Texas Tech University
Center for Pulsed Power & Power Electronics

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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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.-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: 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: S. L. Holt; J. C. Dickens; J. J. Mankowski; G. Rodriquez

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6635187

Abstract: The design of a scalable, burst-mode rapid capacitor charger and a high power battery system will be discussed. The design is scalable up to 40 kW output power and operates at 50 k V. The system is powered by a Lithium Ion Polymer (LiPo) battery pack and includes a battery management system. The topology is derived from a 5 kV, 3 kJ/s battery power capacitor charger that was previously demonstrated. This smaller charger occupied a volume of 1.5 L including batteries and demonstrated a 2 kW/L average power density over a 1 s burst. This power density is five to ten times higher than commercially available systems. The performance of this previous design was limited by strict form factor restrictions (1.5 L, 6" diameter). The new design is intended to match or exceed the power density of the previous design due to reduced form factor restrictions. Electrical and mechanical design, component selection, and circuit simulations will be presented.

IEEE Conferences

Authors: D. V. Reale; J. Mankowski; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6518716

Abstract: Commercially available Electro-Explosive Devices (EEDs), such as blasting caps, use electrical current to initiate a primary charge. Various detonators including bridge wire, match-type, exploding bridge wire, and slapper. The basic operating principle of the match-type device is to heat the ignition element to the ignition temperature of the primary explosive. The normal operation current profiles, both constant current and pulsed excitation, are well known, as is the ignition temperature. However, as safety and reliability are of great concern, both in the operation and storage of EEDs, the susceptibility of these devices to transient or spurious fields is of interest. The susceptibility of match-type EEDs to high pulsed electric fields is examined. A Finite Element Method (FEM) simulation is performed using COMSOL to determine the induced current in the bridgewire due to applied electric fields and the resulting Joule heating of the wire. Several situations are investigated including the EED in conductive and non-conductive media and leads open or terminated representing operational and storage conditions.

IEEE Conferences

Authors: R. W. Karhi; D. A. Wetz; J. J. Mankowski; M. Giesselmann

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6197742

Abstract: The design and experimental results of a 40-stage distributed energy store (DES) plasma arc railgun are presented. The railgun drives a free running hypervelocity plasma arc, one that is not pushing a payload, to velocities in excess of 10 km/s. These high velocities are of interest as they are required to successfully launch payloads into low earth orbit (LEO). The ability to launch payloads into LEO using a hypervelocity electromagnetic launcher has many financial benefits over the more conventional chemical combustion launchers. In collaboration with an Air Force Office of Scientific Research funded Multidisciplinary University Research Initiative project, the Center for Pulsed Power and Power Electronics at Texas Tech University in Lubbock, Texas has been responsible for developing and investigating a functional scale model of a multistage DES railgun to determine its effectiveness to suppress restrike phenomenon and increase plasma armature railgun performance. The distributed energy scheme is theorized to suppress restrike arc formation because the back emf voltage is localized to active stage regions where high gas density and low temperature inhibits breakdown. B-dot sensors positioned along the length of the launcher provide data to measure the plasma arc velocity and detect restrike, arc splitting, or additional secondary arc formation phenomena.

IEEE Journals

Authors: R. W. Karhi; D. A. Wetz; M. Giesselmann; J. J. Mankowski; J. P. Diehl; P. M. Kelly

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5740614

Abstract: The development process pertaining to the design, fabrication, and testing of a 40-stage free-running arc synchronous distributed energy railgun is presented. Research efforts are still ongoing to suppress the restrike phenomenon that is responsible for causing a velocity ceiling around 6 km/s to exist on plasma armature breech-fed railguns. Numerous solutions have been theorized as viable methods of restrike prevention but lack experimental verification. In collaboration on an AFOSR Multidisciplinary University Research Initiative project, the team at Texas Tech University is responsible for characterizing a functional scale model of a synchronous distributed energy railgun to investigate the effectiveness of a distributed energy scheme to suppress the plasma restrike phenomenon and increase plasma armature railgun performance. The distributed energy scheme is theorized to suppress restrike arc formation because the back-EMF voltage is localized to active stage regions. Synchronous operation refers to the speed of an electromagnetic wave in the LC transmission line formed by the rails and capacitors being matched to the velocity of the armature. The railgun drives a hypervelocity (8 km/s) plasma armature, with no payload, to emulate the conditions of a high-altitude microsatellite launch while relieving the financial burden of a large stored energy facility. Experimental data collected from a seven-stage prototype distributed energy system are discussed which will mimic the design and operation of the first seven stages associated with the final 40-stage system, which is currently under construction. The data collected from this prototype as well as the final 40-stage system will be analyzed for secondary arc formation in an attempt to verify the distributed energy scheme's success in suppressing restrike formation.

IEEE Journals

Authors: R. Karhi; D. Wetz; J. Mankowski; M. Giesselmann; I. El-Dana

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6191671

Abstract: The design and experimental results of a 40-stage distributed energy store (DES) plasma arc railgun are presented. The railgun drives a free running hypervelocity plasma arc, one that is not pushing a payload, to velocities in excess of 10 km/s. These high velocities are of interest as they are required to successfully launch payloads into low earth orbit (LEO). The ability to launch payloads into LEO using a hypervelocity electromagnetic launcher has many financial benefits over the more conventional chemical combustion launchers. In collaboration on an Air Force Office of Scientific Research (AFOSR) funded Multidisciplinary University Research Initiative (MURI) project, the Center for Pulsed Power and Power Electronics (P3E) at Texas Tech University in Lubbock. Texas has been responsible for developing and investigating a functional scale model of a multi-stage DES railgun to determine its effectiveness to suppress restrike phenomenon and increase plasma armature railgun performance. The distributed energy scheme is theorized to suppress restrike arc formation because the back emf voltage is localized to active stage regions. B-dot sensors positioned along the length of the launcher provide data to measure the plasma arc velocity and detect restrike, arc splitting, or additional secondary arc formation phenomena.

IEEE Conferences

Authors: D. Reale; J. Mankowski; S. Holt; J. Walter; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6191489

Abstract: Current work at Texas Tech University is focused on enabling technologies for a mobile Pulsed Ring Down Array. A Monte Carlo Simulation was developed to evaluate the potential performance of an array using current capabilities and to determine the future requirements to achieve the desired system performance. Error sources included are the position error of each element, the jitter of the closing switch, and the synchronization error between elements. Error distributions for the simulation are based on device testing. The array is tested by adding an artificial delay that represents an error source. The simulation is verified by running each error source independently and comparing the results to the experimental data. A system of low power radiators based on stacked MOSFET switches were built to simulate the Pulsed Ring Down Source (PRDS) elements for testing purposes. The testing methodology and results are presented.

IEEE Conferences

Authors: P. Kelly; J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6191453

Abstract: The development of a high power in-line limiter utilizing varactor-loaded metamaterial structures is presented. A metamaterial structure is an artificial structure engineered to provide electromagnetic properties not available in nature, more explicitly defined as a material having simultaneously negative permittivity and negative permeability. A singly-negative material (SNG) structure, the split-ring resonator (SRR), is a negative permeability material which acts as a notch filter with resonant frequency f0. The resonant frequency of the SRR filter yields itself to tuning since the capacitance between the SRR and transmission lines is easily changeable through the use of varactors. At nominal power levels, f0 is significantly offset from the receiving frequency such that the receiving frequency is unattenuated. When an in-band high power microwave (HPM) is incident upon the filter, a DC bias is applied to several varactors and shifts the resonant frequency of the filter to that of the receiving frequency due to the change in capacitance of the varactors. This effectively attenuates the incident HPM. The filter uses a microwave rectifying circuit to extract a DC voltage from the in-band HPM, which serves as the DC bias voltage across the varactors. Ansoft's HFSS was used to accurately model and design the SRR structure to minimize the E-field and maximize resonant effects. Both high and low power continuous wave testing verified minimal insertion loss as well as verification that the use of varactors in conjunction with a split ring would effectively shift the resonant frequency of the notch filter.

IEEE Conferences

Authors: Y. Chen; J. Dickens; J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5976084

Abstract: Research efforts at Texas Tech University on impulse antenna phased array has resulted in the development of a dependable high voltage, high repetition rate switch that can minimize jitter into the ps range. To accurately synchronize a phased array to steer and preserve the risetime of a radiated pulse, the jitter can only be a fraction of this risetime. Initial testing with a similar system in produced sub-ns jitter results for operations in different gases and gas mixtures. This paper discusses in detail 50 kV, 100 Hz switch operations using different testing parameters. The switch jitter as a function of triggering conditions is discussed, including a comprehensive evaluation of jitter as a function of operation pressure as well as trigger magnitude. Several phenomenon were observed and discussed to quantify the switch jitter with respect to operation pressure and trigger magnitude. The temperature of gas and its effects on switch jitter is also documented in this paper, with a jitter improvement of ~25% recorded. An empirical formula was determined as a function of the gas density, electric field of the main gap, and electric field for the trigger for the experiments conducted in this manuscript. A 50 Ω, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the switch. The hermetically sealed spark gap, with a modular design composed of copper tungsten electrodes, gas feeds, Kel-F lining, as a well as a G-10 shell is used to house the high pressure gases for the experiments. Trigatron-type triggering is provided by a solid state opening switch voltage source that supplies 75-150 kV, 10 ns risetime pulses at a rep rate up to 100 Hz in burst mode.

IEEE Journals

Authors: Y. Chen; D. Reale; J. Dickens; S. Holt; J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5976097

Abstract: A collaborative effort at Texas Tech University on high power RF transmitters has directly translated to the development of phased array pulsed ring down sources (PRDS). By operating an array of PRDS, peak radiating power on target can theoretically be increased to the squared of N sources. The primary limitation on the application of the array concept is the jitter with which the individual sources can be fired. An ideal jitter of a small fraction of the risetime is required to accurately synchronize the array to steer and preserve the amplitude of the radiated pulse. This paper describes in detail the implementation of a GPS based timing system that will synchronize the operation of each of the elements of a geospatially distributed phased array to maximize the peak power delivered to a single position. Theoretical array performance is shown through Monte Carlo simulations, accounting for switch jitter and a range of GPS timing jitter. Each module will include a control unit, low jitter pulser, low jitter spark gap, antenna element, as well as a GPS receiver. The location of each module is transmitted to a central controller, which calculates and dictates when each element is fired. Low jitter in the timing of the GPS reference signal is essential in synchronizing each element to deliver the maximum power. Testing using a preliminary setup using GPS technology is conducted with both 1 pps and 100 pps outputs. Jitter results between modules are recorded to ~10 ns without any correction factors and 1-2 ns with simple averaging. With the timing and geospatial errors taken into account, the proposed concept will show usable gains at phased array operating frequencies up to several hundred MHz.

IEEE Journals

Authors: D. Realey; J. Mankowski; S. Holt; J. Walter; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6191656

Abstract: An inexpensive and mobile array of Pulsed Ring Down Sources (PRDS) were required to verify previous simulation results. Initial attempts to use a stacked MosFET circuit as a closing switch were unsuccessful due to the additional series inductance of the MosFET stack lowering the frequency of the oscillation on the coaxial radiator. Experimental results showed that by reducing the parasitic inductance due to the geometry of the MosFET stack, the frequency of the oscillation could be increased. Increased series resistance due to the stacked MosFETs was also a concern. In order to minimize the parasitic inductance of the stack and allow for multiple stacks to be connected in parallel, a printed circuit board was designed. Results from testing of the original switch stacks and board assembly are presented and compared with PSPICE simulations. Using the simulation results, the reduction in parasitic inductance can be estimated.

IEEE Conferences

Authors: D. Reale; J. Mankowski; Y. Chen; J. Walter; S. Holt; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5976098

Abstract: The development of mobile Pulsed Ring Down Source (PRDS) arrays requires the ability to accurately determine the relative positions of array elements at distances, and in situations, where discrete measurements are not practical. At the frequencies of interest, centimeter level accuracy is required for the array to localize radiated energy at a given target location. Global Positioning System (GPS) devices and techniques are evaluated for the purpose of position acquisition. Previously a Monte Carlo simulation was developed that takes into account the position error, the GPS timing error, and the switch jitter of the element. The error sources are combined and used a metric to evaluate and predict the array performance. Results of the GPS device testing, as well as previous work, are used as the input parameters of the simulation to determine their viability for use in the implementation of PRDS arrays capable of radiating at frequencies of up to 500 MHz.

Journals

Authors: Y. Chen; J. Dickens; J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5958315

Abstract: Recent research efforts at Texas Tech University on impulse antenna phased array have shown that an ideal jitter of a small fraction of the rise-time is required to accurately synchronize the array to steer and preserve the rise-time of the radiated pulse. This has necessitated the need to develop a reliable high voltage, high repetition rate switch that will operate with very low jitter. This manuscript presents the impact gases and gas mixtures have on switch performance which includes recovery rate and in particular jitter. A 50 Ω, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the different gases. Gases tested include N2, dry air, H2, and SF6, as well as N2-H2, N2-SF6, N2-Ar, and gas mixtures containing Kr85. This manuscript will discuss in detail 50 kV, 100 Hz triggered switch operations of such gases. Switch jitter as a result of triggering conditions is also discussed, also including an evaluation of jitter as a function of formative delay in various gases. An evaluation of switch jitter as a function of operation time and gas temperature is also included. Triggering is provided by a solid state opening switch voltage source that supplies ~150 kV, 10 ns rise-time pulses at a rep-rate up to 100 Hz in burst mode. A hermetically sealed spark gap with a Kel-F -PCTFE (PolyCholoroTriFluoroEthylene) lining is used to house the switch and high pressure gas. It is shown that jitter is strongly dependent on the triggering technique, as well as the trigger magnitude, with ionization rates playing an important role. Sub-ns jitter is seen with a variety of gases and gas mixtures with H2 producing the best results. Varying the gas temperature and addition of radioactive sources are seen to improve the switch jitter.

IEEE Conferences

Authors: Y. Chen; J. Dickens; S. Holt; D. Reale; J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5958436

Abstract: A collaborative effort at Texas Tech University on high power RF transmitters has directly translated to the development of phased array pulsed ring down sources (PRDS). By operating an array of PRDS, peak radiating power on target can theoretically be multiplied by the number of sources. The primary limitation on the application of the array concept is the jitter with which the individual sources can be fired. An ideal jitter of a small fraction of the risetime is required to accurately synchronize the array to steer and preserve the risetime of the radiated pulse. This paper describes in detail the implementation of a GPS based timing system that will synchronize the individual antennas to operate at different geo-locations to function in a coordinated fashion to deliver the peak power of each element to a single position. Theoretical array performance is shown through Monte Carlo simulations, accounting for switch jitter and a range of GPS timing jitter. Each module will include a control unit, low jitter pulser, low jitter spark gap, antenna element, as well as a GPS receiver. The location of each module is transmitted to a central controller, which calculates and dictates when each element is fired. Low jitter in the timing of the GPS reference signal is essential in synchronizing each element to deliver the maxim power. Testing using a preliminary setup using GPS technology is conducted with both 1 pps and 100 pps outputs. Jitter results between modules are recorded to ~10 ns without any correction factors. With the timing and geospatial errors taken into account, the proposed concept will show usable gains of up to several hundred MHz.

IEEE Conferences

Authors: D. Reale; J. Mankowski; S. Holt; Y. Chen; J. Walter; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5958437

Abstract: Current research at Texas Tech University is focused on the development of a High-Power Pulsed Ring-Down Source (PRDS) Antenna Array. Previously, a Monte Carlo based analysis was conducted in order to predict the array performance based upon the estimated switching jitter between elements. This analysis showed good performance for jitter times between 0 to 2 periods of the ringing frequency. Therefore, for ringing frequencies up to 500 MHz, jitter times up to 4 nanoseconds can be tolerated. Subsequently, we have shown practical switching solutions capable of the sub-nanosecond switching performance needed for the frequencies of interest. Taking the analysis a step further, we introduce the uncertainty of the absolute position of each antenna element. To implement a randomly distributed array, where the position of elements is not fixed, a method of accurately resolving element positions relative to each other and the target location is required. The use of a variety of GPS technologies and techniques is explored as a method for position and timing resolution. The relative accuracy between elements and the absolute accuracy of each element is discussed. A Monte Carlo based analysis is conducted to predict array performance based upon GPS positional error, GPS timing error, and switch jitter.

IEEE Conferences

Authors: W. Jiang; B. Weber; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5247063

Abstract: The 23 papers in this special issue were originally presented at the 17th International Conference on High-Power Particle Beams (BEAMS), held in Xi'an China, on July 7-11, 2008.

Journal

Authors: Y. Chen; J. Dickens; J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5211842

Abstract: Recent research efforts at Texas Tech University on impulse antenna phased array has needed to develop a reliable high voltage, high repetition rate switch that will operate with ultra low jitter. An ideal jitter of a small fraction of the risetime is required to accurately synchronize the array to steer and preserve the risetime of the radiated pulse. In, we showed the initial test system with sub-ns results for operations in different gases and gas mixtures. This paper discusses in detail 50 kV, 100 Hz switch operations with different gases. The effects of gases and gas mixtures have on switch performance which includes recovery rate and in particular jitter will be investigated. Gases tested include, dry air, H2, N2, and SF6, as well as H2-N2, and N2-SF6 gas mixtures. Switch jitter as a result of triggering conditions is discussed, also including a comprehensive evaluation of jitter as a function of formative delay in the various gases. The temperature of gas and its effects on switch jitter is also documented in this paper. A 50 ¿, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the different gases. Triggering is provided by a solid state opening switch voltage source that supplies ~150 kV, 10 ns risetime pulses at a rep rate up to 100 Hz in burst mode. A hermetically sealed spark gap with a Kel-F-PCTFE (polychlorotrifluoroethylene) lining is used to house the switch and high pressure gas.

IEEE Journals

Authors: Jiang, WH; Weber, B; Mankowski, J

PDF: https://ieeexplore.ieee.org/document/5247063

Abstract: The 23 papers in this special issue were originally presented at the 17th International Conference on High-Power Particle Beams (BEAMS), held in Xi'an China, on July 7-11, 2008.

Journal

Authors: Y. Chen; J. J. Mankowski; J. C. Dickens; J. Walter; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4743614

Abstract: Recent research efforts at Texas Tech University on impulse antenna phased array has necessitated the need to develop a reliable high voltage, high repetition rate switch that will operate with ultra low jitter. An ideal jitter of a small fraction of the risetime is required to accurately synchronize the array to steer and preserve the risetime of the radiated pulse. In (Y. Chen et al., 2007), we showed the initial test system with sub-ns results for operations in different gases and gas mixtures. This paper presents the impact gases and gas mixtures have on switch performance which includes recovery rate and in particular jitter. A 50 Omega, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the different gases. Triggering is provided by a solid state opening switch voltage source that supplies ~150 kV, 10 ns risetime pulses at a rep rate up to 100 Hz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high pressure gas. This paper discusses in detail 50 k, 100 Hz switch operations with different gases. Gases tested include, dry Air, H2, N2, and SF6, as well as H2-N2, and N2-SF6 gas mixtures. Switch jitter as a result of triggering conditions is discussed, also including a comprehensive evaluation of jitter as a function of formative delay in the various gases.

IEEE Conferences

Authors: Ryan W. Karhi; John J. Mankowski; Magne Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4657655

Abstract: An investigation into arc splitting at distributed energy feed locations is presented. Distributed energy scheme experiments conducted at Texas Tech University reveal secondary arc formation by arc splitting at distributed current feeds. At these locations, dynamic magnetic pressure regions are believed to perturb the plasma and lead to its division into two separate current carrying bodies. Continuing research into this hypothesis and viable methods of prevention are discussed. A two stage distributed energy railgun is utilized for this analysis. Diagnostics including armature B-dots and a photodiode array facilitate an understanding of the plasma dynamics in the complex multi-stage railgun environment. The length of the free- running plasma arc allegedly is believed to be a contributing parameter into the arc splitting phenomenon. These luminosity data collected from the photodiodes provides an arc length measurement of the light emitting particles within the plasma body. This length is observed to expand and contract in accordance with the corresponding magnetic pressure present within the railgun bore.

Conferences

Authors: D. Belt; J. Mankowski; J. Walter; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4743602

Abstract: In recent years, the pulsed ring-down antenna has become of great interest due to its compact size and high power on target potential. Since these systems are fairly new in study, it is often difficult to predict the overall performance without experimental evaluation. A pulsed ring-down antenna operates by charging the single element antenna with a high potential source and then closing a switch to develop transient wave reflections on the antenna, typical CW case analysis does not apply. For this reason, we have constructed a simulation model that allows us to predict the transient behavior of the structure. By utilizing the Comsol RF module transient analysis functions, we are able to characterize various parameters of different antennas, beginning with a dipole pulsed ring-down antenna operating around the 100 MHz range. After examining the simulated results against the experimental results for accuracy, we then moved to more complicated mesoband antenna structures. The simulation model developed within the COMSOL RF module allows us to examine various influential factors such as material losses, transient switching effects, structure capacitance, switch capacitance, and initial charging losses. With this, we are able to examine methods to improve the results in the far field such as capacitive spark gap loading and other capacitive storage methods. Utilizing the pulsed ring-down antenna model, we are able to give a better characterization of mesoband pulsed ring-down structures for implementation into a specific or multi-purpose phased array system.

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: D. Belt; J. Mankowski

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4598926

Abstract: Hall-effect thrusters scaled to power levels below 300 W are of great interest due to their compact size but still require further system optimization. A major component of these thrusters is the free electron source. The majority of the current systems utilize heated element hollow cathodes, but in the event of heater failure, the overall system becomes inoperable. For this reason, a simplistic alternate system such as an arc hollow cathode has been examined. The drawback of utilizing the arc hollow cathode is the reduction in the operational lifetime, especially when the cathode experiences multiple start-up cycles. In order to remedy this, we have developed a soft start-up and continuous operation power supply system. Utilizing this system, we were able to minimize the start-up process from the lifetime influences and examine other factors.

Journal

Authors: J. Walter; J. Mankowski; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4599124

Abstract: Most pulsed high-power microwave sources use explosive electron emission cathodes to generate high current electron beams. In the explosive emission process, the current emitted through small field emission points becomes high enough to cause the cathode material to vaporize and form a plasma. Plasma characteristics, such as uniformity and expansion rate, will affect the performance of the microwave source. High-speed optical imaging can be used to resolve some characteristics of the plasma in time. The images of the cathode plasma during the operation of a triode-geometry virtual cathode oscillator high-power microwave source are presented for three different cathode materials.

IEEE Journals

Authors: Yeong-Jer Chen; John J. Mankowski; John W. Walter; James C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4590809

Abstract: Summary form only given.Recent attention to impulse antenna phased array has necessitated the need to develop a reliable high voltage, high repetition rate switch that will operate with ultra low jitter. An ideal jitter of a small fraction of the rise time is required to accurately synchronize the array to steer and preserve the rise time of the radiated pulse. This paper presents the impact gases and gas mixtures have on switch performance which includes recovery rate and in particular, jitter. A 50 Omega, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source. Triggering is provided by an SOS voltage source that supplies >100 kV, 10 ns rise-time pulses at a rep rate up to 1 kHz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high pressure gas. The system includes a gas mixing chamber that can mix various gases up to 2000 psi. Gases tested include dry air, H2, N2, and SF6. Initial testing with 30 kV, 10 Hz switch operations have shown reliable sub-ns jitter times with pure gases including dry air, H2, N2, and with H2-N2, and N2-SF6 gas mixtures. The system was then modified for 50 kV, 100 Hz operations with no recovery issues. Jitter data for pure gases, H2-N2, N2-SF6, and various Kr85 gas mixtures at the 50 kV, 100 Hz operation is also documented and compared in this paper.

Conferences

Authors: Y. Chen; J. J. Mankowski; J. C. Dickens; J. Walter; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4663175

Abstract: Recent attention to impulse antenna phased array has necessitated the need to develop a reliable high-voltage high-repetition-rate switch that will operate with ultralow jitter. An ideal jitter of a small fraction of the rise time is required to accurately synchronize the array to steer and preserve the rise time of the radiated pulse. This paper presents the impact that gases and gas mixtures have on switch performance which includes recovery rate and, in particular, jitter. A 50-Omega 1-nF pulse-forming line is charged to 30 kV and provides the low inductance voltage source to test the different gases. Triggering is provided by a solid-state opening switch voltage source that supplies >100-kV 10-ns rise-time pulses at a rep rate of up to 1 kHz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high-pressure gas. The system includes a gas-mixing chamber that can mix various gases up to 2000 psi. Gases tested include dry air, H2, N2, and SF6. Switch operations in 30 kV and 10 Hz have shown reliable subnanosecond jitter times with pure gases, including dry air, H2, N2, and with H2 - N2 and N2 - SF6 gas mixtures. The system was then modified for 50-kV 100-Hz operations with data collected for each of the pure gases. Recovery was monitored with no major problems at the 100-Hz operation, and subnanosecond jitter results for H2 , N2, and SF6 are also recorded.

IEEE Journals

Authors: Ryan W. Karhi; John J. Mankowski; James C. Dickens; Magne Kristiansen; David A. Wetz

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4667685

Abstract: Experimental results comparing a breech-fed scheme and two distributed energy schemes for a free-running arc are presented. Analysis and observations of the issues associated with distributed energy switching of a plasma arc in the railgun are explored. The use of a free-running arc allows experiments to emulate the ablation and restrike phenomenon of a plasma armature railgun at high speeds (> 5 km/s) without the requirement of a large amount of stored energy. Numerous experimental tests were conducted to investigate the dynamics of plasma arcs within a distributed energy source railgun. Variations of switch timing, bore pressure, bore material, current amplitude, and current pulse length within each stage have been tested. These data reveal important design parameters for distributed energy railguns. The arc length, stage length, and stage trigger timing play a crucial role in distributed energy railgun performance. Failure to take these parameters into consideration will result in velocity reduction through plasma arc restrike and/or splitting.

Journals

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. Mankowski; J. Dickens; M. Giesselmann; B. McDaniel; B. McHale; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4033078

Abstract: Experimental results of a distributed energy source railgun are presented. Distributed energy source railguns were first proposed by Marshal in an asynchronous scheme and later by Parker synchronously. Both schemes employ a "traveling excitation wave" to push the projectile along the rail. The primary advantages of such a scheme over the common breech-fed is higher efficiency due to less energy remaining in the rail and lower rail resistive loses. Another advantage is the reduction in the probability of re-strike. However, these advantages are achieved at a cost of higher switching complexity. As a proof of principle experiment, we have constructed a bench-top solid armature railgun with distributed energy sources. Instead of a single, capacitive, breech-fed, energy source, the current is supplied by two storage capacitor banks, placed at different positions along the rail. The switching configuration, which requires a dedicated switch at each capacitor, is realized with sold state switches. The railgun is diagnosed in order to evaluate performance and to appropriately trigger the switches. In addition, experimental results are compared to simulation

IEEE Journals

Authors: R. W. Karhi; J. J. Mankowski; J. C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4346322

Abstract: Summary form only given. Experimental results utilizing a distributed energy scheme and free-running arc are presented. Analysis and observations of the issues associated with distributed energy switching of a plasma armature in the railgun will be explored. The use of a free running arc allows experiments to emulate a plasma armature railgun at high speeds (> 5 km/s) without the requirement of a large amount of stored energy. Diagnostics for this examination include rail and plasma current probes as well as independent Rogowski coils for each stage. The distributed system is comprised of four stages spaced uniformly along the rail length. Each stage contains a high voltage capacitor, an inductor, a phase controlled SCR, and a driver board for triggering. The high voltage capacitors used are capable of storing 50 kJ, but are typically charged to store less than 20 kJ. Fiber optic lines are integrated into the system to prevent misfire in the noisy EMI environment. Optimal switch timing is predicted by a computer simulation and tested for accuracy. The assembled railgun is 2.4 m long with a 1.7 cm times 1.7 cm bore cross section. The rails are made of machined UNS C11000 ETP copper. G-10 insulation contains the current probes and separates the Aluminum 6061-T6 outer support structure from the rails. Alumina ceramic inserts are secured by the G-10 and function to reduce in-bore ablation. A PVC chamber encloses the railgun and allows experiments to be conducted within the desired 1-10 torr environment. A plasma source at the breech reliably supplies plasma for each experimental test. The plasma injector is powered by a Marx generator to supply a 40 kV voltage and pulse length of 10 s. The coaxial injector is comprised of a tungsten rod, ceramic insulator, and stainless steel outer casing. Numerous experimental tests were conducted to investigate the dynamics of plasma armatures within a distributed energy source railgun. Variations of switch timing, bore pressure, current amplitude, and current pulse length within each stage have been tested. This data is analyzed to determine the effectiveness of a distributed energy system to suppress the plasma restrike phenomenon and increase plasma armature railgun performance.

IEEE Conferences

Authors: R. Karhi; J. Mankowski; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4652541

Abstract: Experimental results comparing a breech-fed scheme and a distributed energy scheme for a free-running arc are presented. Analysis and observations of the issues associated with distributed energy switching of a plasma armature in the railgun will be explored. The use of a free running arc allows experiments to emulate a plasma armature railgun at high speeds (≫5km/s) without the requirement of a large amount of stored energy. Numerous experimental tests were conducted to investigate the dynamics of plasma armatures within a distributed energy source railgun. Variations of switch timing, bore pressure, current amplitude, and current pulse length within each stage have been tested. These data are analyzed to determine the effectiveness of a distributed energy system to suppress the plasma restrike phenomenon and increase plasma armature railgun performance.

IEEE Conferences

Authors: Y. Chen; J. Mankowski; J. Walter; M. Kristiansen; R. Gale

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4286545

Abstract: We are fabricating and testing several different types of cathodes for the same vircator driven by a single shot Marx generator and pulse forming line (300 kV, 60 ns, 30 Omega). The cathodes types, each with an emitting area of ~32 cm2, include the original velvet with a new geometry, carbon fiber, pin-array, and an array of all metal cathodes. The metal cathodes are made from aluminum and oxygen free copper fashioned to similar geometries with either a chemical etch or a CNC machining process. The vircator is tested with all of these cathodes using both polarity configurations and evaluated for beam voltage, current density, microwave output, and single shot lifetime. In addition to the cathode testing, several stainless steel and tungsten anode meshes with varying transparencies (50% - 80%) are evaluated. The construction and testing of an anode fashioned from Tantalum (70% transparency) is also discussed. Electron beam uniformity of the metal cathodes is investigated with the emitted electron beam current-density distribution evaluated in both time and space. Optimization of output power using resonant effects is also examined.

Journals

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: Yeong-Jer Chen; John J. Mankowski; John W. Walter; James C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4345561

Abstract: Summary form only given. Recent attention in impulse antenna phased array has necessitated the need to develop a reliable high voltage, high repetition rate switch that will operate with ultra low jitter. An ideal jitter of a small fraction of the rise time is required to accurately synchronize the array to steer and preserve the rise time of the radiated pulse. This paper presents the impact, gases and gas mixtures have on switch performance which includes recovery rate and in particular, jitter. A 50 Omega, 2.4 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the different gases. Triggering is provided by an SOS voltage source that supplies >100 kV, 10 ns rise-time pulses at a rep rate up to 1 kHz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high pressure gas. The system will also introduce a high pressure gas mixing chamber that can mix various gases up to 2000 psi. Gases tested include dry air, H2, and SF6.

Conferences

Authors: Yeong-Jer Chen; John J. Mankowski; John W. Walter; James C. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4651833

Abstract: Recent attention to impulse antenna phased arrays has necessitated the need to develop a reliable high voltage, high repetition rate switch that will operate with ultra low jitter. The ideal jitter of such a switch would be a small fraction of the risetime in order to synchronize the array to steer and preserve the risetime of the radiated pulse. This paper presents the impact, gases and gas mixtures have on switch performance which includes recovery rate and in particular, jitter. A 50 Ω, 1 nF pulse forming line is charged to 30 kV and provides the low inductance voltage source to test the different gases. Triggering is provided by an SOS voltage source that supplies ≫100 kV, 10 ns risetime pulses at a rep rate up to 1 kHz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high pressure gas. The system contains a high pressure gas mixing chamber that can mix various gases up to 2000 psi. Gases tested include dry air, H2, N2, and SF6.

Conferences

Authors: D. Belt; J. Walter; J. Mankowski; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4346122

Abstract: Summary form only given. A pulsed ring-down phased array antenna provides substantial energy deposition in the far field region in addition to a broad range main beam with scanning capabilities. This allows remote neutralization of Improvised explosive devices (IEDs) at far field distances and in virtually any direction. The pulsed ring-down antenna operates by charging the single element antenna with a high potential source and closing a switch to develop transient wave reflections on the antenna which then propagate in air. The performance of a pulsed ring-down phased arrav is highly contingent upon the design and performance of the individual antenna elements within the array. Such factors as operating voltage, antenna capacitance, material losses, antenna geometry and closing switch conductance characteristics must be examined for optimal performance to be achieved. By utilizing the Comsol RF module transient analysis functions, we are able to characterize the various parameters beginning with a monopole and a dipole pulsed ring-down antenna operating in the hundreds of MHz range. We have examined and compared the results achieved from the experimental setup to the simulation model in order to better characterize the individual components of the antenna. We have also examined the discrepancies between an ideal closing switch and the experimental setup closing switch, which dramatically affects the far field range of the antenna. We have examined the material properties of the antenna to improve losses and increase system capacitance allowing an increase in the number of RF cycles per antenna discharge. With the results presented, an accurate model of pulsed ring-down antennas is available and will allow future development of more complex geometries that will improve the operation of pulsed ring- down phased array.

Conferences

Authors: D. Belt; J. Mankowski; J. Walter; J. Dickens

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4652443

Abstract: A pulsed ring-down phased array antenna provides substantial energy deposition in the far field region in addition to a broad range main beam with scanning capabilities. This allows remote neutralization of Improvised Explosive Devices (IEDs) at far field distances and in virtually any direction. The pulsed ring-down antenna operates by charging the single element antenna with a high potential source and closing a switch to develop transient wave reflections on the antenna which then propagate in air. The performance of a pulsed ring-down phased array is highly contingent upon the design and performance of the individual antenna elements within the array. Such factors as operating voltage, antenna capacitance, material losses, antenna geometry and closing switch conductance characteristics must be examined for optimal performance to be achieved. By utilizing the COMSOL RF module transient analysis functions, we are able to characterize the various parameters beginning with a monopole and a dipole pulsed ring-down antenna operating in the hundreds of MHz range. We have examined and compared the results achieved from the experimental setup to the simulation model in order to better characterize the individual components of the antenna. We have also examined the discrepancies between an ideal closing switch and the experimental setup closing switch, which dramatically affects the far field range of the antenna. We have examined the material properties of the antenna to improve losses and increase system capacitance allowing an increase in the number of RF cycles per antenna discharge. With the results presented, an accurate model of pulsed ring-down antennas is available and will allow future development of more complex geometries that will improve the operation of pulsed ring-down phased array.

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. Walter; J. Dickens; J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4345788

Abstract: Summary form only given. High-power transient RF transmitters are currently being developed for multiple applications. One potential high- power pulsed source being considered is the pulsed ring- down source (PRDS). In a typical PRDS, electrical energy that is stored in the device structure is discharged through a switch, generating a damped ringing radiated waveform. The magnitude of the transmitted field is limited by parameters such as the overall device geometry and switch characteristics. The potential exists to increase the radiated far field power density by utilizing an array of PRDS transmitters. The performance of such an array depends on the triggering jitter of the device switches and the design of the individual radiating elements. The application of the array concept to pulsed ring-down sources using gas spark gap switches is examined. Theoretical array performance is discussed, taking into account practical switch jitter distributions, practical switch triggering schemes, and individual element characteristics.

Conferences

Authors: J. Walter; J. Dickens; J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4651932

Abstract: High-power transient RF transmitters are currently being developed for multiple applications. One potential high-power pulsed source being considered is the pulsed ring-down source (PRDS). In a typical PRDS, electrical energy that is stored in the device structure is discharged through a switch, generating a damped ringing radiated waveform. The magnitude of the transmitted field is limited by parameters such as the overall device geometry and switch characteristics. The potential exists to increase the radiated far field power density by utilizing an array of PRDS transmitters. The performance of such an array depends on the triggering jitter of the device switches and the design of the individual radiating elements. The application of the array concept to pulsed ring-down sources using gas spark gap switches is examined. Theoretical array performance is discussed, taking into account practical switch jitter distributions.

Conferences

Authors: B. T. McDaniel; J. J. Mankowski; D. Wetz; B. McHale; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4530677

Abstract: The experimental results of a five stage distributed energy source railgun are presented. The advantages of such a scheme are increased efficiency due to less energy remaining in the railgun inductance and lower rail resistive losses. The design is a bench top 81 cm long solid armature railgun with a 1.27 cm × 1.27 cm bore cross-section separating the 1.27 cm × 2.54 cm copper rails. Multiple capacitive storage banks are connected at different positions along the length of the rails. Each bank is composed of electrolytic capacitors, toroidal inductors, SCRs, and free-wheeling diodes. The primary bank (stage 1) can store 11.6 kJ, and the other stages store 5.8 kJ of energy. The diagnostics include Rogowski coils at each bank, a flux ruler along the entire railgun to monitor the armature's velocity, and B-dots. The switches are controlled by pulsers with fiber optic inputs, which accurately trigger the SCRs. To sequence trigger the storage banks, B-dot sensors provide feedback to the armature's position. Additionally, experimental results are compared to PSPICE simulations.

Conference Paper/Presentation

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: S. L. Holt; D. J. Hemmert; J. W. Walter; J. J. Mankowski; J. C. Dickens; M. Kristiansen; L. L. Altgilbers

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4530693

Abstract: Explosively driven ferroelectric generators (FEGs) are reliable, compact, high voltage sources for driving ultra-compact high power devices. When energized by mechanical stress, they release charge from their crystal structure, acting similar to a charged high voltage capacitor. However, due to the relatively low energy of these devices, they must be well matched to their application so that the full potential of the material can be utilized. This requires proper characterization of the material's electrical output as a function of the input shockwave. HEM Technologies and Texas Tech University characterized a ferroelectric ceramic, EDO EC-64, for different pressures to determine possible loss mechanisms and its maximum output power and energy.

Conference Paper/Presentation

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: R. W. Karhi; J. J. Mankowski; D. J. Hemmert; S. L. Holt

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4530676

Abstract: The design and implementation of a real time feedback control system for a distributed energy, bench top, electromagnetic launcher is presented. The feedback control system provides optimum pulse shaping by real time control of solid state switches. Advantages of pulse shaping control include increased energy efficiency and control of armature exit velocity. Lab VIEW 8.0 software¹ is used to program a National Instruments CompactRIO programmable automation controller (PAC). This provides real time processing by use of the reconfigurable I/O (RIO) FPGA technology. The program controls switch timing from analog feedback signals supplied by B-dot probes placed along the rail length. Through signal analysis, real time armature position is derived. The program uses this data to control pulse shape and width. A dedicated B-dot probe is placed at the beginning of each stage which is the desired triggering location. A flux ruler sensor along the bore length provides a secondary velocity calculation excluded from the control system. This sensor provides velocity measurements for every centimeter of bore travel. Collected data is used to characterize the system under test for different load conditions.

Conference Paper/Presentation

Authors: D. A. Wetz; J. J. Mankowski; J. C. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1710025

Abstract: A unique theoretical model of the breakdown mechanism in water has been developed and further tested in both simulation software and experimentation. The conducted experiments test the degree to which electrode material, surface roughness, and surface area impact the dielectric strength of water. Voltage pulses with respective rise times of roughly 200 and 20 ns were applied to a water test gap producing electric fields in excess of 1.5 MV/cm. In experiments testing various electrode materials, thin film coatings of various metallic alloys and oxides were applied to Bruce-profiled stainless steel electrodes, with an effective area of 5 cm2, through ion beam deposition. Similar Bruceprofiled stainless steel electrodes with surface roughness ranging from 0.26 to 1.96 mum and effective areas ranging from 0.5 to 75 cm2 were used in the study of surface roughness and area. Additionally, shadowgraph images of a point plane geometry were taken to further understand the breakdown processes that occur

IEEE Journals

Authors: D. Wetz; J. Mankowski; D. McCauley; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4216144

Abstract: Experimental results are presented on the impact water conductivity, electrode material, and electrode surface roughness have on the dielectric strength of water. A 2 mm water gap was tested under pulsed conditions with maximum electric fields in excess of 1.5 MV/cm. Bruce profiled electrodes manufactured from various materials including molybdenum, aluminum, copper, tungsten, nickel, stainless steel, and zinc oxide, which have work functions ranging from 4.2 eV to 5.3 eV, have been tested. The roughness average of the various surfaces was varied from 0.1 mum to 1.96 mum and the conductivity of the water within the gap was varied from 1muS/cm to 39 muS/cm to determine the impact each has when tested in combination with the various electrode materials and surface roughnesses. Conclusions are made as to the effect each of these factors has on the holdoff strength of a water dielectric system

Conferences

Authors: Y. Chen; J. Mankowski; J. Walter; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4216158

Abstract: Recent research efforts at TTU include a simple aluminum cathode for E-beam generation in a virtual cathode oscillator (vircator). As a continuation of these results, we are fabricating and testing several different types of cathodes for the vircator driven by a Marx generator and a pulse forming line (400 kV, 60 ns, 30 Omega). The cathodes, each with an emitting area of ~32 cm2, are made from aluminum and oxygen free copper fashioned to similar geometries with either a chemical etch or a CNC machine process. The vircator is tested with all of these cathodes and evaluated for beam voltage, current, and HPM generation. In addition to the cathode testing, several stainless steel and tungsten anode meshes with varying transparencies (50%-80%) are evaluated

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: D. Wetz; J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4198346

Abstract: Summary form only given. In the past there has been considerable research performed using electromagnetic plasma thrusters as the primary ion source in sputtering applications. In the experiments conducted here, thin film coatings of various hard metallic alloys and oxides were applied to stainless steel electrodes using ion beam deposition. An SPT type Hall effect thruster capable of producing an ion beam with energies between 150 eV and 350 eV was used as the deposition ion source. The surface coatings were applied to Bruce profiled stainless steel electrodes which have an effective surface area of 5 cm2 and a roughness average of roughly 70 nm. The electrodes were then examined using an optical profiler to accurately measure the roughness average of the thin film coating as well as the effective layer thickness. A comparison of the results obtained from each surface material is performed

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: Xupeng Chen; James C. Dickens; L. L. Hatfield; John Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4084184

Abstract: In virtual cathode oscillator (VIRCATOR) research, multi-frequency and multi-mode microwaves are coupled out and propagate along a waveguide. Identifying the microwave propagating modes has become a priority work for further research on microwave radiation physical mechanisms. Generally, an antenna matrix at the end of an open waveguide, which can shape the radiated microwave power distribution, is used to identify the microwave propagating modes [1-2]. Actual microwave radiation mechanisms in a VIRCATOR are complicated. Multi-propagating modes and multifrequency microwaves coexist simultaneously, which challenges the antenna matrix method. In this paper, based on microwave propagating theory in a waveguide, a new method is proposed to identify multi-frequency and multi-mode microwaves, which is partially justified by the experimental data at Texas Tech University.

Conferences

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: X. Chen; J. Mankowski; L. L. Hatfield; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084185

Abstract: In virtual cathode oscillator (VIRCATOR) research, the radiated microwave pulse is coupled out and propagates along a waveguide. The radiated microwave power is an important parameter to evaluate the oscillator design and is a prerequisite parameter to calculate the energy transferring efficiency. Because the radiated microwaves are an ultra-short pulse of multi-frequency in multi propagating modes, the power measurement is not easily manipulated. In this paper, a broadband calorimeter design with considerations of pulsed microwaves is presented. The design includes its material or medium selection, structure optimization and calibration.

IEEE Conferences

Authors: D. Wetz; J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084430

Abstract: Experimental results are presented on the degree to which electrode surface area and surface roughness impacts the dielectric strength of water. A 2 mm water gap was tested under pulsed conditions with maximum electric fields in excess of 1 MV/cm and maximum currents of more than 5 kA. Six different pairs of stainless steel electrodes, each having a unique Bruce profile and thus a different effective surface area, were used to achieve a uniform electric field across the gap. The differing electrode pair profiles, with effective areas ranging from 0.5 cm2 to 76 cm2, were designed to minimize the change in gap capacitance. Prior to each test, a different roughness average, ranging from 0.26 μm to 1.96 μm, was applied to the electrodes. Conclusions are made as to the effect both electrode surface area and surface roughness has on the holdoff voltage of water dielectric systems. In addition, shadowgraph images of pre-breakdown events are presented.

IEEE Conferences

Authors: D. Wetz; J. Mankowski; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4084372

Abstract: In the experiments presented here, various electrode materials were tested in an effort to determine the impact each has on increasing the dielectric strength of water. Prior investigations have tested materials such as stainless steel, copper, nickel, gold, silver, and cuprous oxide [1-4]. In our experiments, thin film coatings of various metallic alloys and oxides were applied to Bruce profiled stainless steel electrodes with an effective area of 5 cm2. An ion beam sputtering process was used to apply the coatings with thicknesses of several hundred nm. The electrodes were then tested across a water gap, with pulse lengths in both the microsecond and nanosecond time regimes. Electric fields in excess of 8 MV/cm were applied. Conclusions are made as to the impact electrode material has on the pulsed breakdown strength of water.

IEEE Conferences

Authors: D.A. Wetz; K.P. Truman; J.J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1495552

Abstract: Experimental results are presented on the degree to which electrode surface conditioning and surface area impact the dielectric strength of water. The applied pulse to the test gap reached electric field levels greater than 1.5 MV/cm with risetimes around 200 and 50 ns, respectively. The test gap is composed of 304 stainless steel electrodes machined with a Bruce profile. Electrode surface roughness ranged from 0.34 to 1.41 /spl mu/m and effective areas ranged from 0.5 to 75 cm/sup 2/. Additional results are presented on the pulsed breakdown strength of Rexolite with various surface finishes ranging from .025 to 5.715 /spl mu/m. Conclusions are made as to the effect electrode surface area and surface roughness has on the holdoff voltage of water dielectric systems. Conclusions are also made as to the impact of the surface condition of Rexolite has on its bulk breakdown strength.

Journals

Authors: John J. Mankowski; Xupeng Chen; James C. Dickens; Magne Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6220572

Abstract: Experimental findings on a high power Reflex Triode Virtual Cathode Oscillator (Vircator) at Texas Tech University are reported. In order to optimize performance, changes to the AK gap distance were made. The AK gap was varied from 0.5 to 1.5 cm. The type of cathode material employed was velvet. Cathode radius was 2.5 cm. The Reflex Triode Vircator is driven by a 350 kV Marx generator with a 60 ns, 20 ohm pulsed forming line. Typical peak input power is 4 GW. The output microwave power is measured in the far-field with a waveguide to coax adapter. Thus far we have observed microwave peak power output as high as 330 MW corresponding to a peak power efficiency of ~ 11 %. In an effort to provide premodulation to the vircator region, reflecting strips were added in the downstream waveguide. This technique has been employed with success by this group with a coaxial Vircator.

Conferences

Authors: Dickens, JC; Lehr, JM; Mankowski, J

PDF: https://ieeexplore.ieee.org/document/1347223

Abstract: The tradition of chronicling latest advances in pulsed power continues with this fifth edition Special Issue on Pulsed Power Science and Technology. The special issue has proven popular with scientists and engineers. Moreover, the publication of comprehensive peer-reviewed articles in a special issue of a journal devoted to the topic is increasingly recognized as personally and professionally worthwhile. This special issue is a compilation of expanded versions of some of the best papers presented at the 2003 International Pulsed Power Conference (PPC), held in Dallas, TX, June 15–18, 2003. The Proceedings of the Pulsed Power Conference is the premiere forum for exchanging results and ideas in the area of pulsed power and related technologies. In the last decade, the Pulsed Power conferences have been very successful, as is evident from the number of participants and quality papers, with over 25 countries represented. The 2003 PPC is no exception, with 584 registrants from 22 countries and 477 papers. Even with the extraordinary and unprecedented problems in obtaining visas for many foreign participants, the conference still recorded 180 registrants from outside the United States. Pulsed power technology has traditionally been dominated by military applications with some civilian efforts primarily in the area of controlled nuclear fusion. More recently, however, the number of civilian applications has grown considerably. Of particular interest are the areas of biological and medical applications, compact pulsed power, and repetitive pulsed power systems. It is interesting to note that the contributed papers to this special issue are roughly evenly divided between components, fundamental phenomena, systems and applications. All but a few of the papers in this issue are based on the presentations given at the International Pulsed Power Conference. The reader of this Fifth Special Issue on Pulsed Power Science and Technology will also see this truly international participation reflected in the contributed papers. Similarly, the diverse nature of the pulsed power community is reflected in the originating institutions of universities, industry, and government laboratories. Making a clear classification between the topics covered in this issue has been a challenging task since the topics range from flashover phenomena to pulsed power driven sterilizing technology, and from directed weapon technology to explosively driven pulsed power to miniature hole drilling industrial applications. This Fifth Special Issue on Pulsed Power Science and Technology for the IEEE Transactions on Plasma Science has been an extraordinary and enjoyable experience for each of the three Guest Editors. Solicitation of papers is largely conducted through advertisement at the Pulsed Power Conference and, now, tradition. The peer review process, however, was greatly facilitated by the introduction of IEEE's ManuscriptCentral. ManuscriptCentral is a semi-automated electronic database for manuscript control and processing, that was adopted last year by the Transactions on Plasma Science, and championed by Editor-in-Chief Steve Gitomer. It truly has been a revolutionary advance in the synthesis of special issues, a mainstay of the Transactions on Plasma Science, by effectively eliminating the physical distance between the Editor-in-Chief, Guest Editors, referees, and authors. As Guest Editors, we would like to extend our gratitude to the Editor-in-Chief, Steve Gitomer, for his many kindnesses, his patience, and his vigilance. We would like to express our gratitude to both the authors and the referees for their contributions to our profession. In particular, we thank the referees for their understanding, patience, and noble efforts of reviewing the manuscripts. Jane Lehr would like to thank Dr. John Maenchen and Mrs. Beverly Rudys for their encouragement and support for this effort. Jim Dickens and John Mankowski would like to thank their colleagues at Texas Tech University for their many contributions to the success of this special issue. We would also like to extend our thanks and appreciation to our colleagues of the IEEE Nuclear and Plasma Sciences Society, Pulsed Power Science and Technology Standing Committee for their unwavering support and encouragement, for with their vision, and leadership of the pulsed power community, this Biannual Special Issue on Pulsed Power Science and Technology is a tradition.

Journal

Authors: D. Wetz; J. Mankowski; K. Truman; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1433626

Abstract: Experimental results are presented on the impact electrode material and surface treatment has on the dielectric strength of water. A 4 mm water gap was tested under pulsed conditions with pulse widths greater than 1 /spl mu/s, peak electric fields over 2 MV/cm and peak currents over 15 kA. 304 stainless steel electrodes machined with a Bruce profile and an effective area of 5 cm/sup 2/ were tested with surface roughnesses ranging from .34 /spl mu/m to 1.41 /spl mu/m. Results comparing electrodes that have additionally been electropolished are compared to those without an additional treatment. These various surface treatments remove microprotrusions from the electrode's surface which reduce field enhancements across the electrode surface. It is believed that this technique will minimize the number of breakdown initiation points thereby increasing the dielectric strength. Results are given on the degree to which each finish improves the dielectric strength of water.

IEEE Conferences

Authors: Xupeng Chen; J. Dickens; J. Mankowski; L.L. Hatfield; Eun Ha Choi; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1347227

Abstract: Traditionally, the radiated microwave frequency in a coaxial vircator is considered to be determined primarily by the virtual cathode oscillation frequency and the electron reflection frequency. In this paper, some experiments showing different results are reported. In particular, the E-beam is observed to play an important role in the cavity formation. Some possible explanations, including a virtual cavity concept, are proposed. The cavity resonance effect on a coaxial virtual cathode oscillator with different geometries has been investigated in detail. Investigation of the E-beam performance will improve understanding of the interaction between the E-beam and microwaves, which is a key for determining the microwave frequency. These results are helpful in optimizing the design of a cylindrical diode to avoid microwave frequency shifting and mode competition.

Journals

Authors: K. Truman; J. Mankowski; D. Wetz; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1433523

Abstract: Experimental results are presented on the degree to which electrode surface area impacts the dielectric strength of water. A water gap of 4 mm was tested under pulsed conditions with a maximum electric field in excess of 1 MV/cm and a maximum current of more than 20 kA. Stainless steel electrodes with a Bruce profile were used to generate a uniform electric field across the water gap. The profile of the electrodes enabled effective areas ranging from 0.5 cm2 to 75 cm2 while minimizing the change in capacitance of the water gap. Conclusions are made as to the effect electrode surface area has on the holdoff voltage and holdoff time of water dielectric systems

IEEE Conferences

Authors: Xupeng Chen; J. Dickens; Eun Ha Choi; J. Mankowski; L.L. Hatfield; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1278019

Abstract: Traditionally, the radiated microwave frequency in a coaxial vircator is considered to be determined primarily by the virtual cathode oscillation frequency and the electron reflection frequency. However, some experiments at TTU show that the interaction between the virtual cathode oscillation and the cavity is a key in determining the microwave frequency and propagating mode. Particularly, we observe that the E-beam plays an important role in the cavity formation. Some possible explanations, including a virtual cavity concept, are proposed. The cavity resonance effect on a coaxial virtual cathode oscillator has been investigated in detail. The understanding of the E-beam characteristics is critical in optimizing the design of the cylindrical diode to avoid the microwave frequency shifting and mode competition.

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. Mankowski; X. Cheng; J. Dickens; M. Kritiansen; E. Choi

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1277749

Abstract: Experimental findings on the coaxial virtual cathode oscillator (vircator) at Texas Tech University are reported. A major modification to the system extended the pulse forming line several meters. The pulsed power system now delivers up to 750 kV and 60 kA for at least 70 nsec. Additional modifications include diagnostics to measure the diode voltage, current, and radiated field pattern of the output microwaves. In this initial phase of experiments copper reflectors were installed within the diode to provide a simple means of microwave feedback to assist in the e-beam modulation. Thus far we have observed microwave peak power output with feedback reflectors as high as 3.5 GW corresponding to an efficiency of /spl sim/9.5%. Measured radiated field patterns show evidence of multimode operation.

IEEE Conferences

Authors: J. J. Mankowski; E. H. Choi; J. C. Dickens; Xupeng Chen; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1228654

Abstract: Summary form only given, as follows. Summary form only given. Experimental findings on the coaxial virtual cathode oscillator (vircator) at Texas Tech University are reported. Recent modifications to the system include extending the pulse forming line that now provides an applied diode voltage of 600 kV for 70 nanoseconds at 60 kA. Other modifications include diode voltage, current, and radiated field diagnostics. In this initial phase of experiments, copper reflectors were installed within the diode to provide a simple means of microwave feedback to assist in the e-beam modulation. Thus far we have observed microwave peak power output with feedback reflectors as high as 2 GW corresponding to an efficiency of -6%. The mode observed at these high efficiencies is almost entirely TE/sub 11/. At lower efficiencies, both TM/sub 01/, and TE/sub 11/ modes are observed. These results suggest mode competition at lower efficiencies.

IEEE Conferences

Authors: Lara, MB; Mankowski, J; Dickens, J; Kristiansen, M

PDF: https://ieeexplore.ieee.org/document/1278018

Abstract: An eight-stage four-hundred kilovolt Marx bank, in connection with a 60 nanosecond pulse-forming line, is constructed and utilized as a pulsed source to power a planar version of the Virtual Cathode Oscillator (Vircator). Eight 1upsilonF capacitors, charged to 50kV each, are switched in series by dry-air pressurized spark gaps. The energy from the bank charges a 23 ohm oil transmission line, breaking a peaking gap when the maximum voltage is reached, delivering a 60 ns-300kV pulse to the diode. The design of the Planar or Reflex-Triode geometry Vircator is based upon claims of high efficiency by Didenko et al. A previously constructed TTU Vircator includes a unique E-beam source, the brush cathode; in which a circular array of pins is used as an explosive field emission source to produce relatively high beam currents. The anode consists of a round wire mesh through which the E-beam passes, generating a dense cloud of negative charge known as a Virtual Cathode. This initial phase of testing is composed of basic operation of the entire system and baseline output power and efficiency measurements.

Conference Paper/Presentation

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: Mankowski, JJ; Hemmert, D; Krompholz, H

PDF: https://ieeexplore.ieee.org/document/1003944

Abstract: A new type of plasma limiter is being developed which can turn on in less than 1 ns. The approach taken is to initiate streamer breakdown via a micron radius needle tip. Images were taken of the gap region in Argon at several pressures in order to investigate the role of the tip region.

Journal

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: H. Krompholz; L. L. Hatfield; M. Kristiansen; D. Hemmert; B. Short; J. Mankowski; M. D. J. Brown; L. L. Altgilbers

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1178229

Abstract: Gaseous breakdown in the subnanosecond regime is of interest for fast pulsed power switching, short pulse electromagnetics, and for plasma limiters to protect electronic devices from high power microwave radiation. Previous investigations of subnanosecond breakdown were mainly limited to high-pressure gases or liquids, with voltages in excess of 100 kV. In this paper, we investigate subnanosecond breakdown at applied voltages below 7.5 kV in point-plane geometries in argon, with a needle radius <0.5 /spl mu/m. The coaxial setup allows current and voltage measurements with temporal resolutions down to 80 ps. Voltages of 7.5 kV (which are doubled at the open gap before breakdown) produce breakdowns with a delay of about 1 ns. With negative pulses applied to the tip and the same amplitude, breakdown is always observed during the rising part of the pulse, with breakdown delay times below 800 ps, at pressures between 10/sup 2/ and 10/sup 4/ Pa. At lower pressure, a longer delay time (8 ns at 6 Pa) is observed. We expect the breakdown mechanism to be dominated by electron field emission, but still influenced by gaseous amplification.

IEEE Journals

Authors: J. Mankowski; J. Dickens; M. Kristiansen; J. Lehr; W. Prather; J. Gaudet

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1158359

Abstract: Corona discharges in ultrawideband radiating systems can have adverse effects on performance such as reflection, phase dispersion, and significant power losses. A test-bed has been assembled to experimentally observe corona created by voltage pulses similar to ultrawideband systems. The current work involves the attenuation of an incident pulse after propagation through a self-initiated corona and relative measurements of visible light emission from the photoionization produced during streamer development. Several gas dielectrics, including ambient air, N/sub 2/, H/sub 2/, and SF/sub 6/, were tested.

IEEE Journals

Authors: H. Krompholz; L. L. Hatfield; M. Kristiansen; D. Hemmert; B. Short; J. Mankowski; M. Brown; L. Altgilbers

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1002139

Abstract: Gaseous breakdown in the sub-nanosecond regime is of interest for fast pulsed power switching, short pulse electromagnetics and for plasma limiters to protect devices from high power microwave radiation. Previous investigations of sub-nanosecond breakdown were mainly limited to high-pressure gases or liquids, with applied voltages in excess of 100 kV. In this paper, the authors investigate possibilities to achieve sub-nanosecond breakdown at applied voltages below 7.5 kV in point-plane geometries. The setup consists of a pulser (risetime between 400 ps to 1 ns), 50-/spl Omega/ transmission line, axial needle-plane gap with outer coaxial conductor, and a 50-/spl Omega/ load line. The needle consists of tungsten and has a radius of curvature below 0.5 /spl mu/m. The constant system impedance of 50 /spl Omega/ (except in the vicinity of the gap) and a special transmission-line-type current sensors enables current and voltage measurements with a dynamic range covering several orders of magnitude, with temporal resolution down to 80 ps. For pulse amplitudes of 1.7 kV (which are doubled at the open gap before breakdown) delay times between start of the pulse and start of a measurable current flow (amplitude > several milliamperes) have a minimum of about 8 ns, at a pressure of 50 torr in argon. Voltages of 7.5 kV produce breakdowns with a delay of about 1 ns. With negative pulses applied to the tip, at an amplitude of 7.5 kV, breakdown is always observed during the rising part of the pulse, with breakdown delay times below 800 ps, at pressures between 1 and 100 torr. At lower pressure, a longer delay time (8 ns at 50 mtorr) is observed. They authors expect the breakdown mechanism to be dominated by electron field emission, but still influenced by gaseous amplification.

IEEE Conferences

Authors: Mankowski, J; Kristiansen, M

PDF: https://ieeexplore.ieee.org/document/842875

Abstract: Today's ultrafast, pulse generators are capable of producing high-voltage pulses, (>1 kV), with fast, leading-edge rise times, (< 1 ns), A review of generator implementation methods is presented that includes a detailed discussion of the various circuit designs and a list of commercially available high-voltage pulse generators. All of these generators are capable of rise times less than a few ns and voltages greater than several hundred volts. Finally, a brief description of the three primary switch types, reed, spark gap, and solid state is presented.

Journal

Authors: J. Kim; J. Zhang; M. Giesselmann; J. Dickens; J. Mankowski; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=823802

Abstract: In this paper, the simulation and evaluation of energy efficiency and voltage amplification in an inductive energy storage system with resistive, capacitive and diode loads is presented. A numerical simulation with Mathcad shows that by proper reduction of the storage energy inductance and of the exploding fuse interruption time, the energy efficiency and voltage amplification can be greatly improved.

IEEE Conferences

Authors: J. Mankowski; J. Dickens; M. Kristiansen; J. Lehr; W. Prather; J. Gaudet

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=829276

Abstract: Summary form only as given.

IEEE Conferences

Authors: J. Zhang; J. Dickens; M. Giesselmann; J. Kim; E. Kristiansen; J. Mankowski; D. Garcia; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=823610

Abstract: To improve the pulse shape and to obtain /spl mu/s order pulse duration on a diode load in an inductive energy storage system, an oil-submerged compact pulse transformer with diameter of 20 cm and length of 70 cm has been designed and tested. From theoretical calculations, the parameters with primary inductance L/sub p/=3.4 /spl mu/H, secondary inductance L/sub s/=90 /spl mu/H, and coupling coefficient K=0.772 are obtained. To prevent breakdown, a conical secondary design is adopted. Under the condition of 500 kV operation voltage, the maximum electric field in the transformer is 205 kV/cm (in oil). The axial voltage distribution on the secondary is linear. Experiments have been done to test the insulation, the mechanical force and the coupling coefficient. The test results are consistent with the design parameters.

IEEE Conferences

Authors: J. Mankowski; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=700858

Abstract: Present-day ultra-wideband radiation sources produce Megavolt pulses at hundreds of picosecond (ps) risetimes. Empirical data on the breakdown characteristics for dielectric media at these short time lengths and high voltages are either extremely limited or nonexistent. In support of the design of these ultra-wideband sources, we are investigating the breakdown characteristics, at these voltages and time lengths, of several liquids and high-pressure gases. These include air, N/sub 2/, H/sub 2/, SF/sub 6/, and transformer oil. Gap voltages attained were over 700 kV and gas pressures were over 150 atm (15 MPa). Breakdown times achieved were on the order of 600 ps. Electric field strengths observed for given breakdown times were higher than predicted by other investigators. An empirical fit is presented for the data obtained.

IEEE Journals

Authors: J. Mankowski; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=677842

Abstract: Summary form only given, as follows. Present day ultra-wideband (UWB) radiation sources a produce Megavolt pulses at 100's of picosecond (ps) risetimes. Empirical data on the breakdown characteristics for dielectric media at these short time lengths and high voltages are either extremely limited or non-existent. In support of the design of these UWB sources, we are investigating the breakdown characteristics, at these voltages and time lengths, of several liquids and gases. These include air, N/sub 2/, H/sub 2/, He, SF/sub 6/, and transformer oil. The two voltage sources used in the experiments are capable of delivering 400 and 700 kV with a 400 ps risetime into an open load. These pulses are applied to the test gap area, capable of housing various gases and liquids at pressures from less than 1 to 150 atm. An empirical relationship of E-field versus breakdown time for the observed dielectrics is presented. Several other breakdown phenomena at these fast risetimes are observed. Dielectric breakdown strength dependence on polarity is investigated. Streak camera images of arc formation are captured, providing information on gap closure velocity. Also observed is the effect of ultraviolet radiation on the statistical lag time of breakdown for gas dielectrics at various pressures.

IEEE Conferences

Authors: J. Mankowski; J. Dickens; M. Kristiansen

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=679394

Abstract: A high voltage, sub-nanosecond pulser is designed and built for the purposes of investigating dielectric breakdown. The requirement for the pulser is a voltage pulse of several hundred kilovolts, pulsewidth less than a few nanoseconds, and a risetime <400 psec. This is achieved by using pulse sharpening techniques on the output of a 500 kV Marx bank. Originally designed to stimulate a lightning strike, the voltage waveform from the Marx generator has a slow decay time of tens of /spl mu/sec. In order to obtain a more desirable pulse, the Marx bank is modified. By removing a lumped-element resistor a higher peak output voltage with a faster risetime can be obtained. Circuit simulations have shown the capability of achieving an 800 kV output in less than 40 nsec to a charging (pulse forming) line. The 50 /spl Omega/ impedance, oil-filled, pulse forming line consists of a peaking gap and pulse slicing gap. The peaking gap decreases the risetime of the applied pulse down to about 300 psec. The pulse slicing gap is included to short the voltage applied to the delay line and test chamber.

IEEE Conferences

Authors: Agee, FJ; Scholfield, DW; Copeland, RP; Martin, TH; Carroll, JJ; Mankowski, JJ; Kristiansen, M; Hatfield, LL

PDF: https://www.spiedigitallibrary.org/conference-proceedings-of-spie/2843/1/Review-of-catastrophic-electromagnetic-breakdown-for-short-pulse-widths/10.1117/12.255410.short?SSO=1

Conference Paper/Presentation

Authors: J. Mankowski; M. Kristiansen; L. Hatfield

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6308398

Abstract: Breakdown characteristics of various liquids are investigated. Electric field strengths recorded are over 4 MV/cm and at breakdown times ranging from 1 to 4 nanoseconds. Liquids examined are filtered and unfiltered transformer oil, castor oil, and freon 12.

IET Conferences