdown characteristics of such events significantly differ from those observed in standard gas breakdown and a complete understanding of the physics behind ultrafast discharges is far from being clear. As a companion to experimental work, a numerical model is an attractive means of discerning more about the underlying physics behind such events. In this paper, a relativistic, Particle in cell model utilizing Monte-Carlo calculations is discussed as a way to directly simulate the experimental conditions, with similar geometry, background gas, and pulse characteristics. Diagnostic output from the simulation includes space-charge development over time, field and particle energy distributions, and particle number growth rates and spatial distributions. An overview of the structure and formulation behind the simulation code is given followed by a comparison of output data to experimental results. Specific points of interest for comparison include formative and statistical delay times, examination of inhomogeneous ionization regions in the discharge, and the behavior of high-energy particles in the runaway state.

IEEE Conferences

Authors: D. Bolyard; A. Neuber; J. Krile; J. Walter; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4743573

Abstract: Explosively-driven ferroelectric generators are capable of producing single-shot voltage pulses of more than 100 kV, while requiring no seed electrical source, being very compact, and shelf stable. Problems with ferroelectric generators are the low energy output, high dielectric constant of the ferroelectric material, low surface flashover voltage, inconsistent ferroelectric material quality, and uneven or excessive shockwave compression. Initial generator voltage waveforms show that breakdown occurred towards the end of the generator operation time. Several designs and methods have been tested and implemented to prevent surface flashover with varying results. The ferroelectric discs used for the generators were 0.4 inch thick, 1 inch diameter EC-64 PZT ceramic discs. Several six-disc generators were built and tested with resulting open-circuit voltage pulses of 80-140 kV with a FWHM of 2-4 mus. Further improvements to the generators have been designed to prevent surface flashover, improve the explosive driver element and propagating shockwave, as well as increasing the number of discs per generator. Measured output waveforms into varying loads including direct driven antennas will be shown and discussed.

IEEE Conferences

Authors: Jonathan Foster; Greg Edmiston; John Krile; Herman Krompholz; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4743599

Abstract: High power microwave transmission is ultimately limited by window flashover at the vacuum-air dielectric boundary. While surface flashover in the presence of a vacuum has been studied in some detail, the mechanisms associated with flashover in an atmospheric environment need further investigation. For an aircraft based high power microwave system, atmospheric pressures ranging from 760 torr (sea level) to 90 torr (50,000 ft.) are of principal concern. The experimental setup uses a 2.85 GHz, 3 mus microwave pulse with a 10 to 90% rise time of approximately 600 ns from a magnetron capable of producing 5 MW. The slow rise time of the microwave pulse is sharply reduced by using a waveguide spark gap switch used for fast microwave reflection and a high power four port circulator [6]. This reflected pulse has a reduced rise time on the order of 50 ns. The shorter rise time produces a more ideal step waveform that can be more easily compared with theoretical perfect square pulse excitation. Past investigations showed that the delay time for breakdown in air increases with pressure as is expected from the right hand side of the Paschen curve as long as the electron collision frequency is much larger than the microwave frequency. Surface flashover experiments have produced similar results. At a pressure of 155 torr, for instance, the breakdown electric field strength is 6 kV/cm (power density 0.08 MW/cm2) and the overall delay time from HPM pulse application to reaching critical breakdown plasma density is 600 ns. An overview of the experimental setup is given along with a discussion of breakdown delay times as a function of pressure as well as an investigation of surface flashover in the presence of external UV (ultraviolet) illumination.

Conferences

Authors: Jordan Chaparro; Lynn Hatfield; Hermann Krompholz; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4743701

Abstract: For subnanosecond switching, physical phenomena as well as basic breakdown data, such as delay times and breakdown voltages, are of interest. With a RADAN Pulser as source for voltage pulses with up to 180 kV amplitude and risetimes at a test gap of 180 ps, we investigate statistical and formative delays for argon and dry air at sub-atmospheric pressure, for gap widths of 1 and 11 mm. Formative times have minima between 50 and 200 torr, and range from 70 ps at 1.5 MV/cm to 200 ps at 50 kV/cm. For this range of electric fields, this dependence on pressure and applied field can be explained by the behavior of ionization coefficient and electron drift velocity for homogeneous discharges. For higher fields exhibiting a narrow ionization zone in cathode vicinity with pronounced electron runaway conditions, the experimental data agree with results of Monte-Carlo simulations. Statistical delays are about the same as formative delays at fields of 50 kV/cm, and are reduced with increasing field amplitude to less than 50 ps at 1.5 MV/cm. It appears that field emission is the major source for starting electrons, influencing the statistical delay time near the field emission threshold only.

Conferences

Authors: A. Neuber; G. Edmiston; J. Krile; J. Foster; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4591128

Abstract: Summary form only given as follows. For flashover in air, nitrogen, and SF6, we have previously shown that the delay time between microwave pulse application and breakdown increases with pressure in the regime where the elastic electron collision frequency is larger than the microwave frequency (roughly >10 torr for 2.85 GHz microwave frequency), which also coincides with conditions found at the right hand side of the Paschen curve. The specific window flashover geometry was carefully chosen to avoid local field enhancement. That is, no metallic parts are exposed to high fields. Hence, only the window surface itself, the gas, and interaction processes between surface and volume contribute to flashover. A Monte Carlo based electron motion code developed for the flashover conditions predicts formative flashover delay times reasonably well in the pressure regime between 100 to 600 torr (10,000 Pa to 80,000 Pa). However, the statistical delay time, that is the time interval required for the initiatory electron(s) to appear, is unaccounted for in the code. Further computational efforts investigating seed electron production via collisional detachment from, for instance, negative ions in the gas have shown that while effective at unipolar fields, collisional detachment is unlikely to contribute to the production of seed electrons at higher microwave frequencies above several GHz. Experiments show that illuminating the surface with light/photons (180 nm < lambda < 350 nm) reduces the observed statistical delay considerably indicating the importance of seed electron production from the surface. This paper will discuss the key processes of high power microwave surface flashover and present experimental flashover data along with continued investigation into the statistics of possible seed electron sources, including trace contaminates present in the gas or on the dielectric surface.

Conferences

Authors: David W. Belt; John J. Mankowski; Andreas A. Neuber; James C. Dickens; Magne Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4667677

Abstract: Helical flux compression generators (HFCGs) of a 50-mm form factor have been shown to produce output energies on the order of ten times the seeded value and a typical deposited energy of 3 kJ into a 3-muH inductor. One way to drive a high-power microwave source with an HFCG is by power conditioning, such as an inductive energy storage system (IESS). The output performance of the IESS is contingent upon the opening switch scheme, usually an electroexplosive fuse. Our previous work involving fuse parameter characterization has established a baseline for potential fuse performance. In order to optimize the electroexplosive wire fuse, we have constructed a nonexplosive test bed which simulates the HFCG output with high accuracy. We have designed and implemented a capacitor-based magnetic switching scheme to generate the near-exponential rise of the HFCG. The use of the nonexplosive HFCG test bed will allow the verification of scalability of the fuse parameter model and also allow testing of exotic fuse materials. The nonexplosive test bed has provided a more efficient method for electroexplosive switch development and has allowed us to expand the study of opening switches. We will also discuss the a priori calculated baseline fuse design and compare the experimental results of the gold-wire-material with the silver-wire-material baseline design. With the results presented, an accurate PSpice model applicable to our 45-kA HFCG systems will be available.

Journals

Authors: J. Krile; G. Edmiston; J. Dickens; H. Krompholz; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4743719

Abstract: Surface flashover development at the output window of high power microwave (HPM) systems presents a major limitation to the power densities and pulse lengths transmitted through these interfaces. As a result, developing a physical model accurate in predicting surface flashover initiation is of prime interest. A Monte-Carlo type electron motion simulation has been developed to estimate the delay time from initial electron to flashover. Although this approach has shown reasonable agreement with experimental results, the process yielding the initial seed electron(s) was neglected in the model, primarily due to the lack of quantitative and qualitative information on seed electron production. For instance, computational efforts investigating seed electron production via collisional detachment from negative oxygen ions have shown that while effective at DC, the collisional detachment model cannot remain a likely contributor of electrons at high frequencies (Gt ~5 GHz). The key parameters impacting high power microwave surface flashover will be discussed and presented along with continued investigation into the statistics of possible seed electron sources, including trace contaminates present in the gas or on the dielectric surface.

Conferences

Authors: G. Edmiston; A. Neuber; L. McQuage; J. Krile; H. Krompholz; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4286507

Abstract: One of the major limiting factors for the transmission of high power microwave (HPM) radiation is the interface between dielectric-vacuum or even more severely between dielectric-air if HPM is to be radiated into the atmosphere. Surface flashover phenomena which occur at these transitions severely limit the power levels which can be transmitted. It is of major technological importance to predict surface flashover events for a given window geometry, material and power level. When considering an aircraft based high power microwave platform, the effects on flashover formation due to variances in the operational environment corresponding to altitudes from sea level to 50,000 feet (760 to 90 Torr; 1 Torr=133.3 Pa) are of primary interest. The test setup is carefully designed to study the influence of each atmospheric variable without the influence of high field enhancement or electron injecting metallic electrodes. Experimental data of flashover delay times across different materials, such as polycarbonate, Teflonreg, and high density polyethylene as a function of background pressure and gas type, air, N2, argon are discussed. An empirical relationship between flashover field amplitude and delay time is given.

Journals

Authors: Kim Morales; John Krile; Andreas Neuber; Hermann Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4286506

Abstract: Dielectric surface flashover along insulators in atmospheric conditions has only been empirically characterized over the years. The underlying physics involved in atmospheric flashover has yet to be understood, where parameters such as background gas, humidity, surface roughness, and temporal characteristics of the applied voltage play a role. Understanding the fundamental physical mechanisms and the extent to which these parameters influence the discharge behavior is vital to characterizing and modeling surface flashover for various structures and conditions. A solid state high voltage pulser with an adjustable pulse width of ~500 ns at FWHM and amplitudes in excess of 30 kV was developed to replicate the non-standard temporal shape of the transient voltage observed inside a rebar enforced building during a lightning strike. Based on experimental results, the phenomenology of pulsed unipolar surface flashover is discussed, with the emphasis on the influence of external parameters (applied voltage pulse shape/risetime, environment, UV illumination, humidity, etc.) on spatial and temporal discharge channel behavior.

Journals

Authors: John T. Krile; Andreas A. Neuber; Hermann G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4346186

Abstract: Summary form only given. Dielectric surface flashover in an atmospheric environment is an important consideration in the design of insulating support structures for pulsed, high voltage applications. Only recently, increased effort has been invested in characterizing and quantifying the physical processes involved in surface flashover occurring at atmospheric conditions. We have previously shown qualitatively that UV illumination of the surface, either externally or possibly generated by the developing discharge itself, affects the distance between flashover path and surface for small gaps with a nonnegligible field component normal to surface. By studying the effects of UV illumination on the flashover behavior, information was gained about the underlying mechanisms of dielectric surface flashover. Utilizing a solid-state UV source with a much faster turn-off time than gas tubes along with the flashover testing apparatus' generally high temporal resolution enabled us to measure applied UV pulse, voltage, current and flashover self luminosity with high temporal precision. For all experiments, the dielectric flashover sample is placed inside an environmentally controlled chamber with a constant gas flow. Using advanced field simulations the electrode/dielectric geometry was designed to produce an optimized field shape for the testing of surface effects. Besides reducing the flashover delay times by up to 50%, the application of a 1 mW/cm2, 20 mus UV pulse, prior to the voltage pulse, forces the flashover discharge in nitrogen to "hug" the surface rather than develop a few millimeters away from the surface along the field lines. Increasing the time delay between UV and voltage pulse application, it was revealed that the impact of the UV pulse on the flashover path becomes weaker with a time constant, tau, of ~ 3.1 mus. That is, after roughly 10 mus (3 times tau), the flashover path develops as without any UV application at all. In addition to experimental data of dielectric surface flashover with varying degrees of UV illumination, we present a simple model that describes the observed behavior as well as a more advanced analysis utilizing a Monte Carlo type code for the electron collision dynamics.

Conferences

Authors: J. Krile; A. Neuber; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4652476

Abstract: Undesired surface flashover of high voltage support structure can severely limit the compactness of open air high voltage systems. Only recently, increased effort has been invested in characterizing and quantifying the physical processes involved in surface flashover occurring under atmospheric conditions and under the influence of UV illumination. Presently, UV flash lamps as well as a solid-state UV source, for their much faster turn-off times, are utilized in conjunction with a high temporal resolution testing apparatus. The UV pulse, voltage, current and flashover self luminosity will be measured with high temporal precision. A simple model that describes the observed behavior as well as a more advanced analysis utilizing a Monte Carlo type code for the electron collision dynamics will be presented.

Conferences

Authors: D. McCauley; D. Belt; J. Mankowski; J. Dickens; A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4345962

Abstract: Helical Flux Compression Generators (HFCG) of 50 mm form factor have been shown to produce a maximum energy deposit of 3 kJ into a 3 muH inductor from a seed current. A large dl/dt into a coupled load is possible when an electro-explosive fuse is used. Previous work with a non-optimized fuse has produced ~100 kV into a 15Omega load which leads into a regime relevant for High Power Microwave (HPM) systems. It is expected that ~3()0kV can be achieved with the present 2 stage HFCG driving an inductive storage system with an electro-exploding fuse. In order to optimize the electro-explosive fuse design, a non-explosive test bed, which closely simulates the 45 kA HFCG output, is used. To optimize the tiise, effects of fuse material, fuse length, and fuse shape will be examined as well as the effects of various quenching materials. Our previous work has characterized fuse material but we are also looking into the effects of the processes used to create the fuse wire, such as tempered wire versus fully annealed wire. Additionally, to maximize the output voltage and minimize the fuse recovery time, we are optimizing the length of the fuse wire. For shorter fuse lengths, we are optimizing fuse shape as well as fuse length to find the best fuse recovery time. By optimizing the individual parameters of an electro-explosive fuse, the fuse as a whole will be optimized to produce maximum output voltage when used with an HFCG.

Conferences

Authors: Bell, D; Mankowski, J; Neuber, A; Dickens, J; Kristiansen, M

PDF: https://ieeexplore.ieee.org/document/4530702

Abstract: Helical Flux Compression Generators coupled with an inductive energy storage system have shown promising results as a driving source for High Power Microwave (HPM) loads. The output performance of the inductive energy storage system is contingent upon the opening switch scheme, usually an electro-explosive fuse. Our previous work involving fuse parameter characterization has established a baseline for potential fuse performance. By applying this fuse characterization model to an HFCG powered system, a non-optimized fuse has produced 60 kV into an HPM equivalent load with an HFCG output of 15 kA into a 3 mu H inductor. Utilization of a non-explosive HFCG test-bed has produced 36 kV into an HPM equivalent load with an output of 15 kA into a 1.3 mu H inductor. The use of a non-explosive HFCG test bed will allow the verification of scalability of the fuse parameter model and also allow testing of exotic fuse materials. Prior analysis of fuse parameters has been accomplished with various materials including Silver (Au), Copper (Cu), and Aluminum (Al), but particular interest resides in the use of Gold (Ag) fuse material. We will discuss the a-priori calculated baseline fuse design and compare the experimental results of the gold wire material with the silver wire material baseline design. With the results presented, an accurate Pspice model applicable to our 45 kA HFCG systems will be available and allow the development of accurate modeling for higher current systems.

Conference Paper/Presentation

Authors: D. McCauley; D. Belt; J. Mankowski; J. Dickens; A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4652362

Abstract: Helical Flux Compression Generators (HFCG) of 50 mm form factor have been shown to produce a maximum energy deposit of 3 kJ into a 3 μH inductor from a seed current. A large dI/dt into a coupled load is possible when an electro-explosive fuse is used. Previous work with a non-optimized fuse has produced ∼100 kV into a 15 Ω load which leads into a regime relevant for High Power Microwave (HPM) systems. It is expected that ∼300 kV can be achieved with the present 2 stage HFCG driving an inductive storage system with an electro-exploding fuse. In order to optimize the electro-explosive fuse design, a non-explosive test bed, which closely simulates the 45 kA HFCG output, is used. To optimize the fuse, effects of fuse material, fuse length, and fuse shape will be examined as well as the effects of various quenching materials. Additionally, to maximize the output voltage and minimize the fuse recovery time, we are optimizing the length of the fuse wire. For shorter fuse lengths, we are optimizing fuse shape as well as fuse length to find the best fuse recovery time. By optimizing the individual parameters of an electro-explosive fuse, the fuse as a whole will be optimized to produce maximum output voltage when used with an HFCG.

Conferences

Authors: C. Lynnx; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4651971

Abstract: It is known that PMMA and PVC become conductive under shock compression. However, to develop an opening switch, the material’s recovery from conductor back to insulator is critical. Heating must be minimized in order to optimize recovery. Two sources of heat in this switch include shock induced heating and switch loss heating, which should be balanced for optimal results. Furthermore, it is also important to determine if the observed switching behavior is due to shock unloading or intrinsic material relaxation properties. In the extreme case of recovery, bulk breakdown may become an issue. This paper presents experimental data and discusses initial results as they relate to the development of an opening switch.

Conferences

Authors: Andrew J. Young; Thomas A. Holt; Mohamed A. Elsayed; Andreas A. Neuber; M. Kristiansen; L.L. Altgilbers; A.H. Stults

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4346025

Abstract: Compact pulsed power systems require power sources that are small in size yet can produce the necessary electrical energy required to drive the system. Helical magnetic flux compression generators (HFCGs) are attractive for single shot applications due to their rapid conversion of chemical energy to electrical energy. The small total volume of a generator coupled with the energy density of the fast-reacting high explosives makes mid-sized HFCGs an appealing option as sources in single shot compact pulsed power systems. Consistent output current and energy gain from shot to shot are key variables in the ability of an HFCG to drive compact pulsed power systems efficiently.

Conferences

Authors: A. Young; T. Holt; M. Elsayed; A. Neuber; M. Kristiansen; L.L. Altgilbers; A.H. Stults

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4652394

Abstract: Compact Pulsed Power Systems (CPPSs) require power sources that are small in size yet can produce the necessary electrical energy required to drive a given load. Helical Flux Compression Generators (HFCGs) are attractive for single shot applications due to their rapid conversion of chemical energy to electrical energy. Mid-sized generators occupy little total volume (∼4,000-cm3 total with a compressible volume of ∼300-cm3 in the present generator design), while the high explosives used in an HFCG provide an energy density of ∼8,000 MJ/m3. Consistent output current and energy gain from shot to shot are key variables in the ability of an HFCG to drive CPPSs effectively. An investigation into the practicality of using mid-sized HFCGs as the driver for single shot CPPSs is presented. Data and waveforms from generators fired into 3 μH inductive loads are shown, with results measuring the generator’s performance as a driver for an inductive energy storage (IES) system. Results are also shown from adding a power conditioning system to the output of the HFCG, where the measurements demonstrate the ability of an HFCG to drive high impedance loads. The effectiveness of a mid-sized HFCG as drivers for these systems will be evaluated.

Conferences

Authors: Gregory F. Edmiston; Andreas A. Neuber; John T. Krile; Luke McQuage; Hermann Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4345565

Abstract: Summary form only given. Surface flashover formation at dielectric/air interfaces during pulsed high power microwave (HPM) excitation can severely limit the power densities which can be transmitted into atmospheric medium. Previous studies on HPM surface flashover in the S-band at 5 MW power levels have reported on the contributing factors to flashover development including the effects of gas type, pressure and relative humidity. Furthermore, analysis on optical emission spectra collected from the developing discharge has determined that the vibrational and rotational temperatures of the plasma are approximately 2700 degK and 300 degK, respectively. In addition to experimental efforts, a Monte Carlo-type electron motion simulation code, MC, has been developed to calculate the increasing electron density during flashover formation. Results from this code have exhibited a quantitative agreement with experimental data over a wide range of atmospheric conditions. A critical parameter to flashover development is the stochastic process involving the appearance of initiatory or "seed" electrons, as seen by the reduction in flashover delay time by approximately 10-20% in the presence of external UV illumination. While the current version of the MC code seeds the flashover location with electron densities on the order of background ion densities produced by cosmic radiation, it fails to incorporate the field assisted collisional detachment processes which are the primary origin of these electrons on the time scales of interest. Investigation of these processes and development of more accurate seeding in the MC code is a key step towards predicting HPM flashover over a wide range of parameters, particularly in the presence of highly electronegative gasses such as SF6 or O2, in which there is an absence of free electrons. Theoretical results of HPM surface flashover with the improved seeding model will be benchmarked against previously measured data obtained with HPM pulse excitation. Further, the slow rise-time data (~500-600 ns risetime) that revealed a distinct reduced field vs. pressure delay time product dependence will be supplemented by short rise-time pulse data.

Conferences

Authors: G. F. Edmiston; A. A. Neuber; J. T. Krile; L. McQuage; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4651836

Abstract: Surface flashover formation at dielectric/air interfaces during pulsed high power microwave (HPM) excitation can severely limit the power densities which can be transmitted into atmospheric medium. Previous studies on HPM surface flashover in the S-band at 5 MW power levels have reported on the contributing factors to flashover development including the effects of gas type, pressure and relative humidity. In addition to experimental efforts, a Monte Carlo-type electron motion simulation code, MC, has been developed to calculate the increasing electron density during flashover formation. Results from this code have exhibited a quantitative agreement with experimental data over a wide range of atmospheric conditions. A critical parameter to flashover development is the stochastic process involving the appearance of initiatory or “seed“ electrons, as seen by the reduction in flashover delay time by approximately 10–20% in the presence of external UV illumination. While the current version of the MC code seeds the flashover location with electron densities on the order of background ion densities produced by cosmic radiation, it fails to incorporate the field assisted collisional detachment processes which are often assumed to be the primary origin of these electrons on the time scales of interest. Investigation of these processes and development of more accurate seeding in the MC code is a key step towards predicting HPM flashover over a wide range of parameters, particularly in the presence of highly electronegative gasses such as SF6 or O2, in which there is an absence of free electrons with zero applied field.

Conferences

Authors: A. A. Neuber; G. F. Edmiston; J. T. Krile; H. Krompholz; J. C. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4033091

Abstract: The major limiting factor in the transmission of narrowband high-power microwaves (HPM) has been the interface between vacuum-vacuum or even more severely between vacuum-air if HPM are to be radiated into the atmosphere. Extensive studies have identified the physical mechanisms associated with vacuum/dielectric flashover, as opposed to the mechanisms associated with dielectric/air flashover, which are not as well known. Due to the high electron collision frequencies (in the terahertz range) with the background gas molecules, established mitigation methods and concepts of vacuum/dielectric flashover will have to be re-evaluated. The primarily limiting factors of HPM transmission through a dielectric/air interface are presented based on recent experiments at 2.85 GHz. The physics of the involved mechanisms and their practical ramifications are discussed. The potential of surface roughness/geometry for flashover mitigation is addressed as well

IEEE Journals

Authors: Thomas A. Holt; Andrew J. Young; Mohammed A. Elsayed; Andreas A. Neuber; M. Kristiansen; Kevin A. O'Connor; Randy D. Curry

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4346106

Abstract: Summary form only given. A cooperative effort was initiated between Texas Tech University and the University of Missouri-Columbia to develop a single-shot power conditioning system. The purpose of the system is to convert prime power to an output capable of driving a load with an impedance ranging from 15 to 30 Ohms. A helical flux compression generator (HFCG) was chosen as the electrical energy amplification stage due to its portability and high energy density. Certain topologies of IIFCGs are better suited to drive low impedance loads (i.e. short circuits or similar), however, cascaded HFCG systems are capable of driving higher impedance loads, thereby reducing the requirements from subsequent pulse forming stages to match the HFCG output to the load impedance. Therefore, a dual-stage HFCG was chosen to drive a transformer and series fuse in order to step-up the voltage to the 150 k V-300 kV level. A staged fuse has been designed to open in 280 ns or less with 25 kA-40 kA peak currents. The output voltage will be used to drive an RF load or to charge a mesoband oscillator. Both topologies for power conditioning are being considered and tests to date indicate that both types of geometries can be driven by the HFCG and power conditioning system. The results of the experimental tests as well as the energy transfer efficiency will be discussed.

Conferences

Authors: Thomas A. Holt; Andrew J. Young; Mohammed A. Elsayed; Andreas A. Neuber; M. Kristiansen; Kevin A. O'Connor; Randy D. Curry

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4652438

Abstract: A cooperative effort was initiated between Texas Tech University and the University of Missouri-Columbia to develop a single-shot power conditioning system to drive an RF load. The purpose of the system is to convert prime power to an output capable of driving a load with an impedance ranging from 15 to 30 Ohms. A Helical Flux Compression Generator (HFCG) was chosen as the electrical energy amplification stage due to its portability and high energy density. Certain topologies of HFCGs are better suited to drive low impedance loads (i.e. short circuits or similar), however, cascaded HFCG systems are capable of driving higher impedance loads, thereby reducing the requirements from subsequent pulse forming stages to match the HFCG output to the load impedance. Therefore, a dual-stage HFCG was chosen to drive a pulse transformer and series fuse in order to produce voltages on the order of 150 kV to 300 kV across the secondary of the pulse transformer. The fuse has been designed to open in 280 ns or less when a peak current of 25 kA-40 kA is reached. The output voltage will be used to drive an RF load or to charge a mesoband oscillator. Both topologies for power conditioning are being considered and tests to date indicate that both types of geometries can be driven by the HFCG and power conditioning system. The results of the experimental tests as well as the energy transfer efficiency will be discussed.

Conferences

Authors: C. Lynn; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4345862

Abstract: Summary form only given. The feasibility of utilizing shock loaded and unloaded dielectrics as a true closing/opening switch as part of an explosive-driven pulse power system is addressed. While it is known that shock wave compressed PVC and PMMA become conductive, the details of the material's recovery from the conducting back to the insulating state are much less known. To be effective as an opening switch, the recovery time has to be minimized, i.e., for instance, heating of the material must be minimized. The two primary sources of heat in the switch are shock induced heating and switch loss heating. These sources must be balanced for optimal results. Furthermore, it is also important to determine if the observed temporal behavior is due to finite shock unloading or intrinsic material relaxation properties. In the extreme case, bulk breakdown may occur during recovery as voltage increases across the switch. Previous work, performed primarily with a C-4 packed compression rod, has achieved a switch on-state resistance of less than 1 ohm, with an average on-state duration of 80 microseconds. It was also shown that PMMA appears to have a much sharper transition time than PVC, between both insulator to conductor, and conductor to insulator. The results presented here use a timed explosion to more carefully control the intensity and duration of the shock wave. This allows for more control over shock induced heating of the sample. We will present recent experimental data and discuss results as they relate to the development of an opening switch.

IEEE Conferences

Authors: J. T. Krile; R. Vela; A. A. Neuber; H. G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4343168

Abstract: A recently upgraded laser-triggered gas switch at Sandia National Laboratories has developed a failure mode that results in the breakdown spark tracking to the inside of the containment envelope. These breakdowns along the surface, or surface flashovers, degrade the performance of the overall switch, causing the switch to prefire in the successive shot. In the following, experimental results of pulsed surface flashover across different dielectric materials in SF6, primarily at atmospheric pressure, as well as flashover and volume breakdown in at pressures from 1.3 to 365.4 kPa are presented. In addition to fast voltage and current monitoring of the breakdown event, an increased emphasis was put on imaging the event as well as gathering optical emission spectra (~200-700 nm) from it. As much as possible, the small-scale experiments were designed to reproduce, at least partly, the conditions as they are found in the large 5-MV switch. An effort was made to determine what changes could be made to reduce the occurrence of surface flashovers, in addition to some broadly applicable conclusions on surface flashovers in an SF6 environment.

IEEE Journals

Authors: Russell Vela; John T. Krile; Andreas A. Neuber; Herman G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4346336

Abstract: Summary form only given. Volume and surface discharges in SF6 are investigated to support refurbishment of the Z-machine's laser triggered gas switch, LTGS, at Sandia National Laboratories (SNL). The recently upgraded LTGS has exhibited a failure mode which results in surface flashover on the inside of the dielectric switch envelope. These flashover events degrade the LTGS performance and cause a pre-fire in the successive shot. The principal physical mechanisms involved in surface flashover at various pressures of SF6 are therefore vital to the understanding of this specific switch failure mode. A 340 kV system was designed which replicated conditions found in the LTGS between two adjacent backbone electrodes. Besides measurement of flashover delay times for different dielectric materials (Lexan, acrylic, epoxies, etc.), emphasis was put on the detailed characterization of the flashover phenomenology. Hence, in addition to voltage and current diagnostics, fast optical imaging of the flashover along with the acquisition of optical emission spectra (~200 nm to 700 nm) was employed. One possible reason for the LTGS failure was thought to be ultraviolet (UV) radiation emitted from the volume discharge between the backbone electrodes, which could induce surface flashover on the inside of the envelope. Our present setup is utilized to gather evidence that would confirm this possibility by adding a field-stressed dielectric surface directly exposed to the volume arc produced between the model backbone electrodes. With a current amplitude as low as ~ 2 kA, the occurrence of sulfur and fluorine ions as well as hydrogen and carbon during dielectric surface flashover was revealed with optical emission spectroscopy. From the spectra, the plasma temperature was determined to be between 1.5 and 3 eV, largely depending on the SF6 pressure. The presence of carbon and hydrogen is of significance because it indicates removal of material from the dielectric surfaces during discharge. X-ray fluoroscopy has also confirmed sulfur deposits on the surfaces of the dielectrics tested. Experimental results with higher current amplitudes will be discussed as well as the results showing the UV's influence on inducing surface flashover in an SF6 environment.

Conferences

Authors: Russell Vela; John T. Krile; Andreas A. Neuber; Hermann G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4652547

Abstract: Volume and surface discharges in SF6 are investigated to support refurbishment of the Z-machine’s Laser Triggered Gas Switch, LTGS, at Sandia National Laboratories (SNL). The recently upgraded LTGS has exhibited a failure mode which results in surface flashover on the inside of the dielectric switch envelope. These flashover events degrade the LTGS performance and cause a pre-fire in the successive shot. In the following, experimental results of both volume breakdown and surface flashover across different dielectric materials in SF6 primarily at 20 psig are presented. In addition to fast voltage and current monitoring of the breakdown events, there was an emphasis put on imaging and optical emission spectra (∼200 nm to ∼700 nm). As much as possible, the laboratory experiment was designed to reproduce the conditions found in the 5.5 MV LTGS. The principal physical mechanisms involved in surface flashover at various pressures of SF6 are vital to the understanding of this specific switch failure mode.

Conferences

Authors: W. Justis; J. Chaparro; H. Krompholz; L. Hatfield; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4346256

Abstract: Summary form only given. With a RADAN 303 A pulser (risetime 150 ps, maximum voltage 150 kV into matched load), fast breakdown in argon and air is investigated. An oil-filled coaxial transmission line is coupled with a lens to a biconical section and a radial millimeter size gap operated at sub-atmospheric pressure. On the other side of the gap, the arrangement is symmetrically continued to represent a matched load. Pulse risetime at the gap is increased to about 180 ps. With capacitive dividers the voltage across the transmission line separating incident and reflected pulses is measured, which allows to determine voltage across and current through the gap. Temporal resolution is defined by the digitizer (20 Gs/s, 6 GHz). Breakdown usually happens during the rising part of the applied voltage pulse. Breakdown curves, i.e. breakdown voltage or time-to-breakdown vs. pressure, have been measured for different applied dV/dt's (from 2times1014 V/s to 8times1014 V/s) and they resemble Paschen curves with a steep increase toward low pressure, and a slow increase toward high pressure. The major findings, such as shifts of the minimum formative time toward increasing pressure with increasing dV/dt, are discussed in terms of similarity laws. Discharges for this case are characterized by runaway electrons over much of the pressure range, with a strong excitation and ionization layer at the cathode surface, and "free-fall" conditions with negligible gaseous ionization for the rest of the gap. Monte-Carlo simulations for the initial stage of the discharge are expected to confirm and quantify the experimental findings.

Conferences

Authors: W. Justis; J. Chaparro; H. Krompholz; L. Hatfield; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4652506

Abstract: With a RADAN 303 A pulser (risetime 150 ps, maximum voltage 150 kV into matched load), fast breakdown in argon and air is investigated. An oil-filled coaxial transmission line is coupled with a lens to a biconical section and a radial millimeter size gap operated at sub-atmospheric pressure. On the other side of the gap, the arrangement is symmetrically continued to represent a matched load. Pulse risetime at the gap is increased to about 180 ps. With capacitive dividers the voltage across the transmission line separating incident and reflected pulses is measured, which allows to determine voltage across and current through the gap. Temporal resolution is defined by the digitizer (20 Gs/s, 6 GHz). Breakdown usually happens during the rising part of the applied voltage pulse. Breakdown curves, i.e. breakdown voltage or time-to-breakdown vs. pressure, have been measured for different applied dV/dt’s (from 2x1014V/s to 8x1014 V/s) and they resemble Paschen curves with a steep increase toward low pressure, and a slow increase toward high pressure. The major findings, such as shifts of the minimum formative time toward increasing pressure with increasing dV/dt, are discussed in terms of similarity laws. Discharges for this case are characterized by runaway electrons over much of the pressure range, with a strong excitation and ionization layer at the cathode surface, and “free-fall” conditions with negligible gaseous ionization for the rest of the gap. Monte-Carlo simulations for the initial stage of the discharge are expected to confirm and quantify the experimental findings.

Conferences

Authors: Luke McQuage; Gregory Edmiston; John Mankowski; Andreas Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4345564

Abstract: Summary form only given. High power microwave (HPM) surface flashover is investigated in order to gain a better understanding of this phenomenon and reduce the limitations it imposes on transmitted power levels. The experimental setup is designed to produce window flashover without the influence of a triple point. The HPM source for this testing is an experimental virtual cathode oscillator (vircator) capable of producing greater than 50 MW for 100 ns with an adjustable frequency from 3 to 5 GHz. This work builds on previous testing using a magnetron producing 5 MW for 4 mus at 2.85 GHz. The dominant modes of the vircator and magnetron are the circular TE11 and rectangular TE10 modes respectively, with the electric field component in both setups normal to the direction of propagation, yielding comparable conditions. Due to the large differences in output power and pulse length, the two setups operate in different regimes and mechanisms take on differing degrees of importance. Additional factors under investigation include gas pressure, composition, temperature, and air speed. Diagnostic equipment permits the analysis of power levels with sub-nanosecond resolution. Experimental results are compared with data from literature, previous testing, and Monte Carlo simulations.

Conferences

Authors: L. McQuage; G. Edmiston; J. Mankowski; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4651835

Abstract: High Power Microwave (HPM) surface flashover is investigated in order to gain a better understanding of this phenomenon and reduce the limitations it imposes on transmitted power levels. The experimental setup is designed to produce window flashover without the influence of a triple point. The HPM source for this testing is an experimental virtual cathode oscillator (vircator) capable of producing greater than 50 MW for 100 ns with an adjustable frequency from 3 to 5 GHz. This work builds on previous testing using a magnetron producing 5 MW for 4 μs at 2.85 GHz. The dominant modes of the vircator and magnetron are the circular TE11 and rectangular TE10 modes respectively, with the major electric field component in both setups normal to the direction of propagation, yielding comparable conditions. Due to the large differences in output power and pulse length, the two setups operate in different regimes and mechanisms may take on differing degrees of importance. The experimental setup permits study of factors including gas pressure, composition, temperature, and air speed. Diagnostic equipment allows the analysis of power levels with sub-nanosecond resolution. Experimental results are compared with data from literature, previous testing, and Monte Carlo simulations.

Conferences

Authors: J. T. Krile; R. Vela; A. A. Neuber; H. G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4287051

Abstract: The Z-machine, which is located at Sandia National Laboratories, is currently undergoing refurbishment to increase the output drive current. Due to increased switching voltage requirements, some switch failure modes have been identified with the laser-triggered gas switch design, including envelope surface flashover. In order to improve the performance and lifetime of these switches, a basic understanding of the underlying physics of the failure mechanisms is required. A small-scale experimental setup has been constructed to approximate conditions within the switch. The possible impact of the SF6 volume spark between the switch electrodes on the envelope surface flashover is investigated. Measured optical spectra of the SF6 volume spark over a wide pressure range, from rough vacuum to 40 psig overpressure, are analyzed regarding their potential to contribute to switch failure.

IEEE Journals

Authors: J. Chaparro; W. Justis; H. Krompholz; L. Hatfield; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4346257

Abstract: Summary form only given. The X-ray emission of highly overvolted spark gaps under electron runaway conditions is investigated. The pulse source, a RADAN 303 A, is connected to a test chamber through an oil-filled coaxial line, a coupling lens, and a biconical transmission line section, with a symmetrical arrangement attached on the opposite side of the chamber with a matching load. The test chamber allows pressure variation from 10-6-670 torr with argon or dry air used as a background gas. Voltage pulses with amplitudes of 40-150 kV, risetimes less than 200 ps, and FWHM less then 300 ps are applied across hemispherical electrodes with 1 mm spacing. A scintillator-photomultiplier combination with a temporal resolution of 2 ns is used as X-ray detector. Metallic absorber foils of different thicknesses are used to obtain a rough energy spectrum of the x-rays and electrons in the range of about 10 to 150 keV. Results show a high electron-energy component (>60 keV) existing up to atmospheric pressure, and an intense soft component (5 to 20 keV) at pressures around 100 torr. The observations are compatible with gaseous ionization and runaway conditions for extremely high E/p.

Conferences

Authors: J. Chaparro; W. Justis; H. Krompholz; L. Hatfield; A. Neuber; T. Gibson

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4652507

Abstract: The x-ray emission of highly overvolted spark gaps under electron runaway conditions is investigated. The pulse source, a RADAN 303 A, is connected to a test chamber through an oil-filled coaxial line, a coupling lens, and a biconical transmission line section, with a symmetrical arrangement attached on the opposite side of the chamber with a matching load. The test chamber allows pressure variation from 10−6 – 670 torr with argon or dry air used as a background gas. Voltage pulses with amplitudes of 40–150 kV, risetimes less than 200 ps, and FWHM less then 300 ps are applied across hemispherical electrodes with 1 mm spacing. A scintillatorphotomultiplier combination with a temporal resolution of 2 ns is used as X-ray detector. Metallic absorber foils of different thicknesses are used to obtain a rough energy spectrum of the x-rays and electrons in the range of about 10 to 150 keV. Results show a high electron-energy component (≫60 keV) existing up to atmospheric pressure, and an intense soft component (5 to 20 keV) at pressures around 100 torr. The observations are compatible with gaseous ionization and runaway conditions for extremely high E/p.

Conferences

Authors: Thomas A. Holt; Andrew J. Young; Andreas A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4530689

Abstract: Performance reproducibility is a necessity when considering sources for single-shot, high-voltage applications. Helical Flux Compression Generators (HFCGs) are attractive for a variety of single-shot applications and are capable of high energy amplification that can be used in conjunction with other pulse-shaping techniques such as an exploding wire fuse for achieving high output voltages [1,2]. Small scale HFCGs (with active volumes on the order of ~100-200 cm3), however, are known to perform unreliably from shot to shot [3] and can lose as much as 80% of the flux available in the system based on previous experience with small to mid-sized HFCGs [4]. The performance variation is often attributed to erratic armature expansion behavior and/or fabrication methods and tolerances [3, 4]. As the compressible volume increases, HFCGs are known to conserve more flux and perform more reliably [2]. A fabrication method is presented for a midsized (with active volumes on the order of ~300-400 cm3) dual-stage HFCG that aims to improve the reproducibility in shot to shot performance with the goal of increasing the appeal for use of HFCGs in single-shot pulsed-power applications. Results of experiments with inductive loads of ~3μH are discussed.

Conferences

Authors: D. Belt; J. Mankowski; A. Neuber; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4216234

Abstract: Helical flux compression generators (HFCG) of a 50 mm form factor have been shown to produce output energies on the order of ten times the seeded value and a typical deposited energy of 3 kJ into a 3 muH inductor. Our previous work with a non-optimized fuse has produced-100 kV into a 15 load, which leads into a regime relevant for high power microwave (HPM) systems. It is expected that-300 kV can be achieved with the present 2-stage HFCG driving an inductive storage system with electro-exploding fuse. In order to optimize the electro-explosive wire fuse, we have constructed a non-explosive test bed which simulates the HFCG output with high accuracy. We have designed and implemented a capacitor based, magnetic switching scheme to generate the near exponential rise of the HFCG. The varying inductance approach utilizes 4 stages of inductance change and is based upon a piecewise linear regression model of the HFCG waveform. The non-explosive test bed will provide a more efficient method of component testing and has demonstrated positive initial fuse results

Conferences

Authors: M. Butcher; A. A. Neuber; M. D. Cevallos; J. C. Dickens; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1621332

Abstract: With a fast coaxial test setup using high speed electrical and optical diagnostics, prebreakdown current pulses and shadowgraphy images are measured for direct current (dc) breakdown in Univolt 61 transformer oil. Also, dc currents across the gap are measured using a high sensitivity electrometer. The conduction and breakdown mechanisms in transformer oil as function of applied hydrostatic pressures are quantified. Together, this information provides data on the development of current flow in the system. We have identified three stages in the conduction process prior to breakdown for highly nonuniform fields. Stage 1 is characterized by a resistive current at low fields. Increasing the applied electric field lowers the effective barrier at the metal/dielectric interface allowing a "tunneling" mechanism to begin, leading to the rapid rise in the injection current observed in stage 2. In stage 3, at high fields, the current reaches space charge saturation with an apparent mobility of 3/spl middot/10/sup -3/ cm/sup 2//V/spl middot/s prior to breakdown. The processes of final breakdown show a distinct polarity dependence. A strong pressure dependence of the breakdown voltage is recorded for negative needle/plane breakdown; a 50% reduction in breakdown voltage is observed when the hydrostatic pressure is lowered from atmospheric pressure to hundreds of mtorr. Positive needle discharges show a reduction of only about 10% in breakdown voltage for the reduced pressure case. Weak pressure dependence indicates the breakdown mechanism does not have a strong gaseous component. We will discuss possible links between conduction current and dc breakdown.

IEEE Journals

Authors: G. Edmiston; A. Neuber; J. Krile; L. McQuage; H. Krompholz; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4216218

Abstract: One of the major limiting factors for the transmission of high power microwave (HPM) radiation is the interface between dielectric-vacuum, or even more severely, between dielectric-air if HPM is to be radiated into the atmosphere. Surface flashover phenomena which occur at these transitions severely limit the power levels which can be transmitted. It is of major technical importance to predict surface flashover events for a given window geometry, material and power level. When considering an aircraft based high power microwave platform, the effects on flashover formation due to variances in the operational environment corresponding to altitudes from sea level to 50,000 feet (760 Torr to 90 Torr) are of primary interest. The test setup is carefully designed to study the influence of each atmospheric variable without the influence of high field enhancement or electron injecting metallic electrodes.

IEEE Conferences

Authors: Belt, D; Mankowski, J; Neuber, A; Dickens, J; Kristiansen, M

PDF: https://aip.scitation.org/doi/10.1063/1.2336757

Abstract: Helical flux compression generators (HFCGs) of a 50 mm form factor have been shown to produce output energies on the order of ten times the seeded value and a typical deposited energy of 3 kJ into a 3 mu H inductor. By utilizing an electroexplosive fuse, a large dI/dt into a coupled load is possible. Our previous work with a nonoptimized fuse has produced similar to 100 kV into a 15 Omega load, which leads into a regime relevant for high power microwave systems. It is expected that similar to 300 kV can be achieved with the present two-stage HFCG driving an inductive storage system with electroexploding fuse. In order to optimize the electroexplosive wire fuse, we have constructed a nonexplosive test bed which simulates the HFCG output with high accuracy. We have designed and implemented a capacitor based, magnetic switching scheme to generate the near exponential rise of the HFCG. The varying inductance approach utilizes four stages of inductance change and is based upon a piecewise linear regression model of the HFCG wave form. The nonexplosive test bed will provide a more efficient method of component testing and has demonstrated positive initial fuse results. By utilizing the nonexplosive test bed, we hope to reduce the physical size of the inductive energy storage system and fuse substantially. (c) 2006 American Institute of Physics.

Journal

Authors: D. Belt; J. Mankowski; A. Neuber; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4530702

Abstract: Helical Flux Compression Generators coupled with an inductive energy storage system have shown promising results as a driving source for High Power Microwave (HPM) loads. The output performance of the inductive energy storage system is contingent upon the opening switch scheme, usually an electro-explosive fuse. Our previous work involving fuse parameter characterization has established a baseline for potential fuse performance. By applying this fuse characterization model to an HFCG powered system, a non-optimized fuse has produced 60 kV into an HPM equivalent load with an HFCG output of 15 kA into a 3 μH inductor. Utilization of a non-explosive HFCG test-bed has produced 36 kV into an HPM equivalent load with an output of 15 kA into a 1.3 μH inductor. The use of a non-explosive HFCG test bed will allow the verification of scalability of the fuse parameter model and also allow testing of exotic fuse materials. Prior analysis of fuse parameters has been accomplished with various materials including Silver (Au), Copper (Cu), and Aluminum (Al), but particular interest resides in the use of Gold (Ag) fuse material. We will discuss the a-priori calculated baseline fuse design and compare the experimental results of the gold wire material with the silver wire material baseline design. With the results presented, an accurate Pspice model applicable to our 45 kA HFCG systems will be available and allow the development of accurate modeling for higher current systems.

Conferences

Authors: G. Edmiston; J. Krile; A. Neuber; J. Dickens; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1710041

Abstract: The major limiting factor in the transmission of high-power microwave (HPM) has been the interface between dielectric-vacuum or, even more severely, between dielectric-air, if HPM is to be radiated into the atmosphere. Extensive studies have identified the physical mechanisms associated with vacuum-dielectric flashover, as opposed to the mechanisms associated with air-dielectric flashover, which are not as well known. Surface-flashover tests involving high field enhancement due to the presence of a triple point have shown that volume breakdown threshold (dielectric removed) is approximately 50% higher than the flashover threshold with a dielectric interface over the 90-760 torr range. In order to quantify the role of field enhancement in the flashover process independent of electron injection from metallic surfaces, the effects of the triple point are minimized by carefully choosing the geometry, and in some cases, the triple point is "removed" from the flashover location. Experimental results were presented, including the impact of gas pressure and the presence of UV illumination, along with temperature analysis of the developing discharge plasma and temporally resolved images of the flashover formation. These results are compared with literature data for volume breakdown in air, with discussion on the similarities and differences between the data

IEEE Journals

Authors: H.G. Krompholz; L.L. Hatfield; A.A. Neuber; K.P. Kohl; J.E. Chaparro; Han-Yong Ryu

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1643325

Abstract: Volume breakdown and surface flashover in quasi-homogeneous applied fields in 10-5 to 600 torr argon and dry air are investigated, using voltage pulses with 150 ps risetime, <1ns duration, and up to 150 kV amplitude into a matched load. The test system consists of a transmission line, a transition to a biconical section, and a test gap, with gap distances of about 1mm. The arrangement on the other side of the gap is symmetrical. Diagnostics include fast capacitive voltage dividers, for determination of voltage waveforms in the gap, and conduction current waveforms through the gap. X-ray diagnostics use a scintillator-photomultiplier combination with different absorber foils yielding coarse spectral resolution. Optical diagnostics include use of a streak camera to get information on the discharge channel geometry and dynamics, and temporally resolved measurements with photomultipliers. Breakdown delay times are on the order of 100-400 ps, with minima occurring in the range of several 10torr. X-ray emission extends to pressures >100 torr, indicating the role of runaway electrons during breakdown. Maximum X-ray emission coincides with shortest breakdown delay times at several 10 torr. Simple modeling using the average force equation and cross sections for momentum transfer and ionization supports the experimental results

Journals

Authors: H. Veselka; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4216173

Abstract: Summary form only given. Investigations of shock induced conductivity of non-crystalline insulators and crystalline semiconductors were performed. Although we measured the insulator-to-metallic state transition time and conductivity, the focus of this investigation was on the recovery phase of the induced conductivity (i.e. metallic-to-insulator state transition). The recovery time and shock conditions were measured with high speed electrical diagnostics. The goal of this research is to determine the feasibility of using shock induced conductivity as a means of producing a high power opening switch. To minimize switch losses, the insulator-to-metallic transition time and conductance is also important, but has been more widely studied. Initial impact studies have shown that certain insulator can be conductive for 100 microseconds and recover under modest voltage less than one microsecond using a ten gram explosive charge. Various shock intensities are used in the study. The shock is produced primarily with conventional commercial explosives. In addition, the impact of sample thickness and compression duration on the induced the conductivity were also studied. The correlation between modest voltage and high voltage recovery time and shut-off current were studied

Conference Paper/Presentation

Authors: K. Morales; J. Krile; A. Neuber; H. Krompholz; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4216162

Abstract: Dielectric surface flashover along insulators in vacuum has been comprehensively researched over the years. However, the primary mechanisms involved in dielectric flashover at atmospheric pressures have yet to be as extensively analyzed with variable parameters such as electrode geometry, background gas, humidity, and temporal characteristics of the applied voltage. Understanding the fundamental physical mechanisms involved in surface flashover at atmospheric pressures is vital to characterizing and modeling the arc behavior. Previous DC and unipolar excitation experiments have shown distinct arc behavior in air and nitrogen environments for an electrode geometry that produces electric field lines that curve above the dielectric surface. Specifically, flashover arcs in an air environment were observed to develop along the dielectric surface. Experiments conducted in nitrogen revealed that the arc developed along the electric field lines, above the surface of the dielectric. It was also of importance to alter the temporal characteristics of the applied voltage to simulate lightning situations and investigate the impact on the arc behavior and voltage delay times. A solid state high voltage pulser with an adjustable pulse width of ~500 ns at FWHM and amplitudes in excess of 30 kV was specifically developed to replicate the temporal characteristics of a voltage pulse observed when a building structure is hit by a lightning strike. Based on these results, the physical mechanisms primarily involved in pulsed unipolar surface flashover will be discussed. Additional studies regarding the effects of humidity and surface roughness on the flashover arc behavior will also be presented

IEEE Conferences

Authors: K.P. Morales; J.T. Krile; A.A. Neuber; H.G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1667739

Abstract: Dielectric flashover along insulators in vacuum has been comprehensively researched in the past. Less studied, but of similar importance, is surface flashover at atmospheric pressures and the impact of an atypical electrode geometry, humidity, and ultraviolet (UV) illumination. Previous research has shown distinct discharge behavior in air and nitrogen environments for an electrode geometry in which the applied electric field lines curve above the dielectric surface. It was concluded that the discharge development path, whether along the electric field lines or the surface of the dielectric, is related to the oxygen content in the atmospheric background. It is believed that this dependence is due to the discharge's production of UV radiation in an oxygen rich environment. Thus, experiments were conducted in a nitrogen environment employing UV surface illumination in order to observe the affects on the flashover spark behavior. From the experimental data, it can be ascertained that UV illumination and intensity play a significant role in the discharge development path. Based on these results an explanation of the physical mechanisms primarily involved in unipolar surface flashover will be presented. Additional experiments regarding the effects of humidity on the discharge behavior will be discussed as well

Journals

Authors: Giesselmann, M; McHale, B; Neuber, A

PDF: https://ieeexplore.ieee.org/document/4216273

Abstract: We designed and tested several rapid capacitor chargers for rep-rated operation of low-inductance, compact Marx generators with rep-rates ranging from 10 Hz to 100 Hz. All chargers are designed to be packaged in cylindrical volumes with inside diameters in the range of 5 in - 12 in. Our capacitor chargers are based on H-Bridge inverters using ultra fast 600V class IGBTs. The high voltage is obtained by driving step-up transformers with nano-crystalline cores at 30 kHz. These chargers are capable of average DC output power levels of more than 5 kW for short time operation, during which the thermal inertia of the IGBT assembly provides effective cooling (up to seconds). To achieve reliable rep-rated operation of the chargers, we developed HV feedback sensors to monitor the charging process and solid state Marx-style trigger generators to command trigger the discharge of the main Marx.

Conference Paper/Presentation

Authors: Krile, J; Edmiston, G; Morales, K; Neuber, A; Krompholz, H; Kristiansen, M

PDF: https://ieeexplore.ieee.org/document/4084225

Abstract: Mechanisms in vacuum surface flashover caused by rf (f < 10 GHz) or unipolar voltages are virtually identical. Similarities between rf (representing high-power microwave window breakdown on the high-pressure side) and unipolar surface flashover are expected in an atmospheric environment as well. Two separate experimental setups were utilized to investigate both unipolar flashover and rf window flashover under atmospheric conditions while controlling excitation, temperature, pressure, humidity, and type of gas present, all under a similar electric field-surface geometry. The local electric field at the flashover initiating points has been numerically calculated in detail for all test geometries. For both rf and unipolar pulsed excitation, the flashover dynamics are changed by the application of UV light to the dielectric surface. A UV prepulse has a distinct impact on the arc's path and a tendency to decrease the hold-off electric field. The effect of humidity on the hold-off electric field for both pulsed unipolar and rf excitations, along with temporally resolved emission spectroscopy of the flashover event, is discussed.

Journal

Authors: J. T. Krile; A. A. Neuber; G. F. Edmiston; H. G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4216123

Abstract: Flashover along insulators or insulating support structures has to be carefully addressed in the design of any DC, AC, or pulsed high voltage device. Although there is a large body of data on unipolar surface flashover in the atmosphere, which has led to empirical design rules primarily for the power distribution industry, the physics of the involved processes is widely unknown. The major limiting factor in the transmission of high power microwaves (HPM) into the atmosphere has been the vacuum-air interface. Both the unipolar and HPM surface flashover cases have been studied under vacuum conditions and have been found to have the same dominant mechanisms. Similarities between HPM window flashover on the air side and unipolar flashover are observed in an atmospheric environment as well

Conferences

Authors: H.C. Kim; J.P. Verboncoeur; G.F. Edmiston; A.A. Neuber; Y.Y. Lau; R.M. Gilgenbach

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1666169

Abstract: In high-power microwave systems, we investigate the transition of breakdown from single surface vacuum multipactor discharge to collisional rf plasma in argon. As the gas pressure increases, electron-neutral collisions prevail against secondary electron emissions. In addition, the discharge formation time is obtained as a function of the gas pressure

Conferences

Authors: H. Krompholz; L. Hatfield; A. Neuber; J. Chaparro; H.-y. Ryu; W. Justis

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4105497

Abstract: Gas breakdown in quasi homogeneous electric fields with amplitudes of up to 3 MV/cm is investigated. The setup consists of a RADAN 303 A pulser and pulse sheer SN 4, an impedance-matched oil-filled coaxial line with a lens-transition to a biconical line in vacuum or gas, and an axial or radial gap with a width on the order of mm, with a symmetrical arrangement on the other side of the gap. Capacitive voltage dividers allow to determine voltage across as well as conduction current through the gap, with a temporal resolution determined by the oscilloscope sampling rate of 20 GS/s and an analog bandwidth of 6 GHz. The gap capacitance charging time and voltage risetime across the gap is less than 250 ps. Previous experiments at TTU with a slightly larger risetime have shown that breakdown is governed by runaway electrons, with multi-channel formation and high ionization and light emission in a thin cathode layer only. In argon and air, time constants for the discharge development have been observed to have a minimum of around 100 ps at several 10 torr. A qualitative understanding of the observed phenomena and their dependence on gas pressure is based on explosive field emission and gaseous ionization for electron runaway conditions.

Conferences

Authors: A. A. Neuber; Y. J. Chen; J. C. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084440

Abstract: The electrical characteristics and design features of a low inductance, compact, 500 kV, 500 J, 10 Hz repetition rate, Marx generator are discussed. While benefiting from the large energy density of mica capacitors, 4 mica capacitors were utilized in parallel per stage, keeping the parasitic inductance per stage low. Including the spark- gap switches, a stage inductance of 55 nH was measured, which translates with 100 nF capacitance per stage to ~18.5 Omega characteristic Marx impedance. Using solely inductors, ~1 mH each, as charging elements instead of resistors enabled charging the Marx within less than 100 ms with little charging losses. The pulse width of the Marx into a matched resistive load is about 200 ns with 50 ns rise-time. Repetitive HPM generation with the Marx directly driving a small Vircator has been verified. The Marx is fitted into a tube with 30 cm diameter and a total length of 0.7 m. We discuss the Marx operation at up to 21 kV charging voltage per stage, with repetition rates of up to 10 Hz in burst mode primarily into resistive loads. A lumped circuit description of the Marx is also given, closely matching the experimental results.

IEEE Conferences

Authors: J. Mankowski; Y. Chen; J. Dickens; A. Neuber; R. Gale

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084154

Abstract: Recent research efforts at TTU include the testing of a rapidly charged, rep-rated Marx generator driving a reflex triode vircator [1]. As expected, the burning of the cathode material (ordinary cloth velvet), was the primary failure mechanism during repetitive operation. In an effort to achieve a repetitive vircator (>10 Hz), we are exploring a low-cost, all-metal cathode. The cathode is made from aluminum with a patterned surface. A typical pattern is composed of peaks and troughs with dimensions on the order of tens of microns. The pattern is achieved with a simple, low-cost chemical etching process. Results include current, voltage, and microwave waveforms from two solid metal cathodes and a cloth velvet cathode.

IEEE Conferences

Authors: M. Cevallos; M. Butcher; J. Dickens; A. Neuber; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084448

Abstract: The development of cathode initiated low density channel formation and propagation leading to self breakdown in transformer oil is investigated using high speed electrical and optical diagnostics in a coaxial system with a point/plane axial discharge at various hydrostatic pressures. A cathode initiated channel formation and propagation model based upon single bubble dynamics has been presented by Kattan [1]. Experiments based on high-speed shadowgraphy were conducted to decide if the principles that govern single bubble dynamics could be applied to cathode initiated channel formation. These experiments show bubble motion away from the cathode, with separation velocities on the order of 10's m/s. This separation is similar for single bubbles generated at the cathode and for bubble chains developing into low-density channels. Lifetimes of these channels are recorded and show good correlation with the Rayleigh model [2] used to predict lifetimes of single bubbles. Experiments at reduced hydrostatic pressure reveal a critical pressure below which low density channel expansion occurs, further corroborating the presence of a gas phase. Finally, the pressure dependence of the breakdown voltage due to the expansion of the low density channels is examined and a model for this dependence is presented. The experiments conducted confirm the presence of a gas phase channel, its correlation with single bubble dynamics, and its importance to final breakdown.

IEEE Conferences

Authors: M. Cevallos; M. Butcher; J. Dickens; A. Neuber; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084384

Abstract: The physics of cathode initiated discharge formation leading to self breakdown in transformer oil is investigated using high speed electrical and optical diagnostics in a coaxial system with a point/plane axial discharge. Previous research conducted on self breakdown channel formation using high speed shadowgraphy and photography of the emitted light has shown tree-like structures for both cathode and anode initiated discharges, with characteristic differences. Cathode initiated discharges expand faster to a more "bushy" appearance, whereas anode initiated discharges show branching localized channels. So far, the spatial resolution to detect small luminous areas in pre- breakdown discharges and to determine their correlation to low density regions visible in the shadowgraphs was not sufficient in the experiments described in this paper, thus a systematic variation of exposure times, and time delays between luminosity pictures and shadowgraphy pictures has been performed. These experiments confirm that the luminosity emitted during prebreakdown events is generated from the low density regions seen in the shadowgraphy images, indicating charge amplification mechanisms in the gas phase for cathode initiated events. This statement is further supported by the dependence of both the channel dynamics and the light emission at lowered hydrostatic pressure.

IEEE Conferences

Authors: Chen, YJ; Neuber, AA; Mankowski, J; Dickens, JC; Kristiansen, M; Gale, R

PDF: https://aip.scitation.org/doi/10.1063/1.2093768

Abstract: The electrical characteristics and design features of a low inductance, compact, 500 kV, 500 J, 10 Hz repetition rate Marx generator for driving an high-power microwave (HPM) source are discussed. Benefiting from the large energy density of mica capacitors, four mica capacitors were utilized in parallel per stage, keeping the parasitic inductance per stage low. Including the spark-gap switches, a stage inductance of 55 nH was measured, which translates with 100 nF capacitance per stage to similar to 18.5 Omega characteristic Marx impedance. Using solely inductors, similar to 1 mH each, as charging elements instead of resistors enabled charging the Marx within less than 100 ms with little charging losses. The pulse width of the Marx into a matched resistive load is about 200 ns with 50 ns rise time. Repetitive HPM generation with the Marx directly driving a small virtual cathode oscilator (Vircator) has been verified. The Marx is fitted into a tube with 30 cm diameter and a total length of 0.7 m. We discuss the Marx operation at up to 21 kV charging voltage per stage, with repetition rates of up to 10 Hz in burst mode, primarily into resistive loads. A lumped circuit description of the Marx is also given, closely matching the experimental results. Design and testing of a low cost, all-metal Vircator cathode will also be discussed. (c) 2005 American Institute of Physics.

Journal

Authors: G. Edmiston; J. Krile; A. Neuber; J. Dickens; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084224

Abstract: The major limiting factor in the transmission of HPM has been the interface between dielectric-vacuum or even more severely between dielectric-air if HPM is to be radiated into the atmosphere. Extensive studies have identified the physical mechanisms associated with vacuum-dielectric flashover, as opposed to the mechanisms associated with air-dielectric flashover, which are not as well known. Surface flashover tests involving high field enhancement due to the presence of a triple point have shown that volume breakdown threshold (dielectric removed) is approximately 50% higher than the flashover threshold with a dielectric interface over the 90-760 torr range [1]. In order to quantify the role of field enhancement in the flashover process independent of electron injection from metallic surfaces, the effects of the triple point are minimized by carefully choosing the geometry and in some cases the triple point is "removed" from the flashover location. We will present experimental results, including the impact of gas pressure, and discuss possible causes for the difference in the rf-breakdown field with and without the interface/metallic triple point portion.

IEEE Conferences

Authors: M. Giesselmann; B. Palmer; A. Neuber; J. Donlon

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4084329

Abstract: We are reporting on a High-Voltage Impulse Generator, which consists of a step-up transformer, which is driven by new HV-IGBTs (High-Voltage Isolated Gate Bipolar Transistors). The new HV-IGBTs are individually packaged silicon-dies intended for Pulsed-Power Applications. The silicon dies are normally packaged in large modules for locomotive motor drives and similar traction applications. In our work we used the Powerex QIS4506001 discrete IGBT and the QRS4506001 discrete diode, both with a nominal rating of 4500V/60A, derived from continuous- duty applications. Our experiments have shown that the devices are capable of handling currents in excess of 1 kA during pulsed operation.

Conferences

Authors: Cevallos, MD; Butcher, M; Dickens, J; Neuber, A; Krompholz, H

PDF: https://ieeexplore.ieee.org/document/1420543

Abstract: The breakdown physics of transformer oil is investigated using high speed electrical and optical diagnostics. Experiments are done in self-breakdown mode utilizing a needle/plane geometry. Shadowgraphy combined with high-speed electrical diagnostics are aimed at measuring streamer expansion as a function of external pressure. Assuming a breakdown mechanism for negative needle based on bubble formation with subsequent carrier amplification in the gas phase implies a pressure dependence, which is observed in the experiments, i.e. the expansion velocity decreases with increasing pressure.

Journal

Authors: J. Krile; A. Neuber; J. Dickens; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1420431

Abstract: Using a gated intensified digital charge coupled device (ICCD) camera, the development of flashovers across a dielectric surface has been imaged in various gasses at atmospheric pressures. The arc displayed a strong tendency to develop close to the surface, as opposed to following the electric field line leading away from the surface, when oxygen is present in the environment. These findings along with spectroscopy data help to yield a better understanding of the processes involved in surface flashover.

IEEE Journals

Authors: M. Butcher; M. Cevallos; A. Neuber; H. Krompholz; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084425

Abstract: With a fast coaxial setup using a needle/plane geometry and a high sensitivity electrometer, conduction mechanisms in transformer oil at varying temperature and hydrostatic pressure are quantified. There are 3 stages in the conduction process prior to breakdown for highly nonuniform field geometries. Stage I is characterized by a resistive current at low fields. Stage II consists of a rapid rise in the injection current associated with increasing field due to a "tunneling" mechanism through the metal/dielectric interface. The transition from the resistive to tunneling stage occurs when the applied field reduces the barrier at the metal/insulator interface to a point where tunneling of charge carriers through the barrier begins. This transition point is polarity dependent. In stage III, at high fields the current reaches space charge saturation at electron mobilities >100 cm2/V*s prior to breakdown. The processes of final breakdown show distinct polarity dependence. Data for the negative needle exhibits strong pressure dependence of the breakdown voltage, which is reduced by 50% if the hydrostatic pressure is lowered from atmospheric pressure to hundreds of mtorr. Such a strong pressure dependence, at reduced hydrostatic pressure, indicates breakdown is gaseous in nature. This is supported by images of bubble/low density regions forming at the current injection point. Positive needle discharges show a reduction of only about 10% in breakdown voltage for the reduced pressure case. A weak pressure dependence indicates the breakdown mechanism does not have a strong gaseous component. We will discuss possible links between conduction current and DC breakdown.

IEEE Conferences

Authors: Qian, J; Joshi, RP; Kolb, J; Schoenbach, KH; Dickens, J; Neuber, A; Butcher, M; Cevallos, M; Krompholz, H; Schamiloglu, E; Gaudet, J

PDF: https://aip.scitation.org/doi/10.1063/1.1921338

Abstract: An electrical breakdown model for liquids in response to a submicrosecond (similar to 100 ns) voltage pulse is presented, and quantitative evaluations carried out. It is proposed that breakdown is initiated by field emission at the interface of pre-existing microbubbles. Impact ionization within the microbubble gas then contributes to plasma development, with cathode injection having a delayed and secondary role. Continuous field emission at the streamer tip contributes to filament growth and propagation. This model can adequately explain almost all of the experimentally observed features, including dendritic structures and fluctuations in the prebreakdown current. Two-dimensional, time-dependent simulations have been carried out based on a continuum model for water, though the results are quite general. Monte Carlo simulations provide the relevant transport parameters for our model. Our quantitative predictions match the available data quite well, including the breakdown delay times and observed optical emission. (C) 2005 American Institute of Physics.

Journal

Authors: D. Belt; J. Dickens; J. Mankowski; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084267

Abstract: Helical flux compression generators of small dimensions have been shown to produce energy output around 3 kJ into an inductive load. Adding a fuse opening switch has allowed us to produce 300 kV into a 15 Ohm load. We are investigating inductive energy storage with emphasis on an electro-explosive fuse opening switch in order to improve upon previous results. We have designed and constructed a non-explosive test bed composed of two pulse forming networks (PFN). Each PFN provides a linear approximation during two different time ranges of the exponential rise response of a typical HFCG. This approach will be more cost and time effective than to drive the fuse with an explosive generator. Our initial goal will be to simulate a 15 kA HFCG unit followed by the simulation of a 50 kA HFCG.

IEEE Conferences

Authors: K. P. Morales; J. T. Krile; A. A. Neuber; H. G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084426

Abstract: Dielectric flashover along insulators in vacuum has been sufficiently researched in the past. Less studied, but of similar importance, is surface flashover at atmospheric pressures and the impact of various electrode geometries, humidity, and type of gas present. Previous research has shown distinct arc behavior in air and nitrogen for an electrode geometry in which the electric field lines curve above the dielectric surface. Specifically, flashover experiments in nitrogen have shown that the arc path will follow the electric field lines, not the dielectric surface. As a result, it was concluded that the arc development path, whether along the electric field line or the surface of the dielectric, is related to the oxygen content in the atmospheric background. It is believed that this dependence is due to the arc's production of UV radiation in an oxygen rich environment. Further testing, in a pure nitrogen environment with UV illumination of the surface prior to the pulse application, has shown that UV plays a significant role in the arc development path. There is a near linear relationship between the percentage of liftoffs and the time delay between UV application and flashover. Additional studies have also shown a relationship between the UV intensity and the percentage of liftoffs. Based on these results we will discuss the physical mechanisms primarily involved in unipolar flashover at atmospheric pressure. Additional experimental results regarding the effects of humidity on the liftoff phenomenon will be presented as well.

IEEE Conferences

Authors: H. Veselka; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084322

Abstract: Investigations of shock induced conductivity of insulators and semiconductors both crystalline and noncrystalline, were performed. Although the insulator-to- metallic state transition time was measured, the focus of this investigation was on the recovery phase of the induced conductivity (i.e. metallic-to-insulator state transition). The recovery time and shock conditions were measured with high speed electrical diagnostic equipment. The goal of this research is to determine the feasibility of using shock induced conductivity as a means of producing a high power opening switch. To minimize switch losses, the insulator-to-metallic transition time and conductance is also important, but has been more widely studied. Various shock profiles and intensities and used in the study. The shock is produced primarily with conventional commercial explosives. Shock reverberation is planed in some tests to minimize sample heating and to produce tailored shock time profiles. In addition, the impacts of sample thickness and compression duration on the induced conductivity were also studied.

IEEE Conferences

Authors: J. Krile; G. Edmiston; A. Neuber; J. Dickens; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084225

Abstract: Mechanisms in vacuum flashover caused by rf (f<10 GHz) or unipolar voltages are virtually identical. Similarities between rf (representing high power microwave window breakdown on the high pressure side) and unipolar flashover are expected in an atmospheric environment as well. Our experimental setups enable studying both unipolar flashover and rf window flashover at atmospheric conditions while controlling excitation, temperature, pressure, humidity, and type of gas present. The local electric field at the flashover initiating points has been numerically calculated in detail for all test geometries. For both rf and unipolar pulsed excitation, the flashover dynamics are changed by the application of UV light to the dielectric surface. A UV pre-pulse has a distinct impact on the arc's path and a tendency to increase the hold-off electric field. The effect of humidity on the hold-off electric field for both pulsed unipolar and rf excitations, along with temporally resolved emission spectroscopy of the flashover event, will be discussed.

IEEE Conferences

Authors: J. Qian; R. P. Joshi; J. Kolb; K. H. Schoenbach; J. Dickens; A. Neuber; M. Cevallos; H. Krompholz; E. Schamiloglu; J. Gaudet

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084323

Abstract: An electrical breakdown model for liquids in response to a sub-microsecond (~ 100 ns) voltage pulse is presented, and quantitative evaluations carried out. It is proposed that breakdown is initiated by field emission at the interface of pre-existing micro-bubbles. Impact ionization within the micro-bubble gas then contributes to plasma development, with cathode injection having a delayed and secondary role. Continuous field emission at the streamer tip contributes to filament growth and propagation. This model can adequately explain almost all of the experimentally observed features, including dendritic structures and fluctuations in the pre- breakdown current. Two-dimensional, time-dependent simulations have been carried out based on a continuum model for water, though the results are quite general. Monte Carlo simulations provide the relevant transport parameters for our model. Our quantitative predictions match the available data quite well, including the breakdown delay times and observed optical emission.

IEEE Conferences

Authors: H. Krompholz; L.L. Hatfield; A. Neuber; D. Hemmert; K. Kohl; J. Chaparro

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4084242

Abstract: Volume breakdown and surface flashover in quasi homogeneous applied fields in 10-5 to 600 torr argon are investigated, using voltage pulses with 150 ps risetime, < 1 ns duration, and up to 150 kV amplitude into a matched load. The test system consists of a transmission line, a transition to a biconical section, and a test gap, with gap distances of one to several mm. The arrangement on the other side of the gap is symmetrical. An improved system, with oil-filled transmission lines and lens between coax and biconical section to minimize pulse distortion, is being constructed. Diagnostics include fast capacitive voltage dividers, which allow to determine voltage waveforms in the gap, and conduction current waveforms through the gap. X-ray diagnostics uses a scintillator- photomultiplier combination with different absorber foils yielding coarse spectral resolution. Optical diagnostics to obtain information about the discharge channel dynamics is in preparation. Breakdown delay times, and e-folding time constants for the conduction current during the initial breakdown phase, are on the order of 100-400 ps, with minima in the range of several 10 torr. X-ray emission extends to pressures > 100 torr, indicating the role of runaway electrons during breakdown. Maximum X-ray emission coincides with fastest current risetimes at several 10 torr, which is probably related to an efficient feedback mechanism from gaseous amplification to field enhanced electron emission from the cathode.

Conferences

Authors: A. Neuber; J. Krile; J. Dickens; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1433496

Abstract: In a wide variety of high voltage applications surface flashover plays a major role in the system's performance, and yet it has not been studied in great detail for atmospheric conditions with modern diagnostic tools. Environmental conditions to be considered include pressure, humidity, and gas present in the volume surrounding the dielectric. In order to gain knowledge into the underlying process involved in dielectric surface flashover, a setup has been created to produce and closely monitor the flashover event. Within the setup, parameters such as geometry, material, and temporal characteristics of the applied voltage can be altered. Current, voltage, luminosity, and optical emission spectra are measured with nanosecond to subnanosecond resolution. Spatially and temporally resolved light emission data is also gathered along the arc channel

IEEE Conferences

Authors: J. T. Krile; A. A. Neuber; J. C. Dickens; H. G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1347231

Abstract: Surface flashover is a major consideration in a wide variety of high-voltage applications, and yet has not been studied in great detail for atmospheric conditions, with modern diagnostic tools. Environmental conditions to be considered include pressure, humidity, and gas present in the volume surrounding the dielectric. In order to gain knowledge into the underlying process involved in dielectric surface flashover, a setup has been created to produce and closely monitor the flashover event. Within the setup parameters such as geometry, material, and temporal characteristics of the applied voltage can be altered. Current, voltage, luminosity, and optical emission spectra are measured with nanosecond to subnanosecond resolution. Spatially and temporally resolved light emission data is also gathered along the arc channel. Our fast imaging data show a distinct trend for the spark in air to closely follow the surface even if an electrical field with a strong normal component is present. This tendency is lacking in the presence of gases such as nitrogen, where the spark follows more closely the electric field lines and develops away from the surface. Further, the breakdown voltage in all measured gases decreases with increasing humidity, in some cases as much as 50% with an increase from 10% relative humidity to 90% relative humidity.

IEEE Journals

Authors: M. Butcher; A. Neuber; H. Krompholz; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1339891

Abstract: Summary form only given. Any attempt to model the complex interaction of hydrodynamic and electronic processes leading to breakdown in transformer oil suffers from the lack of microscopic transport data. Also, interface processes, such as electron emission from metal electrodes immersed in liquid, are poorly understood. As a first step toward the understanding of breakdown phenomenology, the voltage-current characteristics for pre-discharge conditions are measured. An experimental setup was constructed which allows temperature variations between 10/spl deg/C and 50/spl deg/C, at pressures between 0.5 and 3 bar. DC currents ranging from a few nA with a few kV of applied voltage, to a few /spl mu/A prior to full breakdown are measured using an electrometer. Preliminary results at NTP with a tip-plane geometry indicate Ohmic behavior at low voltages, Schottky emission at intermediate voltages, and saturation due to space charge at high voltages, and allow estimates on the physical parameters governing these effects. The indicated temperature range of the measurements is associated with a variation of the viscosity of a factor of 3, where we anticipate similar relative changes for the transport of electrons. The intermediate voltage range where a Schottky emission process is assumed will be emphasized. For instance, the electron mobility, derived from experimental data to about 0.06 cm/sup 2//Vs at NTP in the intermediate voltage range should distinctly vary with changing temperature and pressure.

IEEE Conferences

Authors: M. D. Cevallos; J. C. Dickens; A. A. Neuber; H. G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1340174

Abstract: Summary form only given. The fundamental breakdown physics of transformer oils is investigated with high-speed electrical and optical diagnostics with temporal resolution down to 500 ps. Univolt 63 and Envirotemp FR3 (biodegradable) are used for this study. The system set up employs a cable discharge into a coaxial system with point/plane axial discharge and load line to providing a matched terminating impedance. Overall, the impedance of the system is matched at 50 ohms throughout with the exception of the very narrow gap region and includes a 50 ohm load resistor terminating the load line. Self breakdown is achieved by applying up to 50 kV to the charging line. Pulsed breakdown is achieved by charging a pulse forming line with a two way transient time of 300 ns, up to 100 kV. The pulse forming line is then fed into the discharge line via an oil spark gap. Transmission line type current sensors and a capacitive voltage divider with fast amplifiers/attenuators are used in order to attain a complete range of information from amplitudes of 0.1 mA to 1 kA with temporal resolutions of 300 ps. Optical measurements are performed on low level light emission using fast photo-multiplier tubes (risetime of 800 ps) spatially resolved, supplemented with high speed and spectroscopic investigations on a nanosecond timescale. Breakdown voltages at gap distances of 5 mm for pre, self, and pulsed breakdown voltages are given, where breakdown with negative needle tips show 20% higher breakdown voltages than the ones with positive needle. Velocities of propagating "tree"-structures for the pre and self-breakdown are a few km/s while pulsed breakdown velocities are several 10's of km/s, with higher velocities for a negative needle. The trees have to reach the opposite electrode before full breakdown occurs. Simultaneous optical measurements for a single breakdown event are presented, such as the luminosity in comparison to shadowgraphy images, which is necessary to describe the complex interaction of hydrodynamic phenomena (channel and tree formation and propagation) and charge carrier multiplication.

IEEE Conferences

Authors: T. Namihira; D. Wang; A. Neuber; M. Butcher; J. Dickens; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1340266

Abstract: Summary form only given. Considering highly stressed dielectric liquids, the role of mechanisms such as photoionization in the liquid volume or photoeffect at the cathode for the development of dielectric breakdown is investigated. We used a pulsed 300 W Xenon light source (25 mm output window, 5 degree divergence) with a broad spectral range of 200 to 1100 nm to study the impact of the light beam focused either solely on the high field region between the breakdown electrodes or including the electrodes. Typical field strengths in the electrode gap (/spl sim/4 mm gap, 3 mm tip radius,) were 15 to 25 kV/cm resulting in a DC current amplitude (without light) of up to 2 nA (apparatus resolution /spl sim/10 pA). Standard transformer oil, Univolt 61, and a biodegradable oil, Environtemp FR3 (natural ester fluid), were examined in the present work. Both oils exhibit strong optical absorption in the UV. However, Univolt 61 has its cut off wavelength at 450 nm, while bio oil easily transmits down to 350 nm. Below the cutoff wavelength, virtually all radiation is absorbed within a few mm. When pulsing the Xenon lamp at /spl sim/500 microsec no increase in DC current amplitude (increase <10 pA) could be detected for either oil. Increasing the pulse length to several seconds lead to a distinct increase in current amplitude (up to 300 pA), however, only for Univolt 61. Such an increase in current amplitude can also be achieved by raising the temperature of the dielectric liquid by external heating (/spl sim/100 pA/K). The temperature levels leading to similar current amplitudes due to heating by the Xenon lamp or external heating are comparable. Since bio oil absorbs only below 350 nm, the temperature rise due to the light irradiation was comparably smaller than in Univolt 61. Thus, any heating and increase in current were less pronounced in bio oil. For both oils, the observed behavior can be entirely explained by thermal effects. Both, photoionization and photoeffect have seemingly a minor impact on breakdown development. The detailed discussions are given in the present work.

Conferences

Authors: A. A. Neuber; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1306688

Abstract: Magnetic flux compression generators offer the largest pulsed power output per unit size or weight when compared with other more conventional systems. They have found widespread use as pulsed power sources for hydrodynamics programs and high magnetic field research at national laboratories or in commercial applications, including exploration for oil and minerals and mine detection. Also, due to their nature as a true one-time-use device with superior energy density, a large portion of applications is defense related. A variety of basic magnetic flux compression generator designs have been developed and tested during the past four decades. All of them rely on the explosive-driven deformation of a system of conductors having an initial, preferably large, inductance. The most successful basic design is the helical flux compression generator, which is capable of producing a high-energy output into large impedance loads, just as it is needed for a practical pulsed power source. This paper will review the advances and state of the art of primarily helical magnetic flux compression generators mainly developed as pulsed power sources and will offer new insights gained as a result of a recently completed five-year AFOSR/DoD Multidisciplinary University Research Initiative program that studied the basic physics and engineering aspects of helical flux compression generators.

IEEE Journals

Authors: E. Schamiloglu; R.J. Barker; M. Gundersen; A.A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1306675

Abstract: In this introduction to the Special Issue on pulsed power and its applications, background information is provided for the nonspecialist to better understand the many challenges in designing pulsed power systems, and the wide diversity of applications that are now emerging. The approach to providing a tutorial on pulsed power technology is to make available to the reader the paper written by J. C. Martin which appeared in a Special Section of the Proceedings of the IEEE on pulsed power technology in June 1992. That paper is supplemented in this introduction with additional information that complements many of the invited papers composing this Special Issue.

Journals

Authors: E. Crull; H. Krompholz; A. Neuber; L. Hatfield

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1339921

Abstract: Summary form only given. Fast gas breakdown with formative times in the sub-nanosecond regime is of interest for pulsed power switching and UWB applications. Use of coaxial transmission lines with conical sections connected to a test gap enables to apply fast voltage pulses to the gap, as well as the simultaneous measurement of voltage across and current through the gap. For small pulse amplitudes, with risetimes of 400 ps, a tip-plane geometry is used, with radii of curvature of 0.5 /spl mu/m. At pulse amplitudes of 5 kV, and macroscopic field enhancements on the order of 1000, delay times between current and voltage of less than 200 ps for pressures larger than 100 torr are observed, in both argon and dry air. Corresponding current risetimes I/(dI/dt) are less than 100 ps. Using a high voltage pulser (RADAN 303B with pulse slicer SN4, risetime 150 ps at 150 kV amplitude) enables the comparison of formative times for the tip-plane geometry with those of more homogeneous field distributions in the gap.

Conferences

Authors: J.C. Hernandez; A.A. Neuber; J.C. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1347243

Abstract: Helical magnetic flux compression generators (MFCGs) are the most promising energy sources with respect to their current amplification and compactness. They are able of producing high current pulses required in many pulsed power applications with at least one order of magnitude higher energy density than capacitive storage with similar discharge characteristics. However, the main concern with MFCGs is their intrinsic flux loss that limits severely their performance and which is not yet well understood. In general, all flux losses have a differing degree of impact, depending on the generator's volume, current and energy amplification, size of the driven load, and angular frequency of armature-helix contact point. Although several computer models have been developed in the open literature, none of them truly quantify, starting from basic physics principles, the ohmic and intrinsic flux losses in helical MFCGs. This paper describes a novel method that provides a separate calculation of intrinsic flux losses (flux that is left behind in the conductors and lost for compression) and ohmic losses, being especially easy to implement and fast to calculate. We also provide a second method that uses a simple flux quantification, making a mathematical connection between the intrinsic flux losses, quantified by the first method, and the intrinsic flux losses observed in the generators. This second method can also be used to a priori estimate the MFCG performance. Further, we will show experimental and calculated data and discuss the physical efficiency limits and scaling of generator performance at small sizes.

Journals

Authors: J.A. Gaudet; R.J. Barker; C.J. Buchenauer; C. Christodoulou; J. Dickens; M.A. Gundersen; R.P. Joshi; H.G. Krompholz; J.F. Kolb; A. Kuthi; M. Laroussi; A. Neuber; W. Nunnally; E. Schamiloglu; K.H. Schoenbach; J.S. Tyo; R.J. Vidmar

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1306684

Abstract: Pulsed power is a technology that is suited to drive electrical loads requiring very large power pulses in short bursts (high-peak power). Certain applications require technology that can be deployed in small spaces under stressful environments, e.g., on a ship, vehicle, or aircraft. In 2001, the U.S. Department of Defense (DoD) launched a long-range (five-year) Multidisciplinary University Research Initiative (MURI) to study fundamental issues for compact pulsed power. This research program is endeavoring to: 1) introduce new materials for use in pulsed power systems; 2) examine alternative topologies for compact pulse generation; 3) study pulsed power switches, including pseudospark switches; and 4) investigate the basic physics related to the generation of pulsed power, such as the behavior of liquid dielectrics under intense electric field conditions. Furthermore, the integration of all of these building blocks is impacted by system architecture (how things are put together). This paper reviews the advances put forth to date by the researchers in this program and will assess the potential impact for future development of compact pulsed power systems.

Journals

Authors: M. Butcher; A. Neuber; H. Krompholz; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1339892

Abstract: Summary form only given. The scattering cross-sections, transport coefficients, and details of the breakdown mechanism needed for describing current conduction in transformer oil at high applied voltages are virtually unknown. This makes it extremely difficult, if not impossible, to properly model electrical breakdown in oil. To address this issue, we have measured the V-I characteristic of transformer oil in the point-plane geometry for /spl sim/3 mm gap widths. Three regimes can be distinguished. (1) For low voltages, V <3 kV, the relationship between voltage and current is linear, exhibiting just resistive behavior within the measurement accuracy. (2) At intermediate voltages, the dependence is linear on a Fowler-Nordheim plot. (3) Between the breakdown voltage V/sub B/ /spl sim/ 30 kV and 10 kV, the current is proportional to V/sup 2/, indicating space charge limited current. Assuming a Schottky-type emission mechanism for electrons injected into the liquid, we derive an electron mobility of /spl sim/6.0/spl times/10/sup -6/ m/sup 2//Vs, which is close to values reported in the literature before. Assuming Fowler-Nordheim emission leads to 3.8/spl times/10/sup -6/ m/sup 2//Vs. The ion mobility is believed to be more than one order of magnitude lower. At this point, it is difficult to gain any more detailed information on charge transport and possible multiplication based on simple analytical methods. Hence, we applied our numerical modeling techniques already proven in analyzing high-field phenomena in polar liquids (e.g., water), to evaluate current conduction and the breakdown process in non-polar oil. By comparing our calculations with the experimental data, we hope to characterize important transport parameters, such as the electron ionization coefficient as a function of the electric field. We also present our attempts to extract details of the field-dependent non-linear processes and electrode effects close to the breakdown regime.

IEEE Conferences

Authors: D. Hemmert; J. Mankowski; J. Rasty; A. Neuber; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1277997

Abstract: Modeling and characterization of a magnetic flux compression generator (MFCG) requires detailed knowledge of the changes in conductivity of the MFCG materials during the shock-loading phase. In the studies reported here, a thin metallic strip is shocked with an explosively generated shock wave produced from a charge of composition C-4. The shock wave is intended to simulate the shock wave and pressures produced in MFCG research currently being conducted at Texas Tech University. These pressures are estimated to be between 1 and 3 GPa. The experimental setup is arranged so that the shapes of the metallic strip and shock front are the same, as confirmed using optical fibers. This was to ensure that the test sample was shocked uniformly. The metallic test strip is pulsed with a 70 A current pulse during application of the shock wave. The current and voltage across the test sample are measured directly to determine the change in conductivity. Pressure measurements are conducted in separate tests under similar conditions using strain gauges. The results are then compared to results determined previously using a split Hopkinson pressure bar apparatus (SHPB).

IEEE Conferences

Authors: J. Rasty; X. Le; J. Dickens; A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1277998

Abstract: Helical magnetic flux compression generators (MFCGs) are capable of producing ultra-high power electric pulses by trapping and compressing a seed magnetic field into a load coil via an explosive-driven armature. The efficiency of helical MFCGs is generally very low, about 10%, due to large magnetic flux losses. One of the main sources of magnetic flux loss is the "turn-skipping" phenomenon, in which the expanding armature fails to establish contact with every turn of the helical coil, resulting in magnetic flux loss in the skipped turns of the coil. The "turn-skipping" phenomenon is related to non-uniform or asymmetric expansion of the armature, as well as detonation end effects. Equations describing the "turn skipping" phenomenon are developed in terms of the eccentricity of the armature with respect to the helical coil, the armature's wall thickness variations and the length of the detonation end effect. Design criteria for prevention of "turn-skipping" are presented in order to achieve optimum MFCG performance.

IEEE Conferences

Authors: N. Schoeneberg; J. Walter; A. Neuber; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1277996

Abstract: An alternative to the use of capacitors as the seed source for flux compression generators (FCG) are permanent magnet based systems. Permanent magnets provide a larger energy-to-volume ratio given that the use of capacitors requires a power source as well as charging and firing circuitry. A recent design developed at Texas Tech University's Center for Pulsed Power and Power Electronics [S.I. Shkuratov, et al., 2002], focused on the demagnetization of a Neodymium-Iron-Boron magnet (Nd/sub 2/Fe/sub 14/B, B/sub r/= 1.23 T) by a shockwave generated from high explosives. The maximum specific energy achieved with this design was only 2.3 J/kg, which may not be sufficient for effectively seeding an FCG (fast capacitors have at least 20 J/kg). The same magnets were used with an alternative design, referred to as a strip FCG, which utilizes opposing magnets to generate initial magnetic field intensity within an air gap. The air gap exists between a central tube, filled with high explosives, and metal strips placed between the magnets a few millimeters off-axis. The detonation of the explosive causes the expansion of the central tube, subsequently compressing the flux into the strips and then into the load. The original strip FCG design [B.A. Bojko, et al., 1994] used oxide-barium magnets (B/sub r/= 0.2T), which produced an estimated specific energy of 5.27 J/kg into a low inductance load. These magnets were replaced with the Nd/sub 2/Fe/sub 14/B (B/sub r/= 1.23 T) magnets in order to achieve better performance, which will be analyzed with respect to the specific energy. In addition, a design utilizing a shocked ferroelectric material, PZT, as a seed current source is discussed. An explosively generated shock wave is passed through a ferroelectric material, generating a current that establishes the initial seed flux for an FCG. Preliminary tests of the ferroelectric sources indicate a possible theoretical specific energy of more than 11 J/kg or 25 J/dm/sup 3/. A comparison of the designs will be discussed with regard to the specific energy produced and the effectiveness of each to seed FCG's.

Conferences

Authors: M. M. McQuage; A. A. Neuber; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1277717

Abstract: The design and experimental testing of a laser-triggered microwave switch with a nanosecond activation time is described. The objectives of the project include, confirming that a nanosecond to subnanosecond risetime is achievable in the X-band waveguide at 9 GHz with the laser-triggered switch and to determine the minimum laser energy necessary to obtain the fastest possible risetime. A 1 kW pulsed X-band source with a 500 ns output pulse provides the microwave power for the system. A variable power Nd:YAG laser with a maximum 450 mJ at 532 nm, 10 ns FWHM output pulse is used in conjunction with an applied high voltage pulse to trigger the microwave switch. The microwave signal is switched with the rapid formation of plasma caused by the breakdown of a gas contained by a quartz tube inserted through a section of waveguide. The centerpiece of the waveguide system is a magic tee, which controls the direction of power flow through the system. Compared to tests in air and N/sub 2/, the best results have been obtained in argon. Risetimes below 2 ns have been obtained using argon at a reduced pressure of 150 Torr and a high voltage pulse of 28 kV from a spark gap. The impact of gas pressure, applied voltage pulse and applied laser pulse on the risetime of the microwave switch are discussed.

IEEE Conferences

Authors: M. Butcher; A. Neuber; H. Krompholz; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1277987

Abstract: An increased need for compact pulsed power systems requiring new switching technologies combined with the benefits of cryogenic properties, such as higher energy density and miniaturization, has lead to increased interest in liquid nitrogen as a switching medium. High hold off voltage, low dielectric constant, and low environmental impact are further advantages of liquid nitrogen. Characterization of breakdown is investigated using high-speed (temporal resolution < 1 ns) optical and electrical diagnostics in a coaxial system with 52 /spl Omega/ impedance. Experiments are done in self-breakdown mode in super-cooled liquid nitrogen with a temperature near 70 K. Discharge current and voltage are determined using transmission line type current sensors and capacitive voltage dividers. Discharge luminosity is measured with photomultiplier tubes (risetime/spl ap/800 ps) that are focused on the negative electrode tips and the center of the channel. Optical investigations of breakdown and pre-breakdown events on a nanosecond time scale will provide a better understanding of the fundamental physics of breakdown formation. Detailed optical and spectroscopic diagnostics combined with high-speed electrical diagnostics are aimed at clarifying the overall breakdown mechanisms, including electronic initiation and bubble formation. The breakdown initiation/development will be discussed.

Conferences

Authors: M. Butcher; M. Cevallos; M. Haustein; A. Neuber; J. Dickens; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1254853

Abstract: In a coaxial test apparatus enabling the measurement of voltage and current at the test gap, dc conduction and breakdown in transformer oil caused by the application of dc voltages are investigated. Current measurements cover the range from 10/sup -1/ A to 1 kA, with temporal resolutions of milliseconds at the lowest current amplitudes to sub-nanoseconds for currents larger than 10/sup -4/ A. The dc current/voltage characteristic for sub-breakdown voltage amplitudes point to the injection of charge carriers, allow us to characterize the transport mechanisms, and the influence of space charges. For voltages approaching breakdown thresholds, quasi dc-currents rising from nanoamperes to microamperes are superimposed by current pulses with amplitudes of milliamperes and above, and durations of nanoseconds. The onset of these current pulses occurs up to 10 /spl mu/s before breakdown. One of these current pulses reaches a critical amplitude causing voltage breakdown and current rise to the impedance-limited value within 2 ns. Additional optical diagnostics using photomultipliers and high-speed photography with gated microchannel plates yield information on hydrodynamic processes and charge carrier amplification mechanisms associated with the current pulses and final breakdown, such as bubble formation, as well as on the development of the spark plasma finally bridging the gap.

IEEE Conferences

Authors: A. A. Neuber; J. -. Hernandez; T. A. Holt; J. C. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1277740

Abstract: Helical magnetic flux compression generators (MFCG) are attractive energy sources with respect to their specific energy output. A variety of one-time use applications would benefit from small inch-sized helical generators with high specific energy output. However, it is widely accepted that the generator performance deteriorates with decreasing size. Previous experimental data have shown that the increase of the ohmic resistance of the MFCG with a reduction in size is the primary cause for the observed behavior when the initial generator inductance is held constant. We will analyze the situation in more depth and quantify how much the efficiency is determined by ohmic losses and intrinsic flux losses (flux that is left behind in the conductors and lost for compression) for different generator sizes and geometries. Our simple constant diameter MFCGs exhibit more intrinsic than ohmic losses (69% compared to 16%), while our MFCGs with tapered armatures display less intrinsic and more ohmic flux losses (13% compared to 66%), however, at increased overall efficiency. We will show experimental and calculated data and discuss the physical efficiency limits and scaling of generator performance at small sizes.

IEEE Conferences

Authors: Krile, J; Neuber, A; Dickens, J; Krompholz, H

PDF: https://ttu-ir.tdl.org/handle/2346/12789

Abstract: The limits of the applicability of dc, ac, or pulsed high voltage are determined by breakdown along insulators or insulating support structures. It is of major technical importance to predict breakdown voltages for given structures, with parameters such as geometry, material, and temporal characteristics of the applied voltage. The impact of atmospheric conditions such as humidity, pressure, temperature, and types of gas present is also important. A setup has been devised to simulate and closely monitor flashover across various gap distances and insulator geometries at atmospheric conditions at different humidities. Current, voltage, luminosity, and optical emission spectra were measured with nanosecond to sub-nanosecond resolution. Spatially and temporally resolved light emission data yielded quantitative information about the charge carrier amplification, the location of this amplification, and its role in the formation of streamers.

Conference Paper/Presentation

Authors: M. Butcher; A. Neuber; H. Krompholz; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1277712

Abstract: The phenomenology of prebreakdown events in transformer oil is investigated using high-speed electrical and optical diagnostics. Data collection using a coaxial test setup terminating into a 50/spl Omega/ load line to simulate a matched impedance system allows very fast risetimes. Transmission line type current sensors and capacitive voltage dividers with temporal resolution of 300 ps provide information about the discharge voltage and current. Steady, DC currents ranging from a few nA with less than 10 kV of applied voltage, to a few /spl mu/A prior to full breakdown are measured using an electrometer. Prebreakdown events are measured with positive and negative charging voltages with respect to ground. Light emission from the discharge is measured using a series of fast photomultiplier tubes, (risetimes 800 ps), that observe positive and negative electrode tips and center of the channel. Preliminary results on self-breakdown (breakdown voltage +44 kV) with a 2.35 mm gap show a DC (seed) current of several hundred nA with prebreakdown spikes of a few mA immediately before final breakdown. Periodicity of the current spikes combined with a general increase in magnitude prior to full breakdown has been observed. Data collection using a negative charging line, with respect to ground with enhanced field at the cathode, indicates current spikes that are typically 25 to 50% faster than spikes using a positive charging line with enhanced field at the anode. Detailed optical diagnostics along with high-speed electrical diagnostics of the pre-breakdown phase will address the physical mechanisms initiating volume breakdown.

IEEE Conferences

Authors: M. D. Cevallos; J. C. Dickens; A. A. Neuber; M. A. Haustein; H. G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1228897

Abstract: Summary form only given, as follows. The fundamental breakdown physics of biodegradable oil is investigated with a set up that employs a cable discharge into a coaxial system with axial discharge and load line to simulate a matched terminating impedance. No discontinuities are created in the system lines when entering the discharge chamber with the implementation of a unique feed-through design. The entire impedance of the system is matched at 50 ohms. A novel design for impedance matching transitions from the discharge cable to the coaxial system to the load line allow for a sub-nanosecond response. Final results are measured on pulsed and self breakdown voltages of up to 200 kV. Self breakdown is achieved by charging the discharge cable and load line to +/- 100 kV respectively. Pulsed breakdown is achieved by charging the discharge cable and load line to +100 kV. Shorting the discharge cable generates a reflected negatively polarized pulse causing breakdown. High speed electrical and optical diagnostics have temporal resolution down to several 100 ps A complete range of information from amplitudes of 0.1 mA to 1 kA with temporal resolutions of 300 ps is achieved by using transmission line type current sensors with fast amplifiers. Capacitive voltage dividers with fast attenuators are also used. Optical measurements are performed on low level light emission using spatially resolved, fast photo-multiplier tubes (risetime of 800 ps), supplemented with high speed photography and spectroscopic investigations on a nanosecond timescale Detailed optical and spectroscopic diagnostic along with high speed electrical diagnostics will address the mechanism initiating/assisting biodegradable oil volume breakdown.

IEEE Conferences

Authors: M.D. Cevallos; J.C. Dickens; A.A. Neuber; M.A. Haustein; H.G. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1277989

Abstract: The fundamental breakdown physics of biodegradable oil is investigated with high-speed electrical and optical diagnostics with temporal resolution down to several 100 ps. The set up employs a cable discharge into a coaxial system with axial discharge and load line to simulate matched terminating impedance. A unique feed-through design creates no discontinuities in the system lines through the discharge chamber. The impedance of the system is matched at 50 ohms including a novel design for impedance matching transitions from discharge cable to coaxial system to load line allowing for a sub-nanosecond response. This paper presents results on self-breakdown with voltages of up to 60 kV. Self-breakdown is achieved by charging the discharge cable and load line to +/-30 kV respectively. Transmission line type current sensors and a capacitive voltage divider with fast amplifiers/attenuators are used in order to obtain a complete range of information from amplitudes of 0.1 mA to 1 kA with temporal resolutions of 300 ps. Optical measurements include high speed photography and shadowgraphy. Detailed optical diagnostics along with high-speed electrical diagnostics will address the mechanism initiating/assisting biodegradable oil volume breakdown.

Conferences

Authors: D. Wetz; K. Truman; J. Dickens; J. Mankowski; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1278009

Abstract: In recent years, researchers have investigated methods of liquid sterilization by applying pulsed high electric fields with some degree of success. The mechanism by which microorganisms are damaged has been shown to be a function of the local electric field and exposure time while independent from thermal and electrolytic effects from the applied pulse. Most published experiments have employed electrical pulses of 10's of kV and microsecond long pulse lengths. We are employing electrical pulses in the 100's of kV range with 100 nanosecond long pulse lengths. This type pulse should be more effective at killing microorganisms and minimize energy losses due to thermal processes in the liquid media.

IEEE Conferences

Authors: J. -. Hernandez; A. A. Neuber; J. C. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1277995

Abstract: End-initiated small volume magnetic flux compression generators (MFCG) have at least one order of magnitude higher energy density (by weight or volume) than capacitive energy storage with similar discharge time characteristics. Since the prime energy is built into the MFCG in form of HE, the capacitor looses even more ground if the necessary prime energy source and the charging supply are included in the weight/volume balance. However, simple MFCGs with a single helix produce high output energy only into low inductance loads, thus producing several 100 kA of current at a voltage level of only a few 10 kV. Many pulsed power devices require less current but a considerably higher voltage level. Two approaches for achieving a higher output voltage level, both utilizing two staged MFCGs, have been reported in the open literature. The first employs a more traditional transformer coupling; the second relies on a dynamic transformer or flux-trapping scheme. Although the traditional transformer coupling has theoretically the better efficiency, we chose the latter approach for our generator design, mostly since it requires a smaller number of components. Our generator has a total length of 250 mm, a helix inner diameter of 51 mm, and is wound with Teflon insulated stranded wire of different sizes in the range from AWG 12 to AWG 22. We have presently achieved an energy gain of /spl sim/ 8 and will discuss the generator performance based on experimental current/voltage waveforms.

IEEE Conferences

Authors: H. Keene; J. Dickens; A. Neuber; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1277716

Abstract: As superconducting technology becomes more viable in the marketplace, especially in high power applications, the need for a well researched high thermal conductivity electrical insulator is needed. The electrical failure mode for these types of insulators is often surface flashover at subatmospheric temperature and pressure. Testing of two such insulators, aluminum nitride and aluminum oxide, for this failure mode is done for two differing electrode geometries. In addition three coats of GE 7031 dielectric varnish are applied to the exposed parts of the insulator for comparison testing with nonvarnished samples. In general the testing shows an increasing breakdown voltage trend with decreasing temperature. These results indicate a temperature related dependence of the secondary electron emission and electron induced outgassing, which is a component in the process of surface flashover. The addition of the varnish results in a lowered breakdown voltage. The research also covers the effect of electrode conditioning, and presents optical diagnostics of the gas species involved during breakdown.

IEEE Conferences

Authors: Andreas A. Neuber; Thomas Holt; Juan-Carlos Hernandez; James C. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6219386

Abstract: While Magneto Cumulative Generators (MCGs) differ widely in size and operating regimes, it is apparent that the helical flux compression generator is the most promising concept with respect to current amplification and compactness. Though the geometry of the helical generator (dynamically expanding armature in the center of a current carrying helix) seems to be basic, it turns out that the understanding of all involved processes is rather difficult. This fact is apparent from the present lack of a computer model that would be solely based on physical principles and manage without heuristic factors. This means, all programs known to the authors utilize an additional parameter that adjusts for the loss in flux that is currently unexplained. Experimental efforts revealed that the unexplained loss in flux becomes smaller the larger the generator volume is. Specifically, for generators with constant diameter helix and armature, the figure of merit, β, for generator performance exhibits a distinct decline with the angular frequency, ω, of the progressing contact between end-initiated armature and helix. Since ω is proportional to the square root of the ratio of initial MCG inductance to compressed volume, it becomes apparent that the generator performance is limited by size. For large generators on a meter scale, a β of approximately 0.95 has been reported (β = 1 means no flux loss at all), and own tests indicate that β drops quickly to about 0.7 to 0.6 for generator on the 10 cm scale. Still, even very small generators with about 30 mm outer diameter can have a β of around 0.6. Tapered generators, with tapered helix or tapered armature might perform differently and can be used to further assess the situation. Preliminary results for MCGs with a tapered helix show a performance that is somewhat superior to the straight helix generator.

Conferences

Authors: H. Krompholz; A. Neuber; M. Haustein; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1022721

Abstract: The phenomenology of breakdown in liquid nitrogen is investigated with high-speed electrical and optical diagnostics (temporal resolution down to several 100 ps). The discharge apparatus uses a cable discharge into a coaxial system with axial discharge, and a load line to simulate a matched terminating impedance. Main experiments are done in self-breakdown mode in supercooled liquid nitrogen. Transmission line type current sensors and capacitive voltage dividers with fast amplifiers/attenuators cover an amplitude range of 0.1 mA to 1 kA with a time resolution of 300 ps, providing complete information about discharge voltage and current. The light emission is measured with fast photomultiplier tubes (risetime 800 ps), and these optical measurements will be supplemented by high-speed photography and spectroscopic investigations on a nanosecond time scale. First results on self-breakdown with a gap width of 1 mm and electrodes with 5 mm radius of curvature (breakdown voltage 42 kV) show a three-phase development: the current rises from an unknown level to several mA during 2 ns, stays approximately constant for 100 ns with superimposed ns-duration spikes, and shows a final exponential rise to the full impedance limited current amplitude during several nanoseconds. Detailed optical and spectroscopic diagnostics along with the high-speed electrical diagnostics will in particular address the physical mechanisms initiating/assisting the liquid nitrogen volume breakdown, such as bubble formation during the pre-breakdown phase.

IEEE Conferences

Authors: A. Neuber; H. Krompholz; M. Haustein; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1030428

Abstract: Summary form only given, as follows. Miniaturization of electrical components along with growing superconductor technology requires a better understanding of the phenomenology of breakdown in liquid nitrogen. It is known that the time delay between breakdown-onset and final impedance-limited arc current can occur within a few nanoseconds. For a temporal resolution down to several 100 ps, a discharge apparatus was built and tested that uses a cable discharge into a coaxial system with axial discharge, and a load line to simulate a matched terminating impedance. Main experiments are done in self-breakdown mode in supercooled liquid nitrogen, pulsed breakdown at high over-voltages in standard electrode geometry is investigated as well. Transmission line type current sensors and capacitive voltage dividers with fast amplifiers/attenuators cover an amplitude range of 0.1 mA to 1 kA with a time resolution of 300 ps, providing complete information about discharge voltage and current. The light emission is measured with fast photomultiplier tubes (risetime 800 ps), and these optical measurements will be supplemented by high-speed photography and spectroscopic investigations on a nanosecond time scale. Preliminary results on self-breakdown in the surface flashover mode with a gap width of 2 mm and electrodes with 5 mm radius of curvature (breakdown voltage /spl sim/ 60 kV) show a three-phase development: the current rises from an unknown level to several mA during 2 ns, stays approximately constant for 100 ns with superimposed ns-duration spikes, and shows a final exponential rise to the full impedance limited current amplitude during several nanoseconds. The detailed optical and spectroscopic diagnostics along with the high-speed electrical diagnostics will in particular address the physical mechanisms initiating/assisting the liquid nitrogen volume breakdown, such as bubble formation during the pre-breakdown phase.

IEEE Conferences

Authors: A. Neuber; N. Schoeneberg; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1189493

Abstract: The feasibility of utilizing the chemical energy stored in high explosives to generate an antenna capable of radiating for several microseconds is studied. Crucial parameters such as conductivity as a function of time, maximum achievable antenna length for given initial device volume and weight, and material dependence are assessed by sub microsecond optical and electrical diagnostics. By utilizing a rotating framing mirror camera with up to 4 Megaframes per second, which produces a color image sequence consisting of 125 frames, possible premature breakup of the transient antenna rod is measured with adequate spatial and temporal resolution, thus revealing a rod growth velocity of a few millimeters/microsecond. Electrical diagnostic, primarily aimed at the conductivity between selected points along the rod propagation, enables to make a connection with the observed behavior based on the optical diagnostics. Maintaining electrical contact at the base of the device while the rod is fully formed is crucial and will be discussed in detail.

IEEE Conferences

Authors: D. Hemmert; A. Neuber; H. Krompholz; J. Mankowski; D. Saeks

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1030653

Abstract: Summary form only given, as follows. A new type of plasma limiter is being developed capable of turnon in less than 1 nsec. The approach taken is to initiate streamer breakdown via a micron radius needle tip. Studies were conducted on breakdown with a variety of gases at pressures from 10/sup 3/ to 10/sup -2/ torr. Gases tested included dry air, neon, argon, and krypton. Studies were also conducted on dc-voltage biasing the needle and its effect on breakdown. The experimental setup uses an S-band traveling wave resonant ring (TWRR) capable of power levels up to 100 MW when coupled to a 2.85 GHz, 4 MW, magnetron. High speed diagnostics with a response on the order of 1 ns record the microwave power, luminosity, and x-rays. A high speed CCD camera with an adjustable exposure time down to 10 nsec records a snapshot of the breakdown sequence. Preliminary results exhibit a reduction in expected gas breakdown levels by over two orders of magnitude.

IEEE Conferences

Authors: M. Cevallos; J. Dickens; A. Neuber; H. Krompholz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6219396

Abstract: Experiments in self-breakdown mode and pulsed breakdown at high over-voltages in standard electrode geometries are performed for liquids to gain a better understanding of their fundamental breakdown physics. Different liquids of interest include liquids such as super-cooled liquid nitrogen, oils, glycerols and water. A typical setup employs a discharge chamber with a cable discharge into a coaxial system with axial discharge, and a load line to simulate a matched terminating impedance, thus providing a sub-nanosecond response. This study is focused on the feed-through design of the coaxial cable into this type of discharge chamber, with the feed-through being the critical element with respect to maximum hold-off voltage. Diverse feedthroughs were designed and simulated using Maxwell 3-D Field Simulator Version 5. Several geometrically shaped feed-through transitions were simulated, including linearly and exponentially tapered, to minimize electrostatic fields, thus ensuring that the discharge occurs in the volume of interest and not between the inner and outer conductor at the transition from the insulation of the coaxial cable to the liquid. All feedthroughs are designed to match the incoming impedance of the coaxial cable. The size of the feedthroughs will vary from liquid to liquid in order to match the coaxial cable impedance of 50Ω. The discharge chamber has two main ports where the feed-through will enter the chamber. Each feed-through is built through a flange that covers the two main ports. This allows the use of the same discharge chamber for various liquids by changing the flanges on the main ports to match the particular liquid. The feedthroughs were designed and built to withstand voltages of up to 200 kV. The feedthroughs are also fitted with transmission line type current sensors and capacitive voltage dividers with fast amplifiers/attenuators in order to attain a complete range of information from amplitudes of 0.1mA to 1 kA with a temporal resolution of 300 ps.

IEEE Conferences

Authors: M. Giesselmann; T. Heeren; A. Neuber; J. Walter; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1003943

Abstract: Explosive flux compression generators generate hundreds of kiloamperes and voltages of a few kilovolts. A power conditioning stage is required since typically voltages in the hundreds of kilovolts range are needed. Inductive energy storage systems with an opening switch provide the necessary voltage gain. In our application, the opening switch has been implemented as an exploding wire fuse. The voltage gain, and hence the performance of the system, is greatly dependent on the opening switch. We utilized high-speed optical imaging (up to 10/sup 7/ pictures/s) to assess the performance of the exploding wire fuse.

Journals

Authors: Dickens, J; Neuber, A; Haustein, M; Krile, J; Krompholz, H

PDF: https://aip.scitation.org/doi/10.1063/1.1530810

Abstract: Compact pulsed power systems require new switching technologies. For high voltages, liquid nitrogen seems to be a suitable switching medium, with high hold-off voltage, low dielectric constant, and no need for pressurized systems as in high pressure gas switches. The discharge behavior in liquid nitrogen, such as breakdown voltages, formative times, current rise as function of voltage, recovery, etc. are virtually unknown, however. The phenomenology of breakdown in liquid nitrogen is investigated with high speed (temporal resolution < I ns) electrical and optical diagnostics, in a coaxial system with 50-Ohm impedance. Discharge current and voltage are determined with transmission line type current sensors and capacitive voltage dividers. The discharge luminosity is measured with photomultiplier tubes. Preliminary results of self-breakdown investigations (gap I nun, breakdown voltage 44 kV, non-boiling supercooled nitrogen) show a fast (2 ns) transition from an unknown current level to several mA, a long-duration (100 ns) phase with constant current superimposed by ns-spikes, and a final fast transition to the impedance limited current during several nanoseconds. The optical measurements will be expanded toward spectroscopy and high speed photography with the aim of clarifying the overall breakdown mechanisms, including electronic initiation, bubble formation, bubble dynamics, and their role in breakdown, for different electrode geometries (different macroscopic field enhancements).

Conference Paper/Presentation

Authors: D. Hemmert; A. Neuber; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1189459

Abstract: Microwave breakdown of various He-N/sub 2/ mixture combinations is investigated utilizing an S-band double window pillbox cavity. The objective is to determine the best combination of the two gases to maximize the mixture's ability to cool a dielectric surface while maintaining high breakdown thresholds. Helium is known to be an excellent source to dissipate heat from a dielectric, but it also has a low microwave breakdown threshold. Nitrogen does not transport heat as well, but has a much higher breakdown threshold. The studies focused on generating a series of E-field vs pressure curves for breakdown to help identify optimum He-N/sub 2/ mixture ratios. The S-band double window pillbox has a 333 cm/sup 3/ cavity with two ports to flow the gas mixture through. The pillbox is placed in a traveling wave resonant ring (TWRR) coupled to a 2.85 GHz, 4 MW, magnetron. This combination of double window pillbox and TWRR allows for testing power levels up to 40 MW. High speed diagnostics are used to measure the incident/reflected power and discharge luminosity. Coupled mass flow controllers maintain the gas mixture ratio and continuous gas flow through the cavity. Investigations can be conducted with single or multiple pulsed operations. Results ranged from a minimum of 5 MW for pure helium at 760 torr, to greater than 40 MW for pure nitrogen at 3040 torr.

IEEE Conferences

Authors: M. Giesselmann; T. Heeren; A. Neuber; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1002018

Abstract: This paper describes an advanced PSpice/sup (R)/ model, which is used to complement the experimental work on explosive flux compression generators. This work is conducted at Texas Tech University in the framework of a MURI program. The results from the model are compared to actual results from generators that have been tested in the authors' laboratory. To achieve better fidelity of the model, the effect of transient current diffusion into the conductors of the generator is modeled by using a ladder network with many concentric layers of each current carrying conductor. For the optimum accuracy, the spacing of the concentric layers is closest at the surface of the conductor. The paper shows results, comparisons with experimental data and applications of the model for the design of power conditioning systems for MFC-generators.

Conferences

Authors: Benton, T; Hsieh, KT; Stefani, F; Neuber, A; Kristiansen, M

PDF: https://www.scopus.com/record/display.uri?eid=2-s2.0-0035197234&origin=inward&txGid=88fa19768efac27451f095b295663564

Abstract: This paper analyzes the transient electromagnetic, thermal, and structural behavior of the stator turns in a simple helical magnetic flux compression generator (MFCG). The main objective is to quantify the losses due to Ohmic heating and flux trapping in the conductors, including the effect of armature motion and armature proximity on the current distribution. The electric current excitation used in the modeling is based on experimental data from tests on a research MFCG. The electromagnetic (EM) and thermal analyses were conducted using EMAP3D, a 3D finite element analysis (FEA) code developed at The University of Texas at Austin (UT), which has the capability to model relative motion and sliding between conductors. Structural analyses were conducted using a version of DYNA3D that allows state data from EMAP3D to be used as input. Details of the analyses include temperature dependence on electrical, thermal, and mechanical properties.

Conference Paper/Presentation

Authors: Xiaobin Le; J. Rasty; A. Neuber; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1001693

Abstract: During the operation of magnetic flux compression generators (MFCG), the gas-plasma, shocked by the rapidly expanding armature, could lead to electrical arcing across the gas between the armature and the stator at locations where physical contact between the armature and stator has not yet occurred. This will result in a loss of magnetic flux and a decrease in the electrical efficiency of the MFCG. Therefore, knowledge of the ensuing gas temperature and pressure histories is necessary for identification of loss mechanisms in an effort to optimize the efficiency of MFCGs. This paper describes the procedure for estimating the air temperature and pressure histories via finite element (FE) simulation of the armature expansion and its ensuing contact with the stator in an MFCG. First, the validity of the FE model was verified by comparing deformation contours obtained from the simulations to those obtained experimentally via high-speed photography. Utilizing the pressure history data obtained from the FE results, the air temperature was theoretically calculated. The results indicate that the air pressure and temperature in an MFCG, having a compression ratio of 1.8, could be as high as 30 MPa and 4000/spl deg/ Kelvin, respectively.

IEEE Conferences

Authors: S. I. Shkuratov; E. F. Talantsev; M. Kristiansen; J. Dickens; J. C. Hernandez; A. Neuber

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1002017

Abstract: The results are presented of tests with compact, explosively driven shock wave ferromagnetic generators. The shock wave from a high explosive charge is passed along the axis of a cylindrical, hard magnet. Two types of permanent magnets were used in the experiments: rare-earth NdFeB cylinders (D = 2.5 cm, L = 1.9 cm) and hard ferrite BaFe/sub 2/O/sub 3/ cylinders (D = 2.2 cm, L = 2.5 cm). The shock wave demagnetizes the cylinder, reducing the flux from the remnant value to zero. This change in flux generates a voltage in the winding. The current generated in the loads of the generators yielded a peak of 0.75 kA. The operation of the shock wave ferromagnetic generators was analyzed by the Maxwell 3D code. An analysis is given on the specific features of pulse generation in a system like this.

IEEE Conferences

Authors: D. Hemmert; J. Rasty; A. Neuber; J. Dickens; X. Le; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1002043

Abstract: Modeling and characterization of a Magnetic Flux Compression Generator (MFCG) requires detailed knowledge of the changes in conductivity of the MFCG materials during the shock-loading phase. A Split-Hopkinson Pressure Bar apparatus (SHPB) and current source/ differential amplifier setup was used to study shock-loading under controlled conditions while monitoring changes in resistivity in armature material samples. The SHPB apparatus was capable of producing strain rates up to the fracture limit of the samples tested. Actual fracturing of samples would not have allowed detailed analysis of thermal and mechanical effects in sample resistivity changes. Sample strain rate levels of up to lx104 strain sec-1 were achieved with the apparatus on OFHC copper and aluminum samples.

Conferences

Authors: A. Neuber; J. Dickens; J. B. Cornette; K. Jamison; E. R. Parkinson; M. Giesselmann; P. Worsey; J. Baird; M. Schmidt; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=940950

Abstract: A variety of basic magnetic flux compression (MFC) generator geometries have been tested during the last three decades. Though size and operating regimes differ widely, it is apparent that the helical flux compression generator is the most promising concept with respect to current amplification and compactness. Though the geometry of the helical generator (dynamically expanding armature in the center of a current carrying helix) seems to be basic, it turns out that the understanding of all involved processes is rather difficult. This fact is apparent from the present lack of a computer model that is solely based on physical principles and manages without heuristic factors. A simple generator was designed to address flux and current losses of the helical generator. The generator's maximum current amplitude is given as a function of the seed current and the resulting "seed-current" spread is compared to the output of state-of-the-art computer models. Temporally resolved current and current time derivative signals are compared as well. The detailed generator geometry is introduced in order to facilitate future computer code bench marking or development. The impact of this research on the present understanding of magnetic flux losses in helical MFC generators is briefly discussed.

IEEE Journals

Authors: J. Rasty; X. Le; A. Neuber; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=960712

Abstract: Summary form only given, as follows.The ability of MFCGs for creating large current pulses is well accepted among the scientific community; however, little is understood regarding the fundamental mechanical and electrical mechanisms, and their interrelationship, governing the operation and efficiency of MFCGs. The main objective of this research was to conduct a series of experimental and numerical studies in an attempt to gain insight into the inner working of MFCGs. As a first step, the expansion characteristic of the exploding armature was selected as one of the major factors affecting the efficiency of MFCGs. Both numerical as well as experimental techniques were employed to capture the explosive-driven expansion behavior of the armature. Numerical results were verified experimentally to determine the expansion-time history, expansion angle, expansion velocity, armature/stator contact velocity and the severity of the end-effect in a typical MFCG. The experimental and numerical results showed excellent agreement paving the way for future simulations using the established FE model. The results indicate that the radial and axial impact velocities between the armature and the stator are almost constant throughout the operation of MFCG. Moreover, the results indicate that the contact velocity between the armature decreases as the post-detonation time increases, reaching a constant value equal to the detonation velocity.

IEEE Conferences

Authors: S. Shkuratov; M. Kristiansen; J. Dickens; A. Neuber; L. L. Altgilbers; P. T. Tracy; Y. Tkach

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=960832

Abstract: Summary form only given. The design of shock wave ferroelectric generators driven by high explosives is presented and experimental data are discussed. The active elements are lead zirconium titanate (PZT) disks with diameter D=25 mm and thickness L=2.5 mm, and PZT cylinders with D=21 mm and L=25 mm. The high explosive charge was varied from 4.2 g to 30 g. Two different ways to initiate shock waves in the active elements were used: explosively driven flyer plates and direct action of high explosives. The data are presented on the maximum power released in the resistive load and on the effect of the load inductance and the load capacitance on the amplitude of the pulse produced. The experimental results are discussed in comparison to numerical simulation with the commercial circuit simulator PSPICE. An analysis is given of the specific features of pulse generation in a system like this.

IEEE Conferences

Authors: M. Giesselmann; T. Heeren; A. Neuber; J. Walter; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1002004

Abstract: This paper presents high-speed optical diagnostics of an exploding wire fuse, which is used in the power conditioning system for an explosive flux compression generator. The images were taken using an IMACON(R) 790 high-speed framing camera utilizing a gated image intensifier tube. For the authors' measurements, the camera was operated in the high-speed multiple frame mode, yielding 8-10 sequential, 2 dimensional pictures with 100 ns between exposures.

Conferences

Authors: T. Holt; A. Neuber; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1001684

Abstract: Explosively driven magnetic flux compression has been the object of research for more than three decades. Recently heightened interest has been focused on the basic physical mechanisms that determine the performance of helical magneto cumulative generators (MCGs). Two single-pitch helical flux compression generators of different sizes have been tested using current-voltage probes and optical diagnostics. The main parameters used to characterize the experimental performance of the flux compression generators were the flux conservation and theoretical current gain of each type of generator. Helices with constant pitch and differing separation between wires as well as wires with different insulation thickness were tested and analyzed with respect to their flux conservation and theoretical current gain. Preliminary results show that the insulation thickness plays only a minor role for a change in flux conservation due to geometry in the range from 0.01 to 0.5 mm provided that no internal breakdown occurred. Additionally, the overall physical dimension of the generator was modified to allow for a substantial increase in initial inductance. The outer diameter of the generator armature was held constant at 1.5 inches and the coil diameter was varied from 2.6 to 3.5 inches (expansion ratio of 1.7 or 2.3, respectively). The results gained from the conservative expansion ratio of 1.7 were used as a base to compare to the generator performance at the more aggressive expansion ratio of 2.3. First results show that an expansion ratio of 2.3 produces viable results for a partially annealed Aluminum armature with a Gurney angle of approximately 15 degrees.

IEEE Conferences

Authors: A. Neuber; T. Holt; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1002003

Abstract: The knowledge of the thermodynamic state of the gas trapped in a helical flux compression generator is crucial for the assessment of flux loss due to internal electrical breakdown/arcing. Besides the helix deformation and armature deceleration at extremely high current amplitudes approaching 1 MA, the thermodynamic state of the shocked and compressed gas causes problems in the prediction of the generator output current vs. time towards the end of generator operation. Such a breakdown is experimentally detected as an abrupt change in the time derivative of the current waveform and it is easily distinguished from partial turn skipping by its sharper fall and nonperiodic occurrence. The thermodynamic state of the generator was measured using primarily optical emission spectroscopy. Three main stages of operation are discussed: (1) the initial stage, which can be represented by a freely expanding armature, that shows fairly low gas temperatures; possibly as low as 2000 K; and (2) the intermediate stage during 14 to 4 microseconds before generator burnout that exhibits mainly an atomic copper line transition at about 0.8 eV; and (3) the last few microseconds that reveal a highly compressed gas with temperatures of about 5000 K and pressures of about 1500 bar. Most experiments were conducted in air, initially at STP, some results are given for SF/sub 6/ initially at one atmosphere. In order to link the thermodynamic state to the breakdown sensitivity, additionally, simple conductivity measurements were conducted in current-free flux compression generator models.

IEEE Conferences

Authors: D. Hemmert; A. Neuber; J. Dickens; H. Krompholz; L. L. Hatfield; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6220112

Abstract: Physical mechanisms leading to microwave breakdown on dielectric/gas interfaces are investigated for power density on the order of 10 MW/cm2 at 2.85 GHz and gas pressures on the order of 10−4 torr to 103 torr. The investigation is focused on an alumina/air interface; other gases are considered for reference purposes. A 3 MW magnetron with 3.5 µs pulse width is coupled to an S-band traveling wave resonator with a pressure adjustable test region. The pre-breakdown phase and the breakdown are monitored by recording the traveling and reflected power, and the spatially integrated luminosity. Electric field probes in the vicinity of the interface are included as well to get information about the local field. Furthermore, the light emission was observed with an image intensifier capable of a minimum gate time of 2.5 ns, in temporal correlation to the other phenomena, or with a framing camera having a 20 ns gate time and 100 ns separation between pictures. The pressure dependent breakdown characteristics, such as appearance, breakdown field, and temporal shape of electric signals, are compared to dielectric/vacuum interface breakdown and volume breakdown, all measured utilizing similar setups.

IEEE Conferences

Authors: A. Neuber; J. Dickens; M. Giesselmann; B. Freeman; D. Dorsey; H. Krompholz; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6220176

Abstract: The design of a helical flux compression generator, driven by 200 g of high explosives, for basic studies is presented and experimental results are discussed. Generator current and dI/dt are measured with Rogowski coils placed on the load. Both crowbar closure and contact velocity have been successfully simulated with CTH, a three-dimensional finite element hydrodynamic code. The generator's magnetic field structure is briefly discussed and magnetic field probe measurements are presented.

IEEE Conferences

Authors: A. Neuber; J. Dickens; M. Giesselmann; B. Freeman; J. Rasty; H. Krompholz; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=855148

Abstract: Summary form only given, as follows. The design of a helical flux compression generator, driven by 150 g of high explosives, for basic studies is presented and experimental results are discussed in comparison to numerical simulations. Simulation of the electric current output with the commercial circuit simulator PSPICE shows that this generator conserves the magnetic flux ideally in the low current mode, <30 kA. At current amplitudes in excess of 100 kA heating and melting of the single wound helix wire, AWG 12, limit the current flow. The volume between armature and stator is spectroscopically probed with fiber optics and valuable insight into the state of the shocked and compressed gas is gained. The same fiber optic probes are used to measure the velocity of the armature-stator contact along the generator axis. This contact velocity is largely affected by armature end effects, mainly due to the pressure loss at the detonator end. Both gas temperature and contact velocity have been successfully simulated with LS-DYNA3D, a three dimensional finite element hydrodynamic code. The generator's magnetic field structure is briefly discussed and magnetic field probe measurements are presented.

IEEE Conferences

Authors: Hemmert, D; Neuber, AA; Dickens, JC; Krompholz, H; Hatfield, LL; Kristiansen, M

PDF: https://spie.org/Publications/Proceedings/Paper/10.1117/12.391818?SSO=1

Abstract: Microwave window breakdown is investigated in vacuum and atmospheric conditions. An S-band resonant ring with a frequency of 2.85 GHz and a power of 80 MW with a 4 MW magnetron as a source is used. Window breakdown on the vacuum side is simulated using a dielectric slab partially filling an evacuated waveguide. Various high-speed diagnostic methods yield a complete picture on the breakdown phenomenology, with far reaching similarities to de surface flashover. During the initiation phase, free electrons are present, which can be influenced by magnetic fields, followed by a saturated secondary electron avalanche with electron-induced outgassing. Final breakdown occurs in the desorbed gas layer above the surface. In order to simulate window breakdown on the gas-side, a segment of the resonant ring separated by two windows was filled with gas at variable pressure, and breakdown was initiated by field-enhancement tips on one of the gas-side surfaces. Threshold power densities for breakdown are measured, and first results on the phenomenology of this gas breakdown are compared with the processes of flashover in vacuum.

Conference Paper/Presentation

Authors: D. Hemmert; A. A. Neuber; J. Dickens; H. Krompholz; L. L. Hatfield; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=887650

Abstract: Microwave window breakdown in vacuum is investigated for an idealized geometry, where a dielectric slab is located in the center of a rectangular waveguide with its normal parallel to the microwave direction of propagation. An S-band resonant ring with a frequency of 2.85 GHz and a power of 60 MW is used. With field enhancement tips at the edges of the dielectric slab, the threshold power for breakdown is observed to be dependent on the direction of the microwaves; i.e., it is approximately 20% higher for the downstream side of the slab than it is for the upstream side. Simple trajectory calculations of secondary electrons in an RF field show a significant forward motion of electrons parallel to the direction of microwave propagation. Electrons participating in a saturated secondary avalanche on the upstream side are driven into the surface, and electrons on the downstream side are driven off the surface, because of the influence of the microwave magnetic field. In agreement with the standard model of dielectric surface flashover for dc conditions (saturated avalanche and electron-induced outgassing), the corresponding change in the surface charge density is expected to be proportional to the applied breakdown threshold electric field parallel to the surface.

IEEE Journals

Authors: Neuber, AA; Dickens, JC; Krompholz, H; Schmidt, MFC; Baird, J; Worsey, PN; Kristiansen, M

PDF: https://ieeexplore.ieee.org/document/823591

Abstract: Explosively driven magnetic flux compression (MFC) has been object of research for more than three decades, Actual interest in the basic physical picture of flux compression has been heightened by a newly started Department of Defense (DoD) Multi-University Research Initiative. The emphasis is on helical flux compression generators comprising a hollow cylindrical metal liner filled with high explosives and at least one helical coil surrounding the liner. After the application of a seed current, magnetic flux is trapped and high current is generated by moving, i.e., expanding, the liner explosively along the finding of the helical coil, Several key factors involved in the temporal development can be addresses by optical diagnostics. 1) The uniformity of liner expansion is captured by framing camera photography and supplemented by laser illuminated high spatial and temporal resolution imaging. Also, S-ray flash photography is insensitive to possible image blur by shockwaves coming from the exploding liner, 2) The thermodynamic state of the shocked gas is assessed by spatially and temporally resolved emission spectroscopy, 3) The moving liner-coil contact point is a possible source of high electric Losses and is preferentially monitored also by emission spectroscopy, Since optical access to the region between liner and coil is not always guaranteed, optical fibers can be used to extract light from the generator. The information so gained will give, together with detailed electrical diagnostics, more insight in the physical loss mechanisms involved in MFC.

Journal

Authors: Neuber, AA; Butcher, M; Krompholz, H; Hatfield, LL; Kristiansen, M

PDF: Not https://ieeexplore.ieee.org/document/825505

Abstract: Results of high-speed electrical and optical diagnostics are used as a basis to discuss a new surface flashover model. Outgassing, caused by electron stimulated desorption, is found to play a crucial role in the temporal flashover development. Dielectric unipolar surface flashover under vacuum is experimentally characterized buy a three-phase development, which covers a current range from 10(-4) A to 100 A. Phase one comprises a fast (several nanoseconds) buildup of a saturated secondary electron avalanche reaching current levels of 10 to 100 mA. Phase two is associated with a slow current amplification reaching currents in the Ampere level within typically 100 ns, The final phase is characterized by a fast current rise up to the impedance-limited current on the order of 100 A. The development during phase tno and three is described by a zero-dimensional model, where electron-induced outgassing leads to a Townsend-like gas discharge above the sur face. This is supported by time-resolved spectroscopy that reveals the existence of excited atomic hydrogen and ionic carbon before the final phase. The feedback mechanism toward a self-sustained discharge is due to space charge leading to an enhanced field emission from the cathode. A priori unknown model parameters, such as outgassing rate and gas density buildup above the surface, are determined by fitting calculated results to experimental data. The significance of outgassing is also discussed with a view to microwave surface flashover.

Journal

Authors: Neuber, A; Butcher, M; Hatfield, LL; Kristiansen, M; Krompholz, H

PDF: https://ieeexplore.ieee.org/document/788752

Abstract: Cryogenic components in high power electrical systems and in power electronics gain more and more importance. The behavior of insulators for cryogenic conditions, however, is virtually unknown. In a fast coaxial setup, dielectric test sample and electrodes in vacuum are cooled to <100 K and flashover is characterized using fast electrical and optical diagnostics. Three consecutive development stages for flashover in self-breakdown mode with a gap distance of 0.5 cm can be distinguished: (1) A fast current rise to mA amplitudes within similar to 2 ns, probably associated with field emission, followed by (2) a slow current rise to similar to 5 to 10 A amplitude with duration of 40 ns to 1 mu s , associated with secondary emission avalanche saturation, and (3) a transition to a rapid gaseous ionization above the sample caused by electron induced outgassing, leading to impedance-limited current amplitudes of less than or equal to 300 A. Phase (1) shows a higher final current at lower temperature, which is probably due to a higher initial velocity of the secondary electrons, the duration of phase (2) is a decreasing function of breakdown voltage and only slightly dependent on temperature, which points to a weak temperature dependence of the outgassing process. Flashover potentials show a slight increase at lower temperature.

Journal

Authors: A. Neuber; D. Hemmert; J. Dickens; H. Krompholz; L. L. Hatfield; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=763092

Abstract: Using two gated intensified digital charge-coupled device cameras, one sensitive in the near infrared to ultraviolet region and one in the soft X-ray region, the temporal development of high-power microwave-induced surface flashover across a vacuum/dielectric interface has been imaged. The emission of X-ray radiation from the interface is caused by field emitted electrons accelerated in the high electromagnetic field impacting the solid. This generation of bremsstrahlung terminates at the moment of full flashover development that is indicated by the optical light emission. A rising plasma density above the dielectric surface due to electron induced outgassing triggers this behavior.

IEEE Journals

Authors: D. Hemmert; A. Neuber; J. Dickens; H. Krompholz; L. L. Hatfield; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=829539

Abstract: Summary form only given. Effects of the microwave magnetic field on window breakdown are investigated at the upstream and downstream side of a dielectric interface. Simple trajectory calculations of secondary electrons in an RF field show significant forward motion of electrons parallel to the microwave direction of propagation. The Lorentz-force due to the microwave magnetic field on high-energy secondary electrons might substantially influence the standard multipactor mechanism. As a result, the breakdown power level for the downstream side of a window would be higher than for the upstream side. This hypothesis was tested utilizing an S-band traveling wave resonant ring, powered by a 3 MW magnetron at 2.85 GHz, leading to a total power greater than 60 MW. Breakdown was studied on an interface geometry consisting of a thin alumina slab in the waveguide, oriented normal to the microwave propagation direction.

IEEE Conferences

Authors: Neuber, A; Hemmert, D; Krompholz, H; Hatfield, L; Kristiansen, M

PDF: https://aip.scitation.org/doi/10.1063/1.370953

Abstract: A simple model of vacuum/dielectric/vacuum interface breakdown initiation caused by high power microwave has been developed. In contrast to already existing models, a spatially varying electron density normal to the interface surface has been introduced. Geometry and parameter ranges have been chosen close to the conditions of previously carried out experiments. Hence, physical mechanisms have become identifiable through a comparison with the already known experimental results. It is revealed that the magnetic field component of the microwave plays an important role. The directional dependence introduced by the magnetic field leads to a 25% higher positive surface charge buildup for breakdown at the interface downstream side as compared to the upstream side. This and the fact that electrons are, in the underlying geometry, generally pulled downstream favors the development of a saturated secondary electron avalanche or a saturated multipactor at the upstream side of the dielectric interface. The previously observed emission of low energy x-ray radiation from the interface is explained by bremsstrahlung generated by impacting electrons having initially a higher energy than the average emission energy. Final breakdown is believed to be triggered by electron induced outgassing or evaporation, generating a considerable gas density above the dielectric surface and eventually leading to a gaseous breakdown. (C) 1999 American Institute of Physics. [S0021-8979(99)09315-9].

Journal

Authors: J. Rasty; Xiaobin Le; A. Neuber; Jiande Zhang; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=823611

Abstract: Characterization of changes in the conductivity during the shock loading process is of paramount importance in estimating the efficiency of magnetic flux compression generators (MFCG). In this study, the main emphasis was the characterization of dynamic conductivity of the armature material during the shock-loading phase. A Split Hopkinson Pressure Bar apparatus was utilized to subject as-received and annealed specimens of OFHC copper to various shock pressures. Experiments conducted to measure the resistivity of Cu specimens indicated that resistivity initially decreases, followed by a sharp increase before decreasing to a steady state value. Depending on the magnitude of the shock pressure, resistivity changes in excess of 200% were recorded.

IEEE Conferences

Authors: A. Neuber; J. Dickens; D. Hemmert; H. Krompholz; L. L. Hatfield; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=677696

Abstract: Summary form only given. Physical mechanisms leading to microwave breakdown on dielectric/vacuum interfaces are investigated for power levels on the order of 100 MW at 2.85 GHz. A 3 MW magnetron with 3.5 /spl mu/s pulse width, is coupled to an S-band traveling wave resonator which is kept at a pressure of 10/sup -8/ Torr. The investigation is focused on an interface geometry comprising a thin dielectric polymer slab in the waveguide, oriented vertical to the direction of wave propagation, and two field enhancement tips placed in the middle of each waveguide broad wall. This ensures an almost purely tangential field at the interface surface and a localized breakdown.

IEEE Conferences

Authors: A. Neuber; J. Dickens; D. Hemmert; H. Krompholz; L. L. Hatfield; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=700757

Abstract: Physical mechanisms leading to microwave breakdown on windows are investigated for power levels on the order of 100 MW at 2.85 GHz. The test stand uses a 3-MW magnetron coupled to an S-band traveling wave resonator. Various configurations of dielectric windows are investigated. In a standard pillbox geometry with a pressure of less than 10/sup -6/ Pa, surface discharges on an alumina window and multipactor-like discharges starting at the waveguide edges occur simultaneously. To clarify physical mechanisms, window breakdown with purely tangential electrical microwave fields is investigated for special geometries. Diagnostics include the measurement of incident/reflected power, measurement of local microwave fields, discharge luminosity, and X-ray emission. All quantities are recorded with 0.21-ns resolution. In addition, a framing camera with gating times of 5 ns is used. The breakdown processes for the case with a purely tangential electric field is similar to DC flashover across insulators, and similar methods to increase the flashover field are expected to be applicable.

IEEE Journals

Authors: Neuber, A; Dickens, J; Hemmert, D; Krompholz, H; Hatfield, LL; Kristiansen, M

PDF: https://ieeexplore.ieee.org/document/604787

Abstract: Physical mechanisms leading to microwave breakdown on windows and in cavities are investigated for power levels on the order of 100 MW at 2.85 GHz. The test stand uses a 3 MW magnetron coupled to an S-band traveling wave resonator. Various configurations of dielectric windows are investigated. In a standard pillbox geometry with a pressure of less than 10(-8) torr, surface discharges on an alumina window and multipactor-like discharges starting at the waveguide edges occur simultaneously. To clarify physical mechanisms, window breakdown with purely tangential electrical microwave fields is investigated for special geometries. Other configurations, such as air filled two window setups, relevant for vacuum-air interfaces, can be investigated as well. Diagnostics include the measurement of incident/reflected power, measurement of local microwave fields, discharge luminosity, and x-ray emission. All quantities are recorded with 0.2 to 1 ns resolution. In addition, a framing camera with gating times of 5 ns is used. Based on the experimental results, methods to increase the power density which can be transmitted through windows, such as surface coatings and window profiles, will be investigated as well.

Conference Paper/Presentation