Texas Tech University
Center for Pulsed Power & Power Electronics

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

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

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

Early Access Articles

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

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

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

Early Access Articles

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

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

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

Journal

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

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

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

Journal

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

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

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

Conferences

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

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

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

Conferences

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

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

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

Conferences

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

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

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

Conferences

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

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

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

Conferences

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

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

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

Conferences

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

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

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

Conferences

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

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

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

Conferences

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

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

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

Conferences

Authors: Keju Yan, Xiangyu Jie, Xiaoqiang Li, Juske Horita, Jacob Stephens, Jianli Hu, Qingwang Yuan

PDF: https://scholars.ttu.edu/en/publications/microwave-enhanced-methane-cracking-for-clean-hydrogen-production

Abstract: Steam methane reforming (SMR) generates about 95% of hydrogen (H2) in the U.S. using natural gas as a main feedstock. However, this technology also generates a large amount of carbon dioxide (CO2), a major greenhouse gas causing global warming. Carbon capture and storage (CCS) technique is required, but the cost and safety of storing CO2 underground are a concern. Here we propose a new approach using microwave/electromagnetic irradiation to produce clean hydrogen from unrecovered hydrocarbons within petroleum reservoirs. Solid carbon or CO2 produced during this process will be simultaneously sequestrated underground without involving CCS. In this paper, we perform a series of experiments to investigate the in-situ hydrogen production from shale gas (methane) conversion by passing a methane stream through a packed shale rock sample heated by microwave. We found that methane conversion was significantly enhanced in the presence of Fe and Fe3O4 particles as catalysts, with a conversion of 40.5% and 100% at reaction temperature of 500 °C and 600 °C, respectively. Methane conversion is promoted at a lower reaction temperature by the catalytic effect of minerals in shale. Additionally, the influences of catalysts, shale rock, and methane flow rate are characterized.

Journal

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

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

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

Conferences

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

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

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

Conferences

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

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

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

Journal

Authors: J. Browning; N. M. Jordan; J. Stephens; P. Zhang

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9771293

Abstract: Modern high-power microwave and millimeter-wave technologies continue to be an active focus of study with considerable interest in the scientific, industrial, defense, and academic communities. Today, these technologies find application in precision radar systems, next-generation particle accelerators, fusion devices, deep-space communications networks, electromagnetic warfare, high-bandwidth communications, microwave-assisted catalysis, and extreme depth drilling, to name a few. The ever-growing needs of these applications call for the production and utilization of electromagnetic radiation with tens of cm to sub-mm wavelength and time-averaged power on the order of tens of megawatts and peak power exceeding 1 GW. Such demands have motivated interdisciplinary innovations for both the production and delivery of high-power microwaves. Advances in modern vacuum electronic microwave and millimeter-wave sources include metamaterial and photonic bandgap interaction circuits, advances in cathode materials, geometries, and modeling capability, improved materials, treatments, and surface coatings for high-power components, and novel manufacturing techniques, such as additive manufacturing. These innovations have preceded unprecedented gains in peak and average microwave power, frequency capability and agility, and efficiency of high-power microwave systems. Complementary to microwave source development, considerable gains have been made in understanding and improving the high power capacity of microwave components, especially with regard to multipactor formation.

IEEE Journals

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

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

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

IEEE Early Access Articles

Authors: Keju Yan, Qingwang Yuan, Xiangyu Jie, Xiaoqiang Li, Juske Horita, Jacob Stephens

PDF: https://onepetro.org/SPEATCE/proceedings-abstract/22ATCE/1-22ATCE/D012S066R002/509233

Abstract: Steam methane reforming (SMR) generates about 95% of hydrogen (H2) in the U.S. using natural gas as a main feedstock. However, this technology also generates a large amount of carbon dioxide (CO2), a major greenhouse gas causing global warming. Carbon capture and storage (CCS) technique is required, but the cost and safety of storing CO2 underground are a concern. Here we propose a new approach using microwave/electromagnetic irradiation to produce clean hydrogen from unrecovered hydrocarbons within petroleum reservoirs. Solid carbon or CO2 produced during this process will be simultaneously sequestrated underground without involving CCS. In this paper, we perform a series of experiments to investigate the in-situ hydrogen production from shale gas (methane) conversion by passing a methane stream through a packed shale rock sample heated by microwave. We found that methane conversion was significantly enhanced in the presence of Fe and Fe3O4 particles as catalysts, with a conversion of 40.5% and 100% at reaction temperature of 500 °C and 600 °C, respectively. Methane conversion is promoted at a lower reaction temperature by the catalytic effect of minerals in shale. Additionally, the influences of catalysts, shale rock, and methane flow rate are characterized.

Journal

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

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

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

IEEE Conferences

Authors: J Stephens, L Pitchford, M Hopkins, B Yee

PDF: https://ieeexplore.ieee.org/abstract/document/9813059/references#references

Abstract: LXCat [1] ( www.lxcat.net ) is an on-line platform for the curation of data needed for modeling the electron and ion components of Low Temperature Plasmas (LTPs). LXCat is open-access and no sign-up is required. The platform is organized in databases which are named and maintained by individual contributors. Nearly 60 people from around the world participate in this project, either by contributing data or by volunteering time to work on other aspects of the project. The data types available now are electron-neutral or ion-neutral scattering cross sections, oscillator strengths, and transport parameters (e.g. mobility, diffusion coefficients) and rate coefficients. The LXCat team does not recommend data and hence data for the same process can appear in one or more of the databases. On-line tools are available that allow visitors to search for specific data, plot and compare data from different databases, download data, or use the available complete sets of electron-neutral scattering cross sections in an on-line Boltzmann solver to calculate transport and rate coefficients in pure gases or gas mixtures. The LXCat team is interested in contacting members of the LTP community about data needs and about how people can volunteer to participate in this project.

Journal

Authors: Tahiyat, MM; Stephens, JC; Kolobov, VI; Farouk, TI

PDF: https://iopscience.iop.org/article/10.1088/1361-6463/ac33da/pdf

Abstract: Plasma stratification has been studied for more than a century. Despite the many experimental studies reported on this topic, theoretical analyses and numerical modeling of this phenomenon have been mostly limited to rare gases. In this work, a one-dimensional fluid model with detailed kinetics of electrons and vibrationally excited molecules is employed to simulate moderate-pressure (i.e. a few Torrs) dc discharge in nitrogen in a 15.5 cm long tube of radius 0.55 cm. The model also considers ambipolar diffusion to account for the radial loss of ions and electrons to the wall. The proposed model predicts self-excited standing striations in nitrogen for a range of discharge currents. The impact of electron transport parameters and reaction rates obtained from a solution of local two-term and a multi-term Boltzmann equation on the predictions are assessed. In-depth kinetic analysis indicates that the striations result from the undulations in electron temperature caused due to the interaction between ionization and vibrational reactions. Furthermore, the vibrationally excited molecules associated with the lower energy levels are found to influence nitrogen plasma stratification and the striation pattern strongly. A balance between ionization processes and electron energy transport allows the formation of the observed standing striations. Simulations were conducted for a range of discharge current densities from similar to 0.018 to 0.080 mA cm(-2), for an operating pressure of 0.7 Torr. Parametric studies show that the striation length decreases with increasing discharge current. The predictions from the model are compared against experimental measurements and are found to agree favorably.

Journal

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

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

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

IEEE Conferences

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

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

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

Journal

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

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

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

Journal

Authors: Carbone, E; Graef, W; Hagelaar, G; Boer, D; Hopkins, MM; Stephens, JC; Yee, BT; Pancheshnyi, S; van Dijk, J; Pitchford, L

PDF: https://www.mdpi.com/2218-2004/9/1/16

Abstract: Technologies based on non-equilibrium, low-temperature plasmas are ubiquitous in today's society. Plasma modeling plays an essential role in their understanding, development and optimization. An accurate description of electron and ion collisions with neutrals and their transport is required to correctly describe plasma properties as a function of external parameters. LXCat is an open-access, web-based platform for storing, exchanging and manipulating data needed for modeling the electron and ion components of non-equilibrium, low-temperature plasmas. The data types supported by LXCat are electron- and ion-scattering cross-sections with neutrals (total and differential), interaction potentials, oscillator strengths, and electron- and ion-swarm/transport parameters. Online tools allow users to identify and compare the data through plotting routines, and use the data to generate swarm parameters and reaction rates with the integrated electron Boltzmann solver. In this review, the historical evolution of the project and some perspectives on its future are discussed together with a tutorial review for using data from LXCat.

Journal

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

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

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

Conferences

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

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

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

Conferences

Authors: 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: Jacob Stephens; Guy Rosenzweig; John Tucek; Ken Kreischer; Michael Shapiro; Richard Temkin

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9520451

Abstract: This paper details recent progress on the experimental demonstration of a W-band klystron amplifier completed at the MIT. The amplifier utilizes a square lattice photonic bandgap (PBG) structure that permits the use of a highly oversized beam tunnel of diameter ~λ/4. Cold test measurements of the PBG klystron cavities revealed successful fabrication of the device. In hot test, a small-signal gain of 26 dB was measured at 93.7 GHz, with a saturated output power of 30 W.

Conferences

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

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

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

Journals

Authors: Picard, JF; Schaub, SC; Rosenzweig, G; Stephens, JC; Shapiro, MA; Temkin, RJ

PDF: https://aip.scitation.org/doi/10.1063/1.5093639

Abstract: A laser-driven semiconductor switch (LDSS) employing silicon (Si) and gallium arsenide (GaAs) wafers has been used to produce nanosecond-scale pulses from a 3 mu s, 110 GHz gyrotron at the megawatt power level. Photoconductivity was induced in the wafers using a 532nm laser, which produced 6 ns, 230 mJ pulses. Irradiation of a single Si wafer by the laser produced 110 GHz RF pulses with a 9 ns width and >70% reflectance. Under the same conditions, a single GaAs wafer yielded 24 ns 110 GHz RF pulses with >78% reflectance. For both semiconductor materials, a higher value of reflectance was observed with increasing 110 GHz beam intensity. Using two active wafers, pulses of variable length down to 3 ns duration were created. The switch was tested at incident 110 GHz RF power levels up to 600 kW. A 1-D model is presented that agrees well with the experimentally observed temporal pulse shapes obtained with a single Si wafer. The LDSS has many potential uses in high power millimeter-wave research, including testing of high-gradient accelerator structures. Published under license by AIP Publishing.

Journal

Authors: Stephens, JC; Rosenzweig, G; Shapiro, MA; Temkin, RJ; Tucek, JC; Kreischer, KE

PDF: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.123.244801

Abstract: This Letter reports the successful experimental demonstration of amplification of subterahertz radiation in a klystron with photonic crystal cavities. The klystron has six cavities, with each cavity having a series of oversized photonic crystal cells made up of a 5 x 3 array of square posts. The center post is removed from each cell to form a highly oversized (0.8 mm similar to lambda/4) beam tunnel, with power coupling from cell to cell through the tunnel. The pulsed electron beam is operated at 23.5 kV, 330 mA in a 0.5 T solenoidal field. At 93.7 GHz, a small-signal gain of 26 dB and a saturated output power of 30 W are obtained. Experimental results are in very good agreement with the predictions of a particle-in-cell code. The successful achievement of high gain operation of a photonic crystal klystron amplifier is promising for the future extension of klystron operation well into the terahertz frequency region.

Journal

Authors: Stephens, JC

PDF: https://aip.scitation.org/doi/10.1063/1.5047809#:~:text=A%20multi-term%20%28MT%29%2C%20multi-harmonic%20%28MH%29%20decomposition%20of%20the,in%20microwave%20and%20THz%20excited%20low%20temperature%20plasmas.

Abstract: A multi-term (MT), multi-harmonic (MH) decomposition of the Boltzmann equation (BE) is developed to describe electron kinetic behavior in microwave and THz excited low temperature plasmas. In the decomposition of the BE, velocity distribution functions retain an arbitrary time dependence enabling the prediction of electron kinetic behavior from an arbitrary initial condition to a steady-state periodic solution. By exploiting the time-periodic nature of the electron swarm, the MTMH-BE model is not restricted to numerically resolving the electric field cycle. The MTMH-BE model is validated via the Reid ramp model gas and the ionization model gas of Lucas and Salee. Following successful validation, the MTMH-BE model is utilized to elucidate the basic electron kinetic behavior in air at atmospheric pressure. Namely, the error associated with the effective field approximation (EFA) is explored, where it is demonstrated that for atmospheric pressure air, given a microwave frequency of 1 GHz, the EFA may result in more than a factor of two errors in the time-averaged ionization rate. In the second part of this study, the MTMH-BE model is demonstrated as a basic modeling tool for low temperature plasmas. First, the MTMH-BE model is utilized to calculate electron heating profiles from a cold initial condition. The MTMH-BE model is demonstrated to be in excellent agreement with strictly time-dependent kinetic models, including a time-dependent MTBE model and a Monte Carlo collision model. To highlight the advantage of this work, the MTMH-BE model is used to predict the formative delay time of 95 GHz high power microwave induced breakdown. In this example, the numerical time step utilized in the MTMH-BE model is approximately six orders of magnitude larger than is possible using a strictly time-dependent MT-BE model. Overall, the MTMH-BE model presents a powerful pathway to modeling temporal kinetic behavior in microwave and THz excited low temperature plasmas.

Journal

Authors: J. C. Stephens; J. C. Tucek; M. A. Basten; K. E. Kreischer; M. A. Shapiro; R. J. Temkin

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8391585

Abstract: This report summarizes the progress of experimental efforts for the development a 94 GHz extended interaction klystron (EIK) with a photonic bandgap (PBG) based interaction structure. Input return loss measurements of the interaction structure are presented, demonstrating the successful fabrication of the input and output cavities of the EIK. Additionally, input return loss and insertion loss measurements of microwave-vacuum feedthrough windows are also presented. Finally, a review of the complete W-band experimental apparatus is given. Hot test experimental efforts for this study are expected to be completed by the 2018IVEC meeting.

Conference Paper/Presentation

Authors: Lu, XY; Stephens, JC; Mastovsky, I; Shapiro, MA; Temkin, RJ

PDF: https://aip.scitation.org/doi/10.1063/1.5016545

Abstract: Experimental operation of a high power microwave source with a metamaterial (MTM) structure is reported at power levels to 2.9 MW at 2.4 GHz in full 1 mu s pulses. The MTM structure is formed by a waveguide that is below cutoff for TM modes. The waveguide is loaded by two axial copper plates machined with complementary split ring resonators, allowing two backward wave modes to propagate in the S-Band. A pulsed electron beam of up to 490 kV, 84A travels down the center of the waveguide, midway between the plates. The electron beam is generated by a Pierce gun and is focused by a lens into a solenoidal magnetic field. The MTM plates are mechanically identical but are placed in the waveguide with reverse symmetry. Theory indicates that both Cherenkov and Cherenkov-cyclotron beam-wave interactions can occur. High power microwave generation was studied by varying the operating parameters over a wide range, including the electron beam voltage, the lens magnetic field, and the solenoidal field. Frequency tuning with a magnetic field and beam voltage was studied to discriminate between operation in the Cherenkov mode and the Cherenkov-cyclotron mode. Both modes were observed, but pulses above 1 MW of output power were only seen in the Cherenkov-cyclotron mode. A pair of steering coils was installed prior to the interaction space to initiate the cyclotron motion of the electron beam and thus encourage the Cherenkov-cyclotron high power mode. This successfully increased the output power from 2.5 MW to 2.9 MW (450 kV, 74 A, 9% efficiency). (C) 2018 Author(s).

Journal

Authors: X. Lu; J. C. Stephens; I. Mastovsky; M. A. Shapiro; R. J. Temkin

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8391535

Abstract: We present the experimental results of a high power microwave source with a metamaterial structure designed at 2.4 GHz. The structure is a waveguide loaded with two metamaterial plates consisting of complementary split ring resonators placed in the waveguide with reverse symmetry. Two backward wave modes can propagate in the waveguide and interact with an electron beam of up to 490 kV, 84 A. Full microsecond long output microwave pulses up to 2.9 MW were measured with an efficiency of 9% from the Cherenkov-cyclotron instability.

Conference Paper/Presentation

Authors: J. C. Stephens; G. Rosenzweig; M. A. Shapiro; R. J. Temkin; J. C. Tucek; M. A. Basten; K. E. Kreischer

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8289729

Abstract: A complete design of a 94 GHz extended interaction klystron (EIK) amplifier has been developed at the MIT Plasma Science and Fusion Center. The device utilizes a novel, mode selective photonic bandgap (PBG) structure which enables the use of a modified PBG coupling scheme. Using particle-in-cell simulation, the EIK is predicted to generate 42 dB of gain and 130 W saturated power.

Conference Paper/Presentation

Authors: G. Rosenzweig; J. C. Stephens; M. A. Shapiro; R. J. Temkin

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8496291

Abstract: The challenge in manufacturing Traveling Wave Tubes (TWTs) at high frequencies is that the sizes of the structures scale with, and are much smaller than, the wavelength. This requires advanced nano-machining techniques or sheet-beam devices and puts strict limits on the peak output power that can be safely handled by the devices. Furthermore, the diameter of the electron-beam tunnel limits the amount of beam current, requires high magnetic fields for beam compression and creates difficulties in alignment.

Conference Paper/Presentation

Authors: A. V. Soane; M. A. Shapiro; J. C. Stephens; R. J. Temkin

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8014466

Abstract: The linear and nonlinear theory of a gyroamplifier using a confocal waveguide is presented. A quasi-optical approach to describing the modes of a confocal waveguide is derived. Both the equations of motion and the mode excitation equation are derived in detail. The confocal waveguide circuit has the advantage of reducing mode competition, but the lack of azimuthal symmetry presents challenges in calculating the gain. In the linear regime, the gain calculated using the exact form factor for the confocal waveguide agrees with an azimuthally averaged form factor. A beamlet code, including velocity spread effects, has been written to calculate the linear and nonlinear (saturated) gain. It has been successfully benchmarked against the MAGY code for azimuthally symmetric cases. For the confocal waveguide, the beamlet code shows that the saturated gain is reduced when compared with results obtained using an azimuthally averaged form factor. The beamlet code derived here extends the capabilities of nonlinear gyroamplifier theory to configurations that lack azimuthal symmetry.

Journal

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

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

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

Conferences

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

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

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

Journal

Authors: N. R. Joseph; M. E. Savage; J. C. Stephens; J. A. Lott; B. A. Lewis; R. D. Thomas; M. A. Torres; E. G. Holman

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7296969

Abstract: The Sandia National Laboratories SPHINX accelerator is used to study the response of electronics to pulsed x-ray and electron environments. The system consists of a Marx generator and an oil-insulated pulse-forming line with self-closing oil switches. SPHINX has a peak load voltage of 2 MV and an adjustable pulse width ranging from 3 to 10 ns. The previous pulsed-power system had reliability and triggering issues with the Marx generator and subsequent undesired variations in voltage output. SPHINX was upgraded to a new Marx-generator system that has solved many of the voltage-output fluctuation and timing issues. The new Marx generator uses recently developed low-inductance 100-kV capacitors and 200-kV spark-gap switches. This paper provides an overview of SPHINX while capturing in detail the design, characterization, and comparative performance of the new Marx generator.

Conference Paper/Presentation

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

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

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

IEEE Conferences

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

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

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

Journal

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

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

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

IEEE Conferences

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

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

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

Journal

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

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

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

IEEE Journals

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

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

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

Journal

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

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

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

IEEE Conferences

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

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

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

IEEE Conferences

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

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

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

Journal

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

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

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

Journal

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

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

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

IEEE Conferences

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

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

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

IEEE Conferences

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

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

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

Journal

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

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

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

Journal

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

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

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

IEEE Conferences

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

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

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

IEEE Conferences

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

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

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

IEEE Conferences

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

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

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

Journals

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

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

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

IEEE Conferences

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

PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5958416

Abstract: Exploding wire arrays as fast switches are of interest for explosive pulsed power applications utilizing magnetic flux compression. This type of opening switch has proven effective in producing pulses of several hundred kilovolts into vacuum diode loads. The research presented here discusses an exploding wire array capable of producing single digit kilo-joules, 100 nanosecond pulses when driven by a 45 kilo-Amp current with a waveform closely resembling typical magnetic flux compression output. For this specific parameter range, the optimal fuse design was developed based on the experimental behavior of the fuse under variation of parameters such as wire spacing, shielding, and quenching medium. Each fuse is composed of several silver wires arranged in a straight wire cylindrical array and is typically pressurized in a chamber filled with about 0.6 MPa of SF6. The tradeoff between wire spacing and voltage output was addressed by designing four different fuse termination pairs each with a diameter that increased wire spacing from 5 to 20 mm in 5 mm increments. A wire shield test was also conducted as an extension to the wire spacing experiment to uncover any mutual radiative effects between wires on fuse opening behavior. The optimum fuse design, including the optimum fuse wire diameter, will be discussed with a 20 Ohm resistive load as well as a vacuum diode load with similar impedance.

IEEE Conferences