Authors: Amanda M Loveless, Samuel J Wyss, William Milestone, Ravi P Joshi, Allen L Garner
PDF: https://ieeexplore.ieee.org/abstract/document/10384882/
Abstract: Calculating pulsed electric field (PEF)-induced pore formation using the Smoluchowski equation (SME) can be computationally expensive, even when reduced to the asymptotic SME (ASME). These issues are exacerbated when incorporating additional physical phenomena, such as membrane temperature gradients or shock waves, or incorporating pore formation into multiscale models starting from an external stimulus at the organism level. This study presents a rapid method for calculating the membrane-level effects of PEFs by incorporating a semi-empirical equation for transmembrane potential (TMP)-dependent membrane conductivity into a single-shell model for calculating the TMP. The TMP calculated using this approach and the ASME agreed well for a range of electric field strengths for various PEF durations and AC frequencies below and above the threshold for pore formation. These results demonstrate the feasibility of rapidly predicting TMP, which is easily measured, during pore formation strictly from electrical properties and dynamics without needing to explicitly calculate pore dynamics, as required when using the SME and ASME.
Authors: M Flynn, L Vialetto, A Fierro, A Neuber, J Stephens
PDF: https://iopscience.iop.org/article/10.1088/1361-6463/ad3477/meta
Abstract: Benchmark calculations are reported for anisotropic scattering in Boltzmann equation solvers and Monte Carlo collisional models of electron swarms in gases. The work focuses on isotropic, forward, and screened Coulomb models for angular scattering in electron-neutral collisions. The impact of scattering on electron swarm parameters is demonstrated in both conservative and non-conservative model atoms. The practical implementation of anisotropic scattering in the kinetic models is discussed.
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.
Authors: YM Pokhrel, SC Shrestha, Y Iqbal, S Portillo, RP Joshi
PDF: https://pubs.aip.org/aip/jap/article/136/4/043303/3304080
Abstract: Thermal driven desorption of surface impurities is probed based on coupled Monte Carlo–heat flow–molecular dynamics simulations. Such adsorbates can lead to plasma formation during the operation of high-power microwave systems with various negative outcomes and so need to be curtailed. Our study attempts to obtain temperature thresholds for desorbing different surface contaminants such as C 2, O 2, CO, and CO 2. The results show that carbon-based adsorbates on copper (chosen as an example anode material) could be ejected at a relatively modest surface temperature of 650 K. On the other hand, reactive species such as oxygen are very stable due to their large cohesive energies. Our calculations further suggest the benefit of using a platinum coating layer, as the noble metal is robust with strong resistance to oxidation.
Authors: Travis Wright; David Saheb; Jacob Hoebelheinrich; John Mankowski; James Dickens; Andreas Neuber; Emily Schrock; James Schrock; Jacob Stephens
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10631717
Abstract: A discrete-element, printed circuit board (PCB)-based nonlinear transmission line was investigated to characterize its RF output characteristics for varying dc bias current and input pulse voltage. The device demonstrated both forward-wave and backward-wave RF formation, with each mode of operation exhibiting RF frequency and power characteristics. The forward-wave operational mode yielded a wider range of frequency tunability via variation of the applied dc bias. In contrast, the backward-wave operational mode depended more strongly on the input pulse voltage. The forward-wave operational mode generated higher instantaneous RF output power; however, the backward-wave operational mode featured longer RF pulse duration. The backward-wave mode produced consistently higher output frequency, achieving an output frequency of approximately 240 MHz for a 2.5 kV input pulse.
Authors: Nathan Fryar, K Schriner, J Stephens, J Dickens, A Neuber, A Young
PDF: https://www.osti.gov/servlets/purl/2375950
Abstract: NovecTM 4710 (C3F7CN) would work as a substitute for sulfur hexafluoride (SF6) in helical flux compression generator (HFCG) applications, a comparative analysis was made between the relative dielectric breakdown properties of air, SF6, and Novec 4710 under the extreme conditions encountered in an HFCG. An experimental apparatus that emulated a helical FCG geometry was developed to allow the measurement of a time-resolved gas resistance using biased wire probes inserted into the device. After initiation, the expanding armature causes interplay between the advancing shocked gas region, armature, and the biased wire probes on the microsecond time scale. This leads to eventual dielectric breakdown on a nanosecond time scale. Incorporating piezoelectric pins, shorting pins, and fiber optic time-of-arrival sensors enabled precise tracking of the armature’s position and the shock layer’s progression, providing a measure of shock layer thickness at critical points. The spatial relationship between the shocked gas region, armature, and wire probes at the moment of breakdown was used to evaluate the relative performance of the different gases. The results demonstrate that NovecTM 4710’s performance closely aligns with SF6, indicating it may be a viable alternative to SF6 in HFCG applications.
Authors: YM Pokhrel, Y Iqbal, SC Shrestha, M Sanati, RP Joshi
PDF: https://pubs.aip.org/aip/jap/article/135/22/223301/3297623
Abstract: Field emission is an important process with a variety of applications. Quantitative predictions of such electron emission need to include details of the internal potentials that shape the electronic wavefunctions (and hence the tunneling probability), predictive analysis of the work function barrier (Φ B), and knowledge of the electron distribution at the surface that constitutes the supply function. Here, these various factors were all collectively considered based on a combined Monte Carlo-density functional theory approach. Results were obtained for both the field-dependent cold electron emission current density as well as photoemission from a short laser pulse. The method also allows for calculations of field-dependent emittance. The technique is general and could be extended to include plasmon–polariton modes, different thicknesses of coatings, and role of surface adsorbates and defects.
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.
Authors: Nathan Fryar, J Stephens, J Mankowski, J Dickens, D Hattz, N Koone, A Neuber
PDF: https://pubs.aip.org/aip/adv/article/14/4/045235/3283645
Abstract: A comparative analysis of the attachment efficacy of sharpened vs blunted air terminals under a strong background DC electric field, similar to conditions found in naturally occurring lightning events, is presented. Testing was conducted using an oil-free, 17-stage, 1.4 MV bipolar Marx generator with a biased discharge plane that emulated the conditions found prior to the lightning return stroke. The biased discharge plane allowed the introduction of a strong DC electric field and subsequent corona formation on a time scale much longer than the brief duration over which the downward leader propagates. Initial observations indicate that despite a higher background electric field at the sharpened terminals, leading to pre-discharge corona formation where none is seen on the blunt terminal, both the sharpened and blunted terminals exhibit similar attachment probabilities during discharge events, with some discharge events attaching to both terminals.
Authors: H Spencer, D Wright, A Gregory, J Mankowski, J Stephens, J Dickens, A Neuber
PDF: https://pubs.aip.org/aip/pop/article/31/3/032108/3271338
Abstract: This study investigates multipactor mitigation techniques in X-band geometries, focusing on surface modifications, surface conditioning, and additive manufacturing. Surface modifications include geometric alterations such as dimpled surfaces. Experimental results demonstrate that 3D-printed copper test pieces can achieve a multipactor onset threshold comparable to traditionally machined test pieces with appropriate sample preparation. Surface conditioning, involving rapid successions of pulses, is explored for its mitigative potential. Among assessed techniques, dimpled configurations demonstrated superior multipactor hold-off compared to non-modified components. Additionally, the conditioning of test pieces significantly increased the single-pulse threshold.
Authors: A Fierro, A Alibalazadeh, J Stephens, C Moore
PDF: https://www.sciencedirect.com/science/article/pii/S0010465524002686
Abstract: A highly parallelizable fluid plasma simulation tool based upon the first-order drift-diffusion equations is discussed. Atmospheric pressure plasmas have densities and gradients that require small element sizes in order to accurately simulate the plasm resulting in computational meshes on the order of millions to tens of millions of elements for realistic size plasma reactors. To enable simulations of this nature, parallel computing is required and must be optimized for the particular problem. Here, a finite-volume, electrostatic drift-diffusion implementation for low-temperature plasma is discussed. The implementation is built upon the Message Passing Interface (MPI) library in C++ using Object Oriented Programming. The underlying numerical method is outlined in detail and benchmarked against simple streamer formation from other streamer codes. Electron densities, electric field, and propagation speeds are compared with the reference case and show good agreement. Convergence studies are also performed showing a minimal space step of approximately 4 μm required to reduce relative error to below 1% during early streamer simulation times and even finer space steps are required for longer times. Additionally, strong and weak scaling of the implementation are studied and demonstrate the excellent performance behavior of the implementation up to 100 million elements on 1024 processors. Finally, different advection schemes are compared for the simple streamer problem to analyze the influence of numerical diffusion on the resulting quantities of interest.
Authors: Tyler Klein, Andrew Eulenbach, Andreas Neuber, James Dickens
PDF: https://pubs.aip.org/aip/rsi/article/95/4/044703/3280717
Abstract: Multiple pulse charging modules were designed and tested for synchronous use in scaled experiments. The pulse chargers are each capable of charging a nanofarad-sized capacitive load to 100 kV in less than 10 µs. This is achieved by initially charging a microfarad-sized capacitor to a maximum of 3 kV, then, using a thyristor switch, discharging the capacitor into two transformers paralleled on the primary side. Each transformer steps the voltage up to 50 kV, with the transformer having opposing polarities and a common neutral on the secondary. Thus, the full 100 kV is obtained by placing the nanofarad-sized capacitor between the +50 and −50 kV terminals. The pulse chargers use a microcontroller to monitor and control the pulse charger in addition to communicating with the user as well as other pulse chargers. For increased battery life, each pulse charger is kept in a low-power state while not in use. Upon startup, the pulse chargers automatically detect the number of modules in the system, the position of itself in the system, and the timing delays associated with each pulse charger in the system. This information is essential for synchronous charging and provides individual information and controls for the pulse chargers.
Authors: Matthew Sokol, C Baker, M Baker, Ravindra P Joshi
PDF: https://iopscience.iop.org/article/10.1088/2057-1976/ad4f90/meta
Abstract: Noise activity is known to affect neural networks, enhance the system response to weak external signals, and lead to stochastic resonance phenomenon that can effectively amplify signals in nonlinear systems. In most treatments, channel noise has been modeled based on multi-state Markov descriptions or the use stochastic differential equation models. Here we probe a computationally simple approach based on a minor modification of the traditional Hodgkin-Huxley approach to embed noise in neural response. Results obtained from numerous simulations with different excitation frequencies and noise amplitudes for the action potential firing show very good agreement with output obtained from well-established models. Furthermore, results from the Mann–Whitney U Test reveal a statistically insignificant difference. The distribution of the time interval between successive potential spikes obtained from this simple approach compared very well with the results of complicated Fox and Lu type methods at much reduced computational cost. This present method could also possibly be applied to the analysis of spatial variations and/or differences in characteristics of random incident electromagnetic signals.
Authors: Raimi Clark, Michael Mounho, William Brooks, Matthew Hopkins, Jacob Stephens, Andreas Neuber
PDF: https://pubs.aip.org/aip/pop/article/31/3/032112/3277894
Abstract: Time-resolved optical emission spectroscopy is used to explore the early development of anode-initiated flashover in vacuum. An experimental apparatus for collecting flashover self-luminosity is introduced, which enables spatially resolved light collection from regions adjacent to the anode and cathode triple junctions. The emitted light is spectrally and temporally resolved utilizing an imaging spectrograph and a fast electronic shutter camera. The first light detectable from flashovers across both Rexolite and polytetrafluoroethylene (PTFE) insulators begins less than 10 ns prior to the flashover gap's impedance collapse and consists of faint broadband feature in primarily visible wavelengths. This suggests luminosity due to excitation in the solid, possibly occurring as a result of field emitted electrons. In the few nanoseconds leading up to and including the impedance collapse, the broadband spectrum grows in intensity and extends into the UV, eventually accompanied by a few emission lines of the insulator material. This is strong evidence that the early stages of anode-initiated flashover include surface layer breakdown of the insulator, as a contrast from cathode-initiated flashover, which is predominately an above-surface process. Spectra accumulated over longer exposures of PTFE flashovers indicate that, in the first few hundred nanoseconds after the impedance collapse, ions from the insulator material constitute the majority of the emission lines. Later, neutral and ionic metal species from the electrode contribute with similar prominence as well as molecular bands from diatomic carbon. Finally, a comparison is provided of the results of these studies to other spectroscopic investigations of vacuum flashover from the literature.
Authors: M Mounho, C Fuksa, R Clark, W Brooks, M Hopkins, A Steiner, A Neuber, J Stephens
PDF: https://pubs.aip.org/aip/pop/article/31/8/080701/3306463
Abstract: This manuscript reports the experimental study of a novel vacuum high-voltage (HV) feedthrough geometry with a recessed anode triple junction (ATJ) relevant to large-scale pulsed power systems. It is shown experimentally that introducing a vacuum gap between the insulator–anode interface and recessing the ATJ farther away from the cathode triple junction significantly enhances high voltage (HV) vacuum surface flashover hold-off. The results for “first pulse” and “consecutive pulse” tests are reported. The physical mechanisms contributing to the electrical failure of this feedthrough geometry appear to be unique to this geometry. These findings show the potential to allow for HV vacuum surface flashover geometries with significantly improved electrical characteristics.
Authors: Ravi Joshi, Avinash Sharma
PDF: https://www.academia.edu/download/110341589/IJETA_V11I1P6.pdf
Abstract: Microstrip patch antennas are the most employable antenna designs as they are having lots of advantages which attract the researchers such as smaller size, cost of fabrication and production is low. Even though Microstrip antennas have a lot of advantages to its name but these antennas come with few limitations as well, such as antenna gain being lower than normal and adequate bandwidth. Metamaterials are engineered designs which are designed, studied and fabricated to acquire unique properties like negative mu, negative epsilon in contrast with the available materials. A brief introduction to Microstrip Patch Antenna, Survey on different basic Metamaterial structures and comparison of various literature papers in terms of important antenna parameters like Resonant Frequency, Negative Scattering parameters, Bandwidth of the Antenna, Gain and VSWR is presented in this paper. Different designs, software platforms used and various applications are also illustrated.
Authors: Sanidhya L Makwana, Kalpesh Vaishnav, Ravi Joshi, Tulsi H Patel, Neil N Vora, Nishit H Sachde, Keyur A Vala, Zalak Raval, Jinsa A Yohannan, Vidhi J Joshi, Tulsi Patel, Neil Vora, Nishit Sachde, Zalak P Raval
Abstract: Zirconia is a widely used restorative material in dentistry due to its superior aesthetic and mechanical properties. The oral cavity is a complex ecosystem with various components, which affect the teeth, as well as artificial restorative materials. Various personal and professional interventions carried out can severely affect the properties of restorative materials, thus altering the longevity of the prosthesis; 1.23% acidulated phosphate fluoride (APF) gel is one such professionally applied topical fluoride agent used to prevent caries. The interaction of this APF gel with highly aesthetic restorative material such as zirconia crowns is unknown.
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.
Authors: Ravi Joshi, Avinash Sharma
PDF: https://www.academia.edu/download/110341375/IJETA_V10I6P4.pdf
Abstract: As 5G continues to evolve and expand its reach, antennas will remain at the forefront of this technological revolution. 5G relies on a diverse array of antenna technologies to deliver enhanced performance. These antennas operate across various frequency bands, including the sub-6 GHz and mmWave (millimeter-wave) frequencies, each presenting unique challenges and opportunities. In this paper give an overview about the microstrip patch antenna design for mmWave and 5G wireless communication and also done the comparative analysis on with the related work done by different authors for microstrip patch antenna design for mmWave and 5G wireless communication.
Authors: Ravi Joshi, Dipti S Shah, Kalpesh Vaishnav, Aneri Patel, Manish Patel, Radhika Agnihotri
Abstract: Dental caries remains a constant problem in clinical practice. The rates of recurrent caries around long-term provisional restorations may be even higher due to poor marginal adaptation and less stable materials. Since provisional crowns luted with provisional cement are susceptible to bacterial infiltration and caries, antibacterial and anticariogenic agents have been added to provisional cement, and retention of the provisional crown has been evaluated in this study.
Authors: A. Gregory; D. Wright ; H. Spencer ; J. J. Mankowski ; J. C. Dickens ; J. Stephens; A. A. Neuber
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.
Authors: Matthew Hopkins, William Brooks, Raimi Clark, Zakari Echo, Christopher Moore, Michael Mounho, Andreas Neuber, Jacob Stephens
PDF: https://ui.adsabs.harvard.edu/abs/2023APS..GECIW5014H/abstract
Abstract: Vacuum insulator flashover is a breakdown process occurring along a dielectric surface separating electrode surfaces. Here, the dielectric is a separation barrier between vacuum and water-containing (or oil-containing) sections in pulsed power systems. While one kind of breakdown process, cathode-initiated flashover, is somewhat understood (an electron cascade resulting in surface charge saturation), other processes are hypothesized for anode-initiated breakdowns. One hypothesized process involves plasma initiation due to high fields at the anode triple junction and subsequent advancement and growth of a breakdown ``spot'' along the insulator towards the cathode. This process has some phenomenological overlap with positive streamers. This work will describe recent efforts to better understand anode-initiated flashover, including experimental investigation of the location and timing of various species emissions and plasma growth. A high-fidelity PIC-DSMC modeling approach using Sandia's massively parallel plasma simulation code, Aleph, will be described, including multiple neutral, excited state, and ion species. Challenges with various emission models will be explored.
Authors: Radhika R Agnihotri, Saloni Naik, Kalpesh Vaishnav, Dipti S Shah, Ravi Joshi, Miloni Bhatt
PDF: https://journals.lww.com/adhb/fulltext/2023/05001/Comparative_Analysis_of_Different_Implant.8.aspx
Abstract: Dental implants have emerged as the treatment of choice for restoring missing teeth in situations that require functional and aesthetic replacements. The aim of the study was to assess the dimensional accuracy of (1) the resultant casts made from different impression techniques for implants, (2) implant impressions using two types of splinting material for open tray technique, auto-polymerising acrylic resin and light-cure acrylic resin and impression techniques, including non-sectioning and sectioning and rejoining with the same splinting material.
Authors: Ravi Joshi, Annapurna Maritammanavar
Abstract: This research paper provides an extensive exploration of deep learning architectures and their diverse applications across various domains. Tailored for researchers, practitioners, and enthusiasts in the field of artificial intelligence, the paper navigates through the foundational concepts of deep learning, state-of-the-art architectures, and real-world applications, showcasing the transformative impact of deep learning in solving complex problems.
Authors: C Baker, A Willis, W Milestone, M Baker, AL Garner, RP Joshi
PDF: https://www.researchsquare.com/article/rs-3504765/latest
Abstract: Most simulations of electric field driven bioeffects have considered spherical cellular geometries or probed symmetrical structures for simplicity. This work assesses cellular transmembrane potential build-up and electroporation in a Jurkat cell that includes the endoplasmic reticulum (ER) and mitochondria, both of which have complex shapes, in response to external nanosecond electric pulses. The simulations are based on a time-domain nodal analysis that incorporates membrane poration utilizing the Smoluchowski model with angular-dependent changes in membrane conductivity. Consistent with prior experimental reports, the simulations show that the ER requires the largest electric field for electroporation, while the inner mitochondrial membrane (IMM) is the easiest membrane to porate. Our results suggest that the experimentally observed increase in intracellular calcium most likely results due to a calcium induced calcium release (CICR) process that is initiated by outer cell membrane breakdown. Repeated pulsing and/or using multiple electrodes are shown to create a stronger poration. The role of mutual coupling, screening, and proximity effects in bringing about electric field modifications is also probed. Finally, while including greater geometric details might refine predictions, the qualitative trends are expected to remain.
Authors: W Milestone, C Baker, AL Garner, RP Joshi
PDF: https://pubs.aip.org/aip/jap/article/133/24/244701/2900330
Abstract: A general, self-consistent scheme for analyzing cellular electroporation for bio-medical applications is developed to probe realistic biological shapes and different length scales ranging from nanometers to hundreds of micrometers. The COMSOL Multiphysics suite is used with suitable embellishments to incorporate the details of the electroporation (EP) process and the inherent internal physics. The results are obtained for the voltage pulse driven electroporation for a Jurkat cell with mitochondria (as an example organelle) where spatial dimensions on the order of a few nanometers become important, to hundreds of cells (with Bacillus as an example) where collective effects and mutual interactions can dominate. Thus, scalable computing to generalized geometries with the ability to include complex organelles is made available. The results obtained for mitochondrial EP in Jurkat cells compare well with available data. In addition, quantitative predictions of field attenuation and shielding in Bacillus clusters are made, which point to highly nonuniform field distributions and a strong need to engineer novel electrode designs.
Authors: M Flynn, J Agan, A Neuber, J Stephens
PDF: https://iopscience.iop.org/article/10.1088/1361-6463/acf3db/meta
Abstract: A complete and consistent set of electron-neutral collision cross-sections for the novel insulating gas C 4 F 7 N is reported. The set is composed of a combination of cross-sections previously reported in literature, optimized via a genetic algorithm in conjunction with a multi-term Boltzmann equation solver, and calculated ab initio using the R-matrix code Quantemol-EC. The finalized set accurately reproduces reported macroscopic rate and transport coefficients as well as Townsend coefficients and critical electric field strengths in C 4 F 7 N and its mixtures with nitrogen, carbon dioxide, and argon.
Authors: L Diaz, A Karkash, S Alsharari, RP Joshi, E Schamiloglu, M Sanati
PDF: https://www.nature.com/articles/s41598-023-34721-8
Abstract: Understanding the relationship between surface adsorbates and secondary electronic emission is critical for a variety of technologies, since the secondary electrons can have deleterious effects on the operation of devices. The mitigation of such phenomena is desirable. Here, using the collective efforts of first-principles, molecular dynamics, and Monte Carlo simulations, we studied the effects of a variety of carbon adsorbates on the secondary electron emission of Cu (110). It was demonstrated that the adsorption of atomic C and C pair layers can both reduce and increase the number of secondary electrons depending on the adsorbate coverage. It was shown that under electron irradiation, the C–Cu bonds can be dissociated and reformed into C pairs and graphitic-like layers, in agreement with experimental observation. It was verified that the lowest secondary electron emission was due to the formation of the graphitic-like layer. To understand the physical reason for changes in number of secondary electrons for different systems from an electronic structure perspective, two-dimensional potential energy surfaces and charge density contour plots were calculated and analyzed. It was shown that the changes are strongly influenced by the Cu surface morphology and depends highly on the nature of the interactions between the surface Cu and C atoms.
Authors: Keju Yan, Xiangyu Jie, Xiaoqiang Li, Juske Horita, Jacob Stephens, Jianli Hu, Qingwang Yuan
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.
Authors: Miranda Maille, NC Dennis, YM Pokhrel, M Sanati, RP Joshi
PDF: https://ttu-ir.tdl.org/handle/2346/92989
Abstract: Secondary electron yields of (110) copper surfaces, covered with either carbon, nitrogen, or their dioxides, have been studied by employing combined first principles methods for the material properties and Monte Carlo simulations for electron transport. Furthermore, by studying electron transport inside the Cu system and modeling the power loss taking account of the inelastic electron scattering within the material, changes in the thermal energy of the system have been modeled. The physical reasons behind the increase and decrease of the yield for each system from an electronic perspective are discussed. In agreement with results observed in studies of secondary electron emission, it is shown that the formation of C2 and N2 monolayers reduce the secondary electron yields, while CO2 and NO2 increase the yield significantly. It is demonstrated that in the case of C2 and N2 formation, changes in the surface electronic barrier reduce the probability of electron escape from the Cu surface, resulting in lower secondary electron emission. Formation of CO2 and NO2, on the other hand, reduce the electronic barrier effects. In addition, due to weak bonding of the CO2 layer with the Cu host, the surface provides an additional source of secondary electrons resulting in higher electronic emission yield. Moreover, the NO2 adsorbate creates a surface electric field that changes the surface electron energy and increases the electron escape probability. Additionally, it is verified that thermal change in the system is negligible and so during secondary electron emission measurements, negligible (if any) surface adsorption or desorption could occur.
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.
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.
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.
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.
Authors: M Brown, L Diaz, A Aslan, M Sanati, S Portillo, E Schamiloglu, RP Joshi
PDF: https://www.nature.com/articles/s41598-022-19924-9
Abstract: First-principles calculations coupled with Monte Carlo simulations are used to probe the role of a surface CO monolayer formation on secondary electron emission (SEE) from Cu, Ag, and Au (110) materials. It is shown that formation of such a layer increases the secondary electron emission in all systems. Analysis of calculated total density of states (TDOS) in Cu, Ag, and Au, and partial density of states (PDOS) of C and O confirm the formation of a covalent type bonding between C and O atoms. It is shown that such a bond modifies the TDOS and extended it to lower energies, which is then responsible for an increase in the probability density of secondary electron generation. Furthermore, a reduction in inelastic mean free path is predicted for all systems. Our predicted results for the secondary electron yield (SEY) compare very favorably with experimental data in all three materials, and exhibit increases in SEY. This is seen to occur despite increases in the work function for Cu, Ag, and Au. The present analysis can be extended to other absorbates and gas atoms at the surface, and such analyses will be present elsewhere.
Authors: C. A. Negri; S. Daneshvardehnavi; K. E. K. Schmitt; A. Esmaeel Nezhad; P. H. J. Nardelli; S. Bayne; M. G. Giesselmann
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9732999
Abstract: The power output of the solar panels follows a power-voltage (P-V) characteristic containing only one Global Maximum Point (GMP) in the normal conditions. However, under Partial Shading Conditions (PSC), the unbalanced irradiance in the panels creates Local Maximum Points (LMP) in the P-V curve. Standard control techniques for Maximum Power Point Tracking (MPPT) can not properly locate the GMP, stagnating in LMPs and generating losses in the energy harvesting. Specific techniques to locate the GMP are presented in the literature. However, the condition to restart the GMP is not widely discussed. The main challenges of global search algorithms are related to the restarting conditions. Avoiding unnecessary searching and providing an assertive GMP restarting condition is crucial for PV systems operation. In every GMP search, the solar inverters oscillate the power exchanged with the grid, causing frequency and voltage variations depending on the size of the PV plant. This paper proposes a novel technique that uses a centralized controller to identify the shaded inverters, creating flags that locally start the GMP searching. The solution minimizes the number of times the search is performed by providing an assertive GMP restarting condition, saving energy, and avoiding unnecessary output power oscillation. The proposed control technique was evaluated using the data of a real 150-kW solar farm containing five inverters with two MPPT trackers each.
Authors: Brown, M; Sanati, M; Joshi, RP
PDF: https://aip.scitation.org/doi/pdf/10.1063/5.0080721
Abstract: Secondary electron yield (SEY) modeling of Ni(110) surface has been carried out with and without the inclusion of wavevector-dependent harmonic corrections (which alter both the inelastic mean free path and stopping power) and is compared to available experimental data. The correction is shown to improve predictions of the inelastic electron mean free path in Ni and yield better agreement with experimental SEY data. It is found that the SEY is strongly dependent on the presence of adsorbates on surfaces. An increase of hydrogen on the surface, for example, is predicted to result in a significant enhancement in the secondary electron yield, with the positional placement of hydrogen layers on or near the Ni surface influencing the SEY. Using first-principles calculations, the permittivities work function and adsorption energy of various Ni systems have also been calculated, and have shown to compare favorably with available experimental data, and have been used in the present Monte Carlo calculations of electron transport. Published under an exclusive license by AIP Publishing.
Authors: William J Milestone, Sergey A Nikishin, RP Joshi
PDF: https://www.mdpi.com/2079-9292/11/19/2997
Abstract: With increases in the demand for faster electronic switching, requirements for higher operating voltages and currents, and the need to perform under harsh environments while operating at even higher frequencies, the research focus in photoconductive semiconductor switch (PCSS) technology has shifted to wide bandgap semiconductors. Here, we examine the possibility of pulse compression in carbon-doped PCSS devices based on the negative differential mobility concept for faster operation. Monte Carlo simulations are used to build in and model various effects on electron transport including degeneracy, charge polarization, and scattering within a three-valley model fitted to bandstructure calculations. The focus is on exploring the density dependence of pulse compression. Thresholds for the biasing fields naturally emerge. Predictive analysis of the output full-width half-maximum (FWHM) current waveforms, as well as the dynamics of the internal charge cloud behavior, and occupancy of the various valleys within GaN are all obtained. Our results show that an increase in carrier density can increase pulse compression and create pulse-widths that are smaller than the FWHM of the input optical excitation. This bodes well for enhanced repetition rates. Variations produced by moving the laser spot along the GaN PCSS length are also examined. Though data for GaN are not yet available, the trends compare well qualitatively with previous reports for GaAs.
Authors: DaneshvarDehnavi, S; Negri, C; Bayne, S; Giesselmann, M
PDF: https://www.sciencedirect.com/science/article/abs/pii/S001905782100207X?via%3Dihub
Abstract: With the advancement of technology, electric equipment and loads have become more sensitive to problems related to power quality, such as voltage sag, swell, imbalances, and harmonics. To detect faults and to protect sensitive loads from these voltage distortions, a Dynamic Voltage Restorer (DVR) series compensator is among the best available cost-effective solutions. One of the main goals of the DVR is to achieve a control structure that is robust, stable, and can handle properly the disturbances (e.g., grid voltage issues, load current, and fluctuations at the DC link voltage) and model uncertainties (e.g., inverters and filter parameters). In this work, a novel framework control strategy based on Uncertainty and Disturbance Estimator (UDE) is proposed to improve the response of the DVR to properly compensate the load voltage under a variety of power quality issues, particularly the ones associated with the grid voltage disturbances. Additionally, the stability of the proposed control system is analyzed and validated using the Lyapunov stability theory. The advantages of the new control system are robustness, simplified design, good harmonic rejection, low tracking error, fast response, and sinusoidal reference tracking without the need for voltage transformations or specific frequency tuning (e.g., abc-dq0 and Proportional-Resonant). This research uses the MATLAB/Simulink software to validate the effectiveness of the proposed scheme under a diverse set of conditions with no control limitations. Moreover, the designed controller is tested under real conditions using Hardware-In-theLoop (HIL) validation with OPAL-RT real-time simulator coupled with a TI Launchpad microcontroller. The results demonstrate a good performance of the proposed control strategy for a quick transient response and a great harmonic rejection when subject to grid voltage distortions. (c) 2021 ISA. Published by Elsevier Ltd. All rights reserved.
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.
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.
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.
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.
Authors: Madeline Brown, William Milestone, RP Joshi
Abstract: Multipactor mitigation is of relevance to microwave applications, and external magnetic fields, surface modifications, and materials engineering have previously been utilized for this purpose. In this contribution, geometric modifications made to rectangular waveguide surfaces in the form of nested grooves are investigated for the suppression of multipactor growth. A time-dependent kinetic scheme is used to simulate electron dynamics that folds in electron trapping at the nested groove structures, with inclusion of the electric field perturbations arising from the presence of various grooved geometries. The charge growth in the system is modeled based on an empirical approach that includes both energy and angular dependencies of secondary electron emission from all the different surfaces. A varying number of grooves, their widths, and their placement (either one sided or dual-sided) within the rectangular waveguide structure are included for a more complete analysis. The results demonstrate that nested grooves can lead to reductions in charge growth by over a factor of 280 when compared with a simple waveguide over the same time period. Furthermore, wider nested grooves are shown to have an advantage, with multiple aligned grooves across two parallel surfaces being especially useful at high external fields. Determining optimal combinations for an arbitrary field, operating frequency, and physical dimensions would require further work.
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.
Authors: W Milestone, Q Hu, AL Garner, RP Joshi
Abstract: Protocols surrounding electroporation have long been based on trapezoidal pulsing of biological cells. Here, we revisit cellular electroporation for bio-medical applications, including tumor treatment, based on a self-consistent electro-thermal analysis with sinusoidal RF excitation. Predictions for the evolution of pores and their surface angular distribution, as well as potential heating and temperature increases, are given. Our results show an optimum frequency range from 5–7 MHz to achieve increased mass transport without detrimental heating in Jurkat cells. Through parametrized frequency sweeps, this work establishes potential optimized regimes that could guide experimental and clinical protocols. More significantly, the optimal frequency for porating healthy B-cells is predicted to be ~ 2.5 MHz, with almost no poration at 7 MHz. This opens up the exciting possibility for treating malignant tissue with a welltuned optimal frequency range for bioeffects, while minimizing deleterious effects on healthy cells and tissues.
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.
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.
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.
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.
Authors: DaneshvarDehnavi, S; Negri, CA; Giesselmann, MG; Bayne, SB; Wollenberg, B
Abstract: Global warming has been a critical issue in recent years. Many leaders and politicians have talked about replacing fossil fuels with Renewable Energy Resources (RES). Some of them even went further and are talking about running a country with 100% RES soon. Is that economically viable? What would be the estimated cost of such a system? In this paper, a city in west Texas with a 100 MW peak load has been assumed to operate with the wind, solar, and Battery Energy Storage System (BESS) totally disconnected from the grid for a cost evaluation. Real generation and load time series data are used to achieve an optimum combination of the installed capacity of those resources by minimizing the total overnight cost, respecting a maximum of 24 h of faults in which the system cannot provide the load during the period. A Monte Carlo simulation is applied to the previous results to evaluate the influence of faulty devices and increase the robustness of the system. The results for both cases are analyzed and compared. Finally, the total cost of install capacity will be compared with other non-renewable resources. (c) 2021 Elsevier Ltd. All rights reserved.
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.
Authors: J. A. Cooper; D. T. Morisette; M. Sampath; C. A. Stellman; S. B. Bayne; M. J. Westphal; C. H. Anderson; J. A. Ransom
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9506992
Abstract: We introduce the concept of constant-gate-charge scaling to increase the short-circuit withstand time of SiC power MOSFETs without increasing their ON-state resistance, gate charge, or oxide field. In gate-charge scaling, we scale the oxide thickness and gate drive voltage, keeping the oxide field constant. Short-circuit measurements on 1200 V SiC double-implanted MOSFETs (DMOSFETs) confirm that short-circuit withstand times can be increased by 2- 4× without increasing ON-resistance, simply by reducing the oxide thickness and the gate drive voltage.
Authors: S. Hahmady; S. Bayne
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9466136
Abstract: We introduce a novel high-voltage SiC p-i-n diode considering a charge plasma approach. This technique facilitates the formation of the anode and the cathode regions within the silicon carbide without requiring any impurity doping by taking advantage of the work-function difference between silicon carbide and metal electrodes. Utilizing the 2-D TCAD simulation, we represent the performance of the proposed doping-less silicon carbide p-i-n diode is analogous to the silicon carbide Schottky diode in terms of forward and reverse characteristics as well as temperature dependency. As opposed to the conventional (doped) silicon carbide p-i-n diode, the doping-less silicon carbide p-i-n diode holds a lower ON-state voltage drop and higher reverse saturation current. Although the doping-less silicon carbide p-i-n diode has the merits of the silicon carbide Schottky diode, but it has leverage over the corresponding counterparts by eliminating the doping and the high thermal budget fabrication processes.
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.
Authors: M. Gaddy, V. Kuryatkov, N. Wilson, A. Neuber, R. Ness, and S. Nikishin
PDF: https://www.mdpi.com/2079-9292/10/13/1600/pdf?version=1625298532
Abstract: The suitability of GaN PCSSs (photoconductive semiconductor switches) as high voltage switches (>50 kV) was studied using a variety of commercially available semi-insulating GaN wafers as the base material. Analysis revealed that the wafers’ physical properties were noticeably diverse, mainly depending on the producer. High Voltage PCSSs were fabricated in both vertical and lateral geometry with various contacts, ohmic (Ti/Al/Ni/Au or Ni/Au), with and without a conductive n-GaN or p-type layer grown by metal-organic chemical vapor deposition. Inductively coupled plasma (ICP) reactive ion etching (RIE) was used to form a mesa structure to reduce field enhancements allowing for a higher field to be applied before electrical breakdown. The length of the active region was also varied from a 3 mm gap spacing to a 600 µm gap spacing. The shorter gap spacing supports higher electric fields since the number of macro defects within the device’s active region is reduced. Such defects are common in hydride vapor phase epitaxy grown samples and are likely one of the chief causes for electrical breakdown at field levels below the bulk breakdown field of GaN. Finally, the switching behavior of PCSS devices was tested using a pulsed, high voltage testbed and triggered by an Nd:YAG laser. The best GaN PCSS fabricated using a 600 µm gap spacing, and a mesa structure demonstrated a breakdown field strength as high as ~260 kV/cm.
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.
Authors: Milestone, W; Guo, D; Sanati, M; Dowling, KM; Hau-Riege, S; Voss, LF; Conway, A; Joshi, RP
PDF: https://aip.scitation.org/doi/10.1063/5.0040173
Abstract: Evaluation of the photoresponse in wurtzite GaN photoconductive switches is presented based on kinetic Monte Carlo simulations. The focus is on electron transport physics and assessment of high frequency operation. The roles of GaN band structure, Pauli exclusion, and treatment of internal fields based on the fast multipole method are all comprehensively included. The implementation was validated through comparisons of velocity-field characteristics for GaN with computational results in the literature. Photocurrent widths of less than similar to 7 ps for the 1 mu m device can be expected, which translates into a 100 GHz upper bound. Photocurrent pulse compression below the laser full width at half maxima at high applied fields are predicted based on the interplay of space-charge effects and the negative differential velocity characteristics of GaN.
Authors: Sami, SN; Sanati, M; Joshi, RP
PDF: https://journals.aps.org/prresearch/pdf/10.1103/PhysRevResearch.3.013203
Abstract: Outgassing remains a pertinent issue as it typically is the first stage of possible plasma formation, and can lead to effects such as breakdown, surface flashover, and pulse shortening in high power systems. Here two pertinent aspects are probed: (i) a model-based assessment of outgassing and associated temperature-dependent rates from a copper electrode based on molecular dynamics simulations, and (ii) calculations for the sticking coefficients of hydrogen gas atoms as a function of incident energy and angle. Our results of temperature dependent diffusion coefficients for hydrogen in copper, agree well with experimental reports over a wide range from 300 to 1350 K, and show reduction in the presence of vacancies. Results also show low reflection coefficients at both high and low energies, with a maxima at around 6.5 eV. A curve fit to the data is predicted to roughly hold for a range of incident angles. Adsorption is predicted to occur for incident energies below 10 eV, with absorption dominating above 10 eV.
Authors: Sami, SN; Islam, R; Khare, R; Joshi, RP
PDF: https://aip.scitation.org/doi/10.1063/5.0054440
Abstract: Outgassing remains a pertinent issue in high-power systems as it can lead to effects such as breakdown, surface flashover, and pulse shortening and is typically the first stage of deleterious plasma formation. In this context, experimental reports suggest that carbon fibers (CFs) may likely be a superior cathode material for low outgassing. Here, model-based assessments of outgassing from CFs are performed based on molecular dynamics simulations. Carbon fibers were generated based on interconnection of an array of graphene sheets resembling ladder-like structures. Our results of temperature-dependent diffusion coefficients for hydrogen in CFs are shown to exhibit Arrhenius behavior and have values smaller than copper by factors of 15.5 and 86.8 at 400 K and 1000 K, respectively. This points to even stronger improvements for operation at high temperatures, with the asymptotic diffusion constant ratio predicted to be similar to 187. With reduced outgassing, higher temperature operation, and durability, our results support CF cathodes as an excellent choice for cathode material in high-power microwave and pulsed power systems.
Authors: R. M. Clark, J. Brinkman, and A. A. Neuber
PDF: https://onlinelibrary.wiley.com/doi/full/10.1002/prep.202000330
Abstract: Polymer-bonded explosive (PBX) 9502 (95 % TATB, 5 % Kel F-800 by weight) is dry-drilled on a CNC milling machine and its thermomechanical response to varying feed rates, drilling speeds, and peck depths with 4 mm and 5 mm diameter drill bits is investigated. The tested samples are affixed to a force sensor that enables recording temporally resolved cutting forces and torques, and a drill-embedded thermocouple yields local temperature data. From the data, an empirical relationship between temperature changes and feed per revolution is developed, which reveals reduced temperatures in higher feed per revolution regimes for PBX 9502. The observed relationship allows extrapolating to temperatures for other hole diameters, indicating increased temperature for smaller diameter drilling across the board. Additional testing was performed with PBX 9501 (95 % HMX, 2.5 % Estane®, 2.5 % BDNPA/BDNPF by weight), albeit over a reduced parameter space, which revealed the opposite behavior for the feed per revolution temperature dependence. It is concluded that both PBX 9502 and PBX 9501 can be dry-drilled efficiently beyond the limits of presently applicable US-DOE standards, where cutting interface temperatures remain far below material critical temperatures. Finally, data reveals that coolant usage in the drilling process for these materials provides a wide safety margin.
Authors: Esser, B; Shaw, ZC; Dickens, JC; Neuber, AA
PDF: https://aip.scitation.org/doi/10.1063/5.0009836
Abstract: A high power, pulsed RF source designated for use in multipactor research is described. Four gallium nitride high electron mobility transistors from Cree/Wolfspeed, capable of 700 W in long pulse mode (500 W rated output), are combined to achieve a maximum rated output of 2.8 kW with a pulse length of similar to 100 mu s. Custom splitters/combiners are used due to the power levels considered in addition to a custom power and sequencing control system to ensure the proper biasing and sequencing of the relatively delicate depletion mode GaN devices. With high efficiency and small size, gallium nitride devices present a good solution for lab based sources, and this paper aims to provide information helpful in the construction of such a source. The multipactor phenomenon itself is studied within a high impedance waveguide section-achieved with a tapered impedance transformer-placed in a WR284 traveling wave ring resonator, which increases the effective power up to a factor of 20, or similar to 40 kW.
Authors: Sugai, T; Shaw, Z; Dickens, J; Neuber, A
PDF: https://aip.scitation.org/doi/10.1063/5.0011641
Abstract: Multipactor is a resonant nonlinear electron multiplication effect that may occur in high power microwave devices at very low pressures, such as those operating in particle accelerators and satellite subsystems. In this research, multipactor of a rectangular waveguide was analyzed using the commercially available, numerical simulation software Spark3D. The electromagnetic wave in the simulation was a TE10 mode-2.85GHz wave of varying power, fed into the impedance transformer waveguide. At the lowest threshold, multipactor is generated in the minimum height region in the impedance transformer and nowhere else. More precisely, the multipacting electrons have a continuous energy distribution since the emitted secondary electrons carry a random initial velocity distribution. We observed that there are cases where the impact electron energy decreases despite an increase in power due to growing non-resonance of the microwave field and electron oscillations, resulting in not only two threshold points where secondary emission yield (SEY)=1 but several more. As a consequence, it was uncovered that when the average SEY in the highest field region is close to or less than one, multipactor may be caused in a lower field region where the SEY is effectively higher than one. The numerical results are compared with data from the experiment. While there is some deviation between the thresholds obtained from Spark3D and the experiment, the results at higher power levels are consistent with the experiment in the view of the SEY for each power level.
Authors: Q. Hu; R. P. Joshi; D. MiklavÄiÄ
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9027918
Abstract: Electric pulses can create pores and/or render cell membranes permeable, and this effect has been studied for decades. Applications include cell membrane permeabilization for gene electrotransfer, drug delivery, and related electrochemotherapy, as well as tissue ablation. Here, we probe the use of time-varying magnetic fields to modulate the transmembrane voltage (TMV) across cell membranes through numerical simulations. This could be a contactless, noninvasive technique. Results show that the induced TMV values exceeding the 1 V threshold for electroporation could be achieved for short duration pulsing with fast rise and fall times. The strongest response is then predicted to occur when the lateral distance between a cell and the center of a current carrying coil equals the coil radius. The induced TMV is shown to peak when the gradient in the magnetic potential is the largest. However, with the more realistic but longer microsecond pulse stimulation systems, the induced TMV is much smaller. Hence, developing shorter pulses or fast rise times is critical for achieving membrane poration based on time-varying magnetic fields. Other effects could also focus on the use of nanoparticles (including magnetic materials) for possible heating for synergistic enhancements of transport through tumor cell membranes.
Authors: He, M; Gu, WX; Kong, Y; Zhang, L; Spanos, CJ; Mosalam, KM
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8864027
Abstract: Predicting blood glucose concentration facilitates timely preventive measures against health risks induced by abnormal glucose events. Advances in IoT devices, such as continuous blood glucose monitors (CGMs) have made it convenient for measurements of blood glucose in real time. However, accurate and personalized blood glucose concentration prediction is still challenging. Previous inference models yield low-inference accuracy due to the ineffective feature extraction and the limited, imbalanced personal training data. The underlying causal correlations among the blood glucose series are scarcely captured by these models. In this article, we propose CausalBG, a causal recurrent neural network (CausalRNN) deployed on an IoT platform with smartphones and CGM for the accurate and efficient individual blood glucose concentration prediction. CausalBG automatically captures the underlying causal relationships embedded in the blood glucose features through CausalRNN, and efficiently shares the limited personal data among users for the sufficient training via the multitask framework. Evaluations and case studies on 112 users demonstrate that CausalBG significantly outperforms the conventional predictive models on the blood glucose dynamics inference.
Authors: Pushpakaran, BN; Subburaj, AS; Bayne, SB
PDF: https://link.springer.com/content/pdf/10.1007/s11664-020-08397-z.pdf
Abstract: Wide bandgap semiconductor technology is gaining widespread acceptance in the area of high-power and high-temperature power electronics. Gallium nitride (GaN) not only has a wide bandgap of 3.4 eV and all the associated superior electronic properties but also enables the development of high-mobility power devices which is critical in increasing the power density of a power electronics system. Since a commercial GaN power transistor has a lateral structure as opposed to the traditional vertical device structure, commercially available devices are rated below 1000 V breakdown voltage with a maximum value of 900 V and typical value around 650 V. The primary focus of this review will be to introduce readers to the commercially available power electronic systems developed by various manufacturers which employ GaN-based power devices and highlight their remarkable performance which surpasses existing technology. This review also includes a brief introduction on GaN technology followed by current market study showing the roadmap of integration of GaN-based power electronics in the power industry.
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.
Authors: M. Kim; J. J. Forbes; E. A. Hirsch; J. Schrock; S. Lacouture; A. Bilbao; S. B. Bayne; H. K. O’Brien; A. A. Ogunniyi
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9238402
Abstract: Silicon carbide (SiC) is becoming a preferred technology of choice for power dense application compared with silicon (Si). A more comprehensive analysis of the long-term pulsed power reliability of SiC is necessary so that the technology can make the transition commercially. In this article, a testbed is utilized to evaluate research grade 15-kV SiC MOSFETs and 20-kV SiC IGBTs manufactured by Wolfspeed, a Cree Company. A testbed was developed here at Texas Tech University (TTU), Lubbock, TX, USA, to test these two devices. The narrow pulse testbed's capacitor bank can be charged up to 10 kV and output square waveform pulse up to 2.0 μs. The waveform has a fullwidth at half-maximum pulse and is tested at a repetition pulse rate of three seconds. The electrical characteristics of the forward conduction and reverse breakdown of the device under test (DUT) are measured periodically during the experiment. The DUTs were pulsed at different current levels, up to 340 A (1.06 kA/cm2) for the IGBTs and 74 A (296 A/cm2) for MOSFETs, while the electrical device degradation was monitored. This work discusses the results of the long-term pulsed power reliability, failure modes, and their robustness in overcurrent operations of highpower SiC MOSFETs and IGBTs.
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.
Authors: S. Bayne; R. Allen; B. Srinivasan
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9218223
Abstract: The Guest Editor team of this Special Issue of the IEEE Transactions on Plasma and Science (TPS) is very excited to present this technical issue that encompasses various topics currently being studied within the field of pulsed power science and technology (PPST). Many articles for this Special Issue came from contributors (much like past Special Issues) who participated in the 2019 IEEE Pulsed Power and Plasma Science Conference (PPPS 2019), which comprised the 22nd Biennial PPC Conference and the 46th Annual ICOPS conference. The combined conference was held in Orlando, FL, USA, in June 23–28, 2019 (http://www.ppps2019.org/). Both PPC and ICOPS conferences continued a strong history of serving their community by providing a platform for members to give presentations and have discussions in their field of research. Although this Special Issue was prepared during the COVID-19 Pandemic, the editorial team and reviewers worked very hard to make this a very successful Special Issue.
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.
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.
Authors: S. S. L. Chukkapalli; S. Mittal; M. Gupta; M. Abdelsalam; A. Joshi; R. Sandhu; K. Joshi
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9187931
Abstract: Cyber-Physical Systems (CPS) and Internet of Thing (IoT) generate large amounts of data spurring the rise of Artificial Intelligence (AI) based smart applications. Driven by rapid advancements in technologies that support smart devices, agriculture and farming sector is shifting towards IoT connected ecosystem to balance the increase in demand for food supply. As the number of smart farms reach critical mass, it is now possible to include AI assisted systems at a cooperative (co-op) farming level. Today, in the United States alone there are about 1,871 co-ops serving 1,890,057 member farmers. Hence, such advanced technologies and infrastructure when incorporated in the co-op farming ecosystem can immensely benefit small member farmers who operate and maintain these independent co-op entities. In this paper, we develop a connected cooperative ecosystem which defines sensors and their communication among different entities along with cloud supported co-op hub. We develop member farm and co-op ontologies to capture data and various interactions that happen between shared resources, member farms, and the co-op that are stored in the cloud. These can then help generate AI supported insights for farmers and the cooperative. Several co-op farming use case scenarios have been discussed to demonstrate the functioning of our smart cooperative ecosystem. Finally, the paper describes various AI applications that can be deployed at the co-op level to aid member farmers.
Authors: S. Hahmady; S. Bayne
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9194759
Abstract: In this article, a novel approach is used for the first time to design a high-voltage PIN diode without any chemical doping process of cathode and anode region. This approach favors "p" and "n" plasma region formation through various metal contacts with appropriate work-functions for anode and cathode respectively. In this study, the forward and reverse characteristics, as well as the switching performance (reverse recovery) of this novel device, charge plasma (CP) PIN diode, were compared with the Schottky diode and the conventional PIN diode using TCAD simulation.
Authors: C. A. Negri; S. D. Dehnavi; M. G. Giesselmann
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9186303
Abstract: The high power and energy density of synchronous machines turn them into reliable sources of energy for pulsed power applications. In this article, two mathematical models using both actual and normalized (per-unit) parameters have been developed and simulated in LTspice, which is a powerful and a free tool to simulate electrical/electronic circuits. In this article, an efficient solver based on the flux and current equations is presented. The model has been validated against published results with both actual and per-unit parameters. It has been observed that for models with normalized parameters, the simulation time is significantly reduced. The validated model of the machine has been used to show the effects of nonlinear loads on the voltages and currents, in particular the reaction currents in the damper winding inside of the generator. In addition, the model has been used to study the effect of the RC time constant of the load on the peak power provided by a synchronous machine used to store energy for pulsed power applications.
Authors: Castillo, R; Bayne, S; Pol, S; Westergaard, C
PDF: https://journals.sagepub.com/doi/epub/10.1177/0309524X19852350
Abstract: Wind farm control has demonstrated power production improvements using yaw-based wake steering compared to individual turbine optimization. However, slower yaw actuation rates in response to rapid inflow changes lend to impracticality of yaw-based steering, as it causes time-varying downstream rotor-wake overlap, power production fluctuation, and consequent reduction. Therefore, closed-loop wake control is required to mitigate wake deflection uncertainty. To respond to rapid inflow variations, rotor speed actuated is investigated here. Furthermore, wake position information is required as feedback for closed-loop control function. For field-installed turbines, nacelle-based Light detection and ranging (LIDAR) is expected to provide this information. So far, LIDAR-derived wake position has been determined through model-based field reconstruction of scattered LIDAR data. However, this requires sophisticated, economically prohibitive LIDARs. To incorporate inexpensive, two-beam LIDAR for wake detection, a tip vortex-based approach was developed and is also presented here. These contributions can be considered as intermediate steps toward realization of a novel closed-loop wake control.
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.
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.
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.
Authors: Ramabhotla, S; Bayne, SB
PDF: https://journals.sagepub.com/doi/epub/10.1177/0309524X18820019
Abstract: A microgrid, an emerging technology in the electric power systems, has various benefits due to the implementation of distributed energy sources along with the loads. A microgrid utilizing the wind energy, solar energy, combined heat and power, natural gas generator, diesel generator, and battery storage is considered in an islanded mode of operation. The economic dispatch optimization is implemented using a reduced gradient algorithm to optimize the Operation and Maintenance cost in the islanded mode of the microgrid. The cost of each energy source is evaluated for every hour of the day using MATLAB code. Then, the availability of each energy source in the microgrid is evaluated. The results obtained are validated by comparing the Operation and Maintenance cost and the availability of each energy source in the microgrid. The optimal solution is achieved by considering the change in wind forecast and battery energy storage profiles.
Authors: Russo, J; Litz, M; Ray, W; Bayne, S; Rosen, GM; Cho, H; Yu, J; Bigio, DI; Thomas, C; Alam, TR
Abstract: Unattended, compact, terrestrial and space sensors require sources that have high energy and power densities to continuously operate for 3 to 99 years depending on application. Currently, chemical sources cannot fully satisfy these applications, especially in solid state form. Betavoltaic (beta V) nuclear batteries using beta(-)-emitting radioisotopes possess energy densities 1000 times greater than conventional chemical sources. Their power density is a function of fl flux saturation point relative to the planar (2D) configuration, beta(-) emission range, and the semiconductor converter, the betavoltaic (beta V) cell, properties. The figure of merit is the beta (beta(-))-flux surface power density (P-beta- in mu W-n per cm(2) footprint), where an optimal portion of incident beta particles penetrates the surrounding semiconductor depletion region. Tritiated nitroxides are favorable radioisotope sources with the potential to have the highest specific activity (A(m) in Ci/g) and P-beta- for an organic compound in solid form. The goal of this research is to demonstrate a tritiated nitroxide nuclear battery using the planar (2D) coupling configuration. The reproducible tritiation procedure produced stable product with a A(m) of approximately 635 Ci/g, which was 70% of the theoretical A(m). For the nuclear battery demonstration, the tritiated nitroxide, dissolved in methanol, was deposited on a 4H-SiC beta V and InGaP photovoltaic (PV) cell using a dispensing apparatus and micropipette. Both devices' characteristics were measured beforehand using a controlled electron beam source to approximate the surface radioactivity from the deposited radioisotope. The maximum power point (MPP) of the 4H-SiC and InGaP were 7.77 nW/cm(2) and 1.63 nW/cm(2) with 100 mCi and 67 mCi, respectively. The power and total efficiency were lower than expected due to partial solvent evaporation and droplet thickness. Numerical models using MCNP6 Monte Carlo code were used to simulate an optimal nuclear battery prototype. The models' accuracy was confirmed with the device calibration curves and a previous metal tritide model based on empirical results. Based on optimal model results, the tritiated nitroxide saturation layer thickness (D-0.99) and P-beta-(D-0.99) were 10 mu m and 558 nW/cm(2), respectively, using a 4H-SiC.
Authors: Shaw, ZC; Garcia, A; Powell, M; Dickens, JC; Mankowski, JJ; Neuber, AA
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.
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.
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.
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.
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.
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.
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.
Authors: M. Giesselmann; V. Roy
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8662233
Abstract: This paper is showing simulations of power supplies for repetitive power modulators using LTspice. We are presenting power supplies with single-phase and three-phase AC input and the effects of the rectifier stages on AC input current waveforms in both cases. We are also presenting the resulting power factor including circuits for electronic power factor correction. LTspice models for electronic power factor correction using cycle-by-cycle switching as well as time-averaged models are presented. The time-averaged models replace the PWM (pulse width modulation) switch with a transformer model with variable duty cycle. They are valid for a range of DC-DC converters. We are extending the time-averaged model for operation in continuous (CCM) and discontinuous (DCM) conduction mode and presenting results that show the fidelity of these models by comparison to cycle-by-cycle results. We are providing the detailed mathematical derivation for the DCM model expansion in the appendix. Time-averaged models do not have to keep track of thousands of steep switching transitions and can run orders of magnitude faster than cycle-by-cycle switch-mode models. Results using the time-averaged model are presented for a power supply with primary (wave-shaping) and cascaded secondary feedback control that regulates the output voltage of the converter.
Authors: Hu, Q; Zhang, L; Joshi, RP
PDF: https://aip.scitation.org/doi/10.1063/1.5085677
Abstract: Synergistic applications of an electric field combined with nanojet-based mechanical pressure, have recently been shown to help create larger pores and provide control of the aspect ratio in biological membranes. The nanojets are formed by the collapse of nanobubbles in the vicinity of biomembranes upon being subjected to external shockwaves. Here we analyze the effects produced by the collapse of multiple nanobubbles in the presence of an electric field. Our simulations, based on molecular dynamics, show that not only would multiple nanobubbles make it possible to create larger pores, but also increase the pore density on the surface of biological cells. Both aspects could aid in the transport of drugs and genes for bio-medical applications. (C) 2019 Author(s).
Authors: A. R. Chowdhury; S. Nikishin; J. Dickens; A. Neuber; R. P. Joshi; R. Ness
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8662228
Abstract: Time-dependent photocurrent response in semi-insulating GaN is simulated with a focus on the "Lock-On" phenomenon. A one-dimensional, time-dependent model based on drift-diffusion theory is used. The model was first tested for GaAs and shown to yield good agreement with data, before applying it to GaN simulations. The main findings are that compensated GaN with deeper traps nearer the midgap at higher densities, and/or multiple levels around the mid-gap would aid in driving the PCSS towards Lock-On. The initial average threshold field for Lock-On in GaN is predicted to be around 150 kV/cm, though this would be strongly dependent on the trap parameters of a sample.
Authors: F. Hegeler; A. Neuber; C. Hatfield
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8689446
Abstract: Recounts the career and contributions of Lynn LaMar Hatfield.
Authors: Nguyen, HKA; Sanati, M; Joshi, RP
PDF: https://aip.scitation.org/doi/10.1063/1.5113642
Abstract: There is considerable interest in mitigating secondary electron emission (SEE) from surfaces and electrodes produced by incident electrons, due to the deleterious effects of SEE in vacuum electron devices, accelerators, and other technologies. Since surface conditions are known to affect SEE, here the role played by crystal orientation and a vacancy (which is a simple example of a surface defect) is probed through Monte Carlo simulations. The effect of the lattice imperfection on the frequency-dependent permittivity, which then influences inelastic energy losses, mean free paths, and secondary generation profiles, is obtained on the basis of density-functional theory. The Monte Carlo simulations are in good agreement with previous experimental reports. The results indicate that the secondary electron yield for pure copper is the highest for the 110 orientation and the lowest for the 111 case, with a relatively higher differential predicted between a single vacancy and ideal copper for the 111 orientation. The results underscore the benefit of annealing or reducing inhomogeneities through laser or charged particle beam surface treatments.
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.
Authors: Stephens, JC
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.
Authors: Hu, Q; Hossain, S; Joshi, RP
PDF: https://iopscience.iop.org/article/10.1088/1361-6463/aaca7a
Abstract: Electric pulse driven membrane poration finds applications in the fields of biomedical engineering and drug/gene delivery. Shock waves are known to permeabilize cell membranes as well. Here we focus on and analyze the synergistic effects of both inputs in concert based on molecular dynamics simulations. Our results show that shockwaves could be used for pretreating cell membranes for electroporation. The dual strategy would either reduce the external voltage requirements (leading to more compact external circuitry) or help create larger pores. Furthermore, shockwaves could form pores at any desired membrane site location, and suitable combinations of nanojets and electric pulses would help control the aspect ratio and size as desired.
Authors: A. R. Chowdhury; R. Ness; R. P. Joshi
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8423685
Abstract: The time-dependent photocurrent response in semi-insulating GaAs and InP was studied based on 1-D, time-dependent simulations with a focus on the Lock-On phenomenon. The results underscore the role of trap-to-band impact ionization from deep traps in rapid charge creation and its subsequent propagation much like a streamer. The numerical results compare well with the actual data. The main findings are that deeper traps nearer the valence band at higher densities, materials with larger high-field drift velocity, and cathode-side illumination would all aid in attaining Lock-On. These could be useful guidelines for producing Lock-On in new materials such as GaN for high-power applications.
Authors: Chowdhury, AR; Dickens, JC; Neuber, AA; Joshi, RP
PDF: https://aip.scitation.org/doi/10.1063/1.4972968
Abstract: Simulation studies of the electrical response characteristics of 4H-SiC switches containing traps are reported in the absence of photoexcitation. The focus is on trap-to-band impact ionization and the role of hole injection from the anode. Simulations show that hole-initiated ionization can be more important than the electron-initiated process. The results also underscore the role of hole injection at the high applied voltages. Our one-dimensional, time-dependent model yielded reasonable agreement with measured current-voltage data spanning over three orders of magnitude, but only when impact ionization was taken into account. Finally, the simulations predicted undulations in the device conduction current density with respect to time, due to the dynamic interplay between impact ionization, spatial electric field values, and occupancies of the trap levels.
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).
Authors: S. Abedi; M. He; D. Obadina
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8352818
Abstract: Large-scale and ubiquitous penetration of wind power generation to power systems necessitates more conservative provision of system reliability by ensuring adequately committed reserve and observance of transmission constraints. In addition, wind power curtailment due to the technical limitations of system operations, such as transmission congestion, should be efficiently mitigated. To this aim, this paper presents a congestion risk-aware unit commitment formulation in a two-settlement market environment. The uncertainty impact of multicorrelated wind power and contingencies on the risk of transmission congestion for each line, called the Line Transfer Margins (LTM), is incorporated using basic statistical data on the nodal wind power forecast and probability of credible line-outages across the system. The LTMs, formulated free of any distributional assumptions, collectively provide a measure for transmission reserves, which effectively mitigate the likelihood of transmission congestion, reserve undeliverability, and wind power curtailment in the real-time economic dispatch. The effectiveness of the proposed approach is verified through comparative case studies on IEEE RTS-96 for various wind power and LTM scenarios.
Authors: Woodrum, RB; Barnett, DH; Dickens, JC; Neuber, AA
PDF: https://onlinelibrary.wiley.com/doi/10.1002/prep.201700285
Abstract: This paper presents the work performed on dry-lathing PBX 9501 to gather and analyze cutting force as well as temperature data during the machining process. The data is compared to present USA federal-regulation-constrained machining limits of high explosives. The effects of machining parameters depth of cut, surface meters per minute, and feed per revolution on cutting force and cutting interface were evaluated. Cutting tools of tip radius 0.013cm and 0.127cm were tested to determine the effect of the tool shape on the machining process. Empirically, a pronounced dependence of the maximum tool temperature on the depth of cut and surface meters per minute was found, while the dependence on the feed per revolution was found much weaker. It is elucidated that rapid, shallow cuts optimize machining time for a billet of PBX 9501 while minimizing temperature increase and cutting force.
Authors: Hinojosa, M; Ogunniyi, A; Bayne, S; Van Brunt, E; Ryu, SH
PDF: https://www.scientific.net/MSF.858.949
Abstract: This section presents the current progress in the development of an electro-thermal numerical model for 22 kV 4H-silicon carbide IGBTs. This effort involved the creation of a TCAD model based on doping profiles and structural layers to simulate the steady-state and switching characteristics of recently-fabricated experimental devices. The technical challenge of creating this high voltage SiC IGBT model was incorporating semiconductor equations with sub-models representing carrier mobility, generation, recombination, and lattice heat flow effects with parameters conditioned for 4H-silicon carbide material. Simulations of the steady-state and switching characteristics were performed and later verified with laboratory measurements for an N-type SiC IGBT rated for 22 kV with an active area of 0.37 cm(2) and a drift region of 180 mu m.
Authors: S. Bayne; B. Novac; H. O"™Brien; H. Li
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8481639
Abstract: This special issue of the IEEE TRANSACTIONS ON PLASMA SCIENCE (TPS) mainly contains works presented at the 21st IEEE International Pulsed Power Conference (PPC) held in Brighton, U.K., between June 18"“22, 2017 (
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).
Authors: Chowdhury, AR; Dickens, JC; Neuber, AA; Ness, R; Joshi, RP
PDF: https://aip.scitation.org/doi/10.1063/1.5013248
Abstract: The time-dependent photoconductive current response of semi-insulating GaAs is probed based on one-dimensional simulations, with a focus on the lock-on phenomenon. Our results capture most of the experimental observations. It is shown that trap-to-band impact ionization fuels local field enhancements, and photon recycling also plays an important role in pushing the device towards lock-on above a 3.5 kV/cm threshold field. The results compare well with actual data in terms of the magnitudes, the rise times, and the oscillatory behavior seen at higher currents. Moving multiple domains are predicted, and the response shown depended on the location of the photoexcitation spot relative to the electrodes. Published by AIP Publishing.
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.
Authors: He, M; Hao, M; Chen, G; Chen, X; Li, WP; Zhang, C; Wang, HT; Zhou, MY; Lei, XZ
PDF: https://www.emerald.com/insight/content/doi/10.1108/COMPEL-07-2017-0297/full/html
Abstract: PurposeHigh voltage direct current (HVDC) cable is an important part in the electric power transmission and distribution systems. However, very little research has been carried out on partial discharge under direct current (DC) conditions. Niemeyer's model has been widely used under alternating current (AC) conditions. This paper aims to intend to modify the Niemeyer's model considering both electric field and charge dynamics under DC conditions, and therefore proposes a numerical model describing partial discharge characteristics in HVDC cable. Design/methodology/approachThis paper intends to understand partial discharge characteristics under DC conditions through numerical modelling. Niemeyer's model that has been widely used under AC conditions has been modified, taking both electric field and charge dynamics under DC conditions into consideration. The effects of loading level or current through the conductor, cavity location and material properties on partial discharges have also been studied. FindingsElectrical conductivity is important in determining the characteristics of partial discharge under DC conditions and discharges tend to happen in short when the cavity field exceeds the inception level under the parameter values studied in the paper. Research limitations/implicationsBuilding the numerical model is the purpose of the paper, and there is lack in experiment and the comparison between the simulation results and experiment. Practical implicationsThe proposed model provides the numerical model describing partial discharge in HVDC cable and helps understand the partial discharge mechanism under DC voltage. Originality/valueTo the best of the author's knowledge, this paper is a very early research on the numerical modelling work on partial discharge under DC voltage.
Authors: He, M; Hao, M; Chen, G; Li, WP; Zhang, C; Chen, X; Wang, HT; Zhou, MY; Lei, XZ
PDF: https://www.emerald.com/insight/content/doi/10.1108/COMPEL-08-2017-0323/full/html
Abstract: Purpose For the dramatically developed high voltage direct current (HVDC) power transmission, HVDC cables play a vital role in the power transmission across seas and connections with renewable power sources. However, the condition monitoring of HVDC cables is still a challenging research topic. This paper aims to understand the influence of external factors, namely, current, cavity location and material properties, on partial discharge (PD) characteristics in HVDC cable in a numerical way referring to the refined Niemeyer's model. Design/methodology/approach The influences of the three external factors are studied by a proposed numerical model for DC PDs based on the modification of a conventional PD model for AC voltage via a finite element analysis method. Findings The external factors can influence the discharge magnitude and discharge repetition rate via affecting the electrical conductivity of the material: DC PD is more frequent and with higher discharge magnitude when the cavity is closer to the conductor or the current through the conductor is higher. Both DC PD repetition rate and average discharge magnitude in long-term aged material are lower than virgin material. The effect of discharge on insulation degradation becomes decreasingly significant. Research limitations/implications The current work is based on the numerical modelling of DC PDs. Further experimental validations and comparisons are essential for improving the model. Practical implications The studies of the influence factors for PDs under HVDC voltage can benefit the research and practical power transmission on DC PDs, contributing the design and test of DC PDs in HVDC cables, exploring the understandings of the DC PDs' mechanism. Originality/value This paper, to the best of author's knowledge, first studies the influence factors on DC PDs based on the numerical modelling work.
Authors: Kelley, MD; Pushpakaran, BN; Bilbao, AV; Schrock, JA; Bayne, SB
Abstract: The high-voltage silicon carbide MOSFET is a state-of-the-art solution for increasing power density and efficiency in power electronics; nonetheless, a full-scope of failure modes during extreme operating condition has not been established. Past efforts evaluated short-circuit capability of 10-kV silicon carbide MOSFET, however, in this manuscript, the single-pulse avalanche mode operation of a research-grade 10-kV/10-A silicon carbide MOSFET is explored for the first time. A decoupled undamped inductive circuit was selected for evaluation, and avalanche energy was increased until catastrophic failure occurred. The maximum tolerable avalanche energy was measured to be 2.84 J corresponding to an energy density of 8.8 &cm(-2). This result was compared with 1.2 kV silicon carbide MOSFETs to evaluate device robustness. Post failure analysis included: estimation of junction temperature, scanning electron microscopy, and focused ion beam cut. Peak junction temperature of 1010 degrees C was estimated using a thermal RC model and measurement results suggested gate degradation as the primary mechanism responsible for device destruction. Microscopy of the device validated gate failure which occurred at, or beneath, the gate metallization. A narrow cavity with-in the failure region was discovered during failure analysis and is hypothesized to have protruded the epitaxial region of the semiconductor.
Authors: Gaddy, M; Kuryatkov, V; Meyers, V; Mauch, D; Dickens, J; Neuber, A; Nikishin, S
PDF: https://link.springer.com/article/10.1557/adv.2018.234
Abstract: Characterization of three vendor's bulk semi-insulating GaN:Fe wafers, grown by either hydride vapor phase epitaxy or the ammonothermal method, was performed using: scanning electron microscopy, secondary ion mass spectroscopy, high resolution X-ray diffraction, cathodoluminescence, photoluminescence, and high voltage testing. Although the Fe doping level is significantly different for each growth method, both are promising for the fabrication of PCSS devices operating in the lock-on mode.
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.
Authors: Shaw, Z; Feilner, W; Esser, B; Dickens, JC; Neuber, AA
PDF: https://pubmed.ncbi.nlm.nih.gov/28964249/
Abstract: A software controllable system which generates and transmits user defined RF signals is discussed. The system is implemented with multiple, modular transmitting channels that allow the user to easily replace parts such as amplifiers or antennas. Each channel is comprised of a data pattern generator (DPG), a digital to analog converter (DAC), a power amplifier, and a transmitting antenna. All channels are controlled through a host PC and synchronized through a master clock signal provided to each DAC by an external clock source. Signals to be transmitted are generated through the DPG control software on the PC or can be created by the user in a numerical computing environment. Three experiments are discussed using a two-and four-channel antenna array incorporating Chebyshev tapered TEM horn antennas. Transmitting distinct sets of nonperiodic bipolar impulses through each of the antennas in the array enabled synthesizing a sinusoidal signal of specific frequency in free space. Opposite to the standard phased array approach, each antenna radiates a distinctly different signal rather than the same signal simply phase shifted. The presented approach may be employed as a physical layer of encryption dependent on the position of the receiving antenna. Published by AIP Publishing.
Authors: Lacouture, S; Schrock, J; Hirsch, E; Bayne, S; O'Brien, H; Ogunniyi, AA
PDF: https://pubmed.ncbi.nlm.nih.gov/28964219/
Abstract: Of all of the material parameters associated with a semiconductor, the carrier lifetime is by far the most complex and dynamic, being a function of the dominant recombination mechanism, the equilibrium number of carriers, the perturbations in carriers (e.g., carrier injection), and the temperature, to name the most prominent variables. The carrier lifetime is one of the most important parameters in bipolar devices, greatly affecting conductivity modulation, on-state voltage, and reverse recovery. Carrier lifetime is also a useful metric for device fabrication process control and material quality. As it is such a dynamic quantity, carrier lifetime cannot be quoted in a general range such as mobility; it must be measured. The following describes a stand-alone, wide-injection range open circuit voltage decay system with unique lifetime extraction algorithms. The system is initially used along with various lifetime spectroscopy techniques to extract fundamental recombination parameters from a commercial high-voltage PIN diode. Published by AIP Publishing.
Authors: Meyers, V; Chowdhury, AR; Mauch, D; Dickens, JC; Neuber, AA; Joshi, RP
PDF: https://iopscience.iop.org/article/10.1088/1361-6463/aa59aa/pdf
Abstract: We report on the intensity-dependent behavior of the absorption coefficient (alpha) in semiinsulating 4H-SiC material. Data from as-received samples show a monotonic decrease in a with incident energy density, with a pronounced change in slope at around 10 mJ cm(-2). Annealed samples, on the other hand, exhibit an experimental trend of increasing alpha with intensity. Qualitative explanation of the observed behavior is presented that probes the possible role of spontaneous and stimulated emission for as-received samples. With annealing, trap related recombination is strongly reduced leading to higher carrier densities and increased free-carrier absorption with incident intensity. The role of band-filling and permittivity changes are shown to be inconsequential, while changes in internal fields could contribute to decreases in absorption.
Authors: Hu, Q; Joshi, RP
PDF: https://aip.scitation.org/doi/10.1063/1.4994310
Abstract: Electric pulse driven membrane poration finds applications in the fields of biomedical engineering and drug/gene delivery. Here we focus on nanosecond, high-intensity electroporation and probe the role of pulse shape (e.g., monopolar-vs-bipolar), multiple electrode scenarios, and serial-versus-simultaneous pulsing, based on a three-dimensional time-dependent continuum model in a systematic fashion. Our results indicate that monopolar pulsing always leads to higher and stronger cellular uptake. This prediction is in agreement with experimental reports and observations. It is also demonstrated that multipronged electrode configurations influence and increase the degree of cellular uptake. Published by AIP Publishing.
Authors: Song, J; Garner, AL; Joshi, RP
PDF: https://journals.aps.org/prapplied/pdf/10.1103/PhysRevApplied.7.024003
Abstract: The use of nanosecond-duration-pulsed voltages with high-intensity electric fields (similar to 100 kV/cm) is a promising development with many biomedical applications. Electroporation occurs in this regime, and has been attributed to the high fields. However, here we focus on temperature gradients. Our numerical simulations based on molecular dynamics predict the formation of nanopores and water nanowires, but only in the presence of a temperature gradient. Our results suggest a far greater role of temperature gradients in enhancing biophysical responses, including possible neural stimulation by infrared lasers.
Authors: Bejoy N. Pushpakaran; Stephen B. Bayne; Aderinto A. Ogunniyi
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7792177
Abstract: Pulsed power applications are characterized by very high instantaneous power due to the high voltage and current involved. Power diodes used as a closing switch in pulsed power circuits must be able to withstand the high current operation well above the continuous device rating, for a transient duration. Due to the superior electrothermal properties of wide bandgap Silicon Carbide (SiC) material, it is feasible to develop high Blocking Voltage (BV) Schottky and Junction Barrier Schottky (JBS) diodes besides p-i-n rectifiers. In order to evaluate the device performance under high current density pulsed operation, 2-D models of SiC p-i-n, Schottky, and JBS diodes rated for 3.3-kV BV and 100 A/cm2 current density were developed using Silvaco ATLAS TCAD software. The diode structures were simulated electrothermally to study the device behavior and compare the performance under high current density pulsed operation. The power dissipation and the lattice temperature profile of the SiC diodes were analyzed to compare the magnitude of heat loss and formation of thermal hot spot in the diode structure to predict the suitability of the device for pulsed power applications.
Authors: Zhang, Z; Giesselmann, M; Mankowski, J; Dickens, J; Neuber, A; Joshi, RP
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.
Authors: Liu, S; Zou, W; He, MZ; Kurths, J; Zhan, M
PDF: https://epubs.siam.org/doi/epdf/10.1137/16M1086005
Abstract: Synchronization (or sync) is a basic problem in nature and engineering. One decade ago, the problem of the size of the sync (as a global geometric quantity) was first proposed by Wiley, Strogatz, and Girvan (WSG). In studying a ring of N coupled identical phase oscillators with each interacting equally with its d nearest neighbors on either side, they found that the sync state becomes globally stable when d/N is greater than a critical constant [D. A. Wiley, S. H. Strogatz, and M. Girvan, Chaos, 16 (2006) 015103]. Based on our previous results on the local stability of splay states in a ring of coupled Landau-Stuart oscillators, in this paper we further study the problem of global stability of the sync with amplitude effects, and we find that for any sufficiently large N the condition for the critical coupling range d(c) satisfying global stability of sync now becomes d(c) alpha N-gamma (gamma = 2/3), whereas when the coupling strength becomes weaker, the condition gradually changes to d(c) similar or equal to 0.3405N coincidental with the former result of the WSG in coupled phase oscillators. These relations are indicative of the favorable effect of amplitude for global stability of the sync. All these findings are based on rigorous theoretical analysis and have been well verified by numerical simulation; they are expected to be of potential significance for theory as well as application of synchronization in networked oscillatory systems.
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.
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.
Authors: Meyers, V; Mauch, D; Dickens, J; Neuber, A
PDF: https://aip.scitation.org/doi/10.1063/1.4978768
Abstract: The intensity-dependent light absorption in bulk high-purity semi-insulating 4H-SiC at above band gap photon energies has been studied. In particular, 3.49 eV (355 nm) UV absorption of 160 lmthick samples of varying recombination lifetimes in the intensity range of 1 mJ/cm(2) -30 mJ/cm(2) is addressed. The effective absorption coefficient was found to vary up to 30% within this range. Assuming deep level trapping, interband absorption, and free carrier absorption as dominant processes, a four energy level model reproduces the experimentally observed absorption behavior. While nonlinearities in the optical absorption behavior of SiC have been studied previously as function of wavelength alpha(lambda), temperature alpha(T) and, to a very limited extent, at below bandgap optical intensities, the presented elucidates the UV intensity-dependent nonlinear absorption behavior, alpha(I), of SiC at above bandgap photon energies. Published by AIP Publishing.
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.
Authors: Wilson, N; Mauch, D; Meyers, V; Feathers, S; Dickens, J; Neuber, A
PDF: https://pubmed.ncbi.nlm.nih.gov/28863629/
Abstract: The electrical and optical characteristics of a high-power UV light emitting diode (LED) (365 nm wavelength) were evaluated under pulsed operating conditions at current amplitudes several orders of magnitude beyond the LED's manufacturer specifications. Geared towards triggering of photoconductive semiconductor switches (PCSSs) for pulsed power applications, measurements were made over varying pulse widths (25 ns-100 mu s), current (0 A-250 A), and repetition rates (single shot-5 MHz). The LED forward voltage was observed to increase linearly with increasing current (similar to 3.5 V-53 V) and decrease with increasing pulse widths. The peak optical power observed was >30 W, and a maximum system efficiency of 23% was achieved. The evaluated LED and auxiliary hardware were successfully used as the optical trigger source for a 4H-SiC PCSS. The lowest measured on-resistance of SiC was approximately 67 k Omega. Published by AIP Publishing.
Authors: J. Zhao; S. Abedi; M. He; P. Du; S. Sharma; B. Blevins
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7872512
Abstract: Hourly wind power ramps in ERCOT are studied by applying extreme value theory. Mean excess plot reveals that the tail behavior of large hourly wind power ramps indeed follows a generalized Pareto distribution. The location, shape, and scale parameters of generalized Pareto distribution are then determined by using mean excess plot and the least square technique, from which risk measures including α quantile value at risk and conditional value at risk are calculated.
Authors: M. D. Kelley; B. N. Pushpakaran; S. B. Bayne
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7676377
Abstract: Commercialization of 1200-V silicon carbide (SiC) MOSFET has enabled power electronic design with improved efficiency as well as increased power density. High-voltage spikes induced in applications such as solenoid control, solid-state transformer, boost converter, and flyback converter can drive the MOSFET into avalanche mode operation due to high di/dt coupled with parasitic inductance. Avalanche mode operation is characterized by high-power dissipation within the device due to the high voltage and current crossover. This study focuses on the evaluation of two commercially available SiC MOSFETs from different manufacturers, each rated for 1200 V with an ON-state resistance of 80 mΩ, during unclamped inductive switching (UIS) mode operation. To determine device reliability, a decoupled UIS testbed was developed to evaluate the avalanche energy robustness at 22 °C and 125 °C during two specific conditions: high current and low energy, and low current and high energy. The SiC MOSFETs were evaluated using a load inductance of 1.42, 5.1, 10.5, and 15.8 mH to understand the effect of current and avalanche energy on device failure. To correlate the experimental results with the failure mechanism, estimated junction temperature and static device characteristics are presented; additionally, MOSFETs were decapsulated to examine the failure sites on the semiconductor die.
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%.
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.
Authors: Fan, ZY; Islam, N; Bayne, SB
Abstract: Electrochemical capacitors (ECs) are slow devices with charging and discharging rates limited below 1 Hz. They run at direct current and function as power source, but cannot afford the role of a conventional capacitor for current ripple filtering or pulse energy harvesting. Recently, developing ultrafast ECs that work at hundreds to kilohertz (kHz) frequency scope have attracted great interests, with the aim to replace the traditional aluminum electrolytic capacitors (AECs) that have bulky size and large equivalent series resistance. Compact kHz ECs would produce huge impacts on power design, power electronics and environmental pulse energy harvesting. Towards such a goal, the electrode material and its nanostructure are the keys to boost the response frequency of an EC from below 1 Hz to above 1 kHz. In this Review, we summarize guidelines on the electrode nanostructure design for kHz response, discuss the various carbonaceous materials and other highly conductive materials based electrode structures for kHz ECs. The configurations of higher voltage kHz ECs, and their dimension advantage over AECs are critically evaluated, followed with the outlook on the further study and development in this promising area.
Authors: Lacouture, S; Bayne, S
PDF: https://aip.scitation.org/doi/full/10.1063/1.4941938
Abstract: With the advent of modern power semiconductor switching elements, the envelope defining high power is an ever increasing quantity. Characterization of these semiconductor power devices generally falls into two categories: switching, or transient characteristics, and static, or DC characteristics. With the increasing native voltage and current levels that modern power devices are capable of handling, characterization equipment meant to extract quasi-static IV curves has not kept pace, often leaving researchers with no other option than to construct ad hoc curve tracers from disparate pieces of equipment. In this paper, a dedicated 10 V, 500 A curve tracer was designed and constructed for use with state of the art high power semiconductor switching and control elements. The characterizer is a physically small, pulsed power system at the heart of which is a relatively high power linear amplifier operating in a switched manner in order to deliver well defined square voltage pulses. These actively shaped pulses are used to obtain device's quasi-static DC characteristics accurately without causing any damage to the device tested. Voltage and current waveforms from each pulse are recorded simultaneously by two separate high-speed analog to digital converters and averaged over a specified interval to obtain points in the reconstructed IV graph. (C) 2016 AIP Publishing LLC.
Authors: K. Eldridge; A. Fierro; J. Dickens; A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7462994
Abstract: Destructive and constructive interference of multiple time-shifted and amplitude-adjusted higher frequency signals (wavelet signals) is exploited in order to reproduce a desired signal at a given point in the far-field regime of radiating antennas. The number of individual wavelets is intentionally kept small in keeping with a realistic antenna array size, where each antenna would emit wavelets at conceivably very high power levels. Wavelet decomposition theory is coupled with particle swarm optimization to determine the necessary time shifts and amplitude adjustments of the wavelet signals. In this application, the reconstructed signal can be specified by a desired frequency or arbitrary shape. A pyramidal horn antenna array is used in the analysis of the far-field propagation of the wavelet signals due to its relatively large bandwidth and known analytical electric field solutions. It is found that when the wavelet signals are appropriately superpositioned and added in the far field, the desired signal may be reconstructed with the quality of reconstruction mostly governed by the intentionally low number of wavelets. The reconstructed signal is solely found on the centerline while the signal drastically changes off the centerline or at distances too close or too far from the antenna array.
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.
Authors: A. S. Subburaj; S. B. Bayne; M. G. Giesselmann; M. A. Harral
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7175004
Abstract: This paper provides steady-state and transient analysis of the equivalent circuit of the 1 MWh battery tied to the grid for wind integration. It also discusses the installation of a 1 MWh battery system at Reese Technology Center (RTC) in Lubbock, Texas. The research involves deploying energy storage devices for application with wind turbine model to understand the transient behavior of the system under three phase fault conditions. A 1 MW/1 MWh battery storage system at the RTC is connected to the South Plains Electric Cooperative (SPEC) grid. The batteries are used for energy storage and for mitigation of transient conditions grid dynamics. In this paper, the equivalent circuit of the 1 MWh battery is modeled in PSCAD and analyzed for its charge and discharge characteristics under transient fault conditions when it is tied to the grid for wind integration.
Authors: Chowdhury, AR; Dickens, JC; Neuber, AA; Joshi, RP
PDF: https://aip.scitation.org/doi/10.1063/1.4972968
Abstract: Simulation studies of the electrical response characteristics of 4H-SiC switches containing traps are reported in the absence of photoexcitation. The focus is on trap-to-band impact ionization and the role of hole injection from the anode. Simulations show that hole-initiated ionization can be more important than the electron-initiated process. The results also underscore the role of hole injection at the high applied voltages. Our one-dimensional, time-dependent model yielded reasonable agreement with measured current-voltage data spanning over three orders of magnitude, but only when impact ionization was taken into account. Finally, the simulations predicted undulations in the device conduction current density with respect to time, due to the dynamic interplay between impact ionization, spatial electric field values, and occupancies of the trap levels. Published by AIP Publishing.
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.
Authors: T. Flack; C. Hettler; S. Bayne
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7470502
Abstract: This paper details the experimental evaluation and simulation of a 4-kV n-type gate turn-OFF thyristor (GTO) designed for pulsed power applications. The primary criteria of evaluation are rate of current rise (dI/dt), turn-ON delay time (TD), and resistance of the device during turn-ON transients [RON(t)]. The device under test (DuT) is an n-type asymmetric-blocking GTO manufactured by Silicon Power (Part No. 14N40A10) with a rated dc blocking voltage of 4 kV. A test circuit was specifically designed to minimize stray inductance in order to capitalize on the dI/dt capabilities of the DuT. Experimental data collected from resistance measurements are used to develop a single-switch approximate model for use in simulation. The results of dI/dt experiments provide a profile of DuT dI/dt operation beyond rated values; specifically dI/dt values >70 kA/μs were readily achieved. The turn-ON delay time of the DuT is also characterized and determined to be ~225 ns on average.
Authors: A. R. Chowdhury; D. Mauch; R. P. Joshi; A. A. Neuber; J. Dickens
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7505601
Abstract: We focus on a simulation study to probe the mitigation of electric fields, especially at the edges of metal contacts to SiC-based photoconductive switches. Field reduction becomes germane given that field-induced failures near contacts have been reported. A dual strategy of extending metal contacts to effectively spread the electric field over a larger distance and to employ HfO2 as a high-k dielectric, is discussed. Simulation results show that peak electric fields can be lowered by up to ~67% relative to a standard design. Finally, our calculations predict that the internal temperature rise for a ~7-ns laser pulse and applied voltages around 20 kV (typical experimental values) would also be effectively controlled.
Authors: Cao, Y; Jiang, T; He, M; Zhang, JS
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7147770
Abstract: The transmission of simultaneous and latency-sensitive data puts forth a significant challenge for the smart grid communications. In this paper, we investigate the application of device-to-device (D2D) communications for the energy management in the electric distribution network. Specifically, we develop a D2D-assisted relaying framework to exploit the spatial diversity and the differentiated data rate requirements, which improves the spectral efficiency, especially for the scenarios that there are faults in the electric distribution network. We study the data transmission scheduling problem under the proposed D2D-assisted relaying framework, aiming to minimize the overall information loss rate, while taking into account the uncertainties in the communication latency. To this end, we first cast the data transmission scheduling problem as a two-stage stochastic programming problem and derive the solution. Then, we develop a real-time distributed data transmission scheduling scheme based on the sample path realizations. Extensive simulation results show significant performance improvement by using the proposed D2D-assisted relaying framework compared with two baseline frameworks for a variety of different cases.
Authors: Fierro, A; Stephens, J; Beeson, S; Dickens, J; Neuber, A
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.
Authors: J. A. Schrock; B. N. Pushpakaran; A. V. Bilbao; W. B. Ray; E. A. Hirsch; M. D. Kelley; S. L. Holt; S. B. Bayne
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7180402
Abstract: SiC MOSFETs are a leading option for increasing the power density of power electronics; however, for these devices to supersede the Si insulated-gate bipolar transistor, their characteristics have to be further understood. Two SiC vertically oriented planar gate D-MOSFETs rated for 1200 V/150 A were repetitively subjected to pulsed overcurrent conditions to evaluate their failure mode due to this common source of electrical stress. This research supplements recent work that demonstrated the long term reliability of these same devices [1]. Using an RLC pulse-ring-down test bed, these devices hard-switched 600 A peak current pulses, corresponding to a current density of 1500 A/cm2. Throughout testing, static characteristics of the devices such as BVDSS, RDS (on), and VGS(th) were measured with a high power device analyzer. The experimental results indicated that a conductive path was formed through the gate oxide; TCAD simulations revealed localized heating at the SiC/SiO2 interface as a result of the extreme high current density present in the device's JFET region. However, the high peak currents and repetition rates required to produce the conductive path through the gate oxide demonstrate the robustness of SiC MOSFETs under the pulsed overcurrent conditions common in power electronic applications.
Authors: J. A. Schrock; E. A. Hirsch; S. Lacouture; M. D. Kelley; A. V. Bilbao; W. B. Ray; S. B. Bayne; M. Giesselmann; H. O'Brien; A. Ogunniyi
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7482767
Abstract: SiC SGTO thyristors are an advanced solution for increasing the power density of medium voltage power electronics. However, for these devices to replace Si thyristor technology in industrial applications their characteristics and failure modes must be understood. This letter presents the failure modes of two 15-kV SiC SGTO thyristors during repetitive overcurrent conditions. The devices were evaluated with 2-kA (3.85 kA/cm2) square pulses of 100 μs duration using a pulse forming network. Throughout testing, each devices' static characteristics were analyzed for signs of degradation; upon degradation, testing was ceased and the physical failure mode was determined through imaging with a scanning electron microscope (SEM) in conjunction with a focused ion beam. The electrical results demonstrate the failure modes of both SiC SGTO thyristors during pulsed overcurrents electrically manifested themselves as a conductive path through the gate-anode junction and an increased device on-state voltage. SEM imaging revealed one SiC thyristor formed an approximately 10-μm wide cylindrical void, and the second SiC thyristor formed an approximately 200-μm long crack. However, the experimental results demonstrate these 15-kV SiC SGTO thyristors' robust ability to repetitively switch at extreme high current density for tens of thousands of cycles.
Authors: Flack, TJ; Pushpakaran, BN; Bayne, SB
PDF: https://link.springer.com/content/pdf/10.1007/s11664-016-4435-3.pdf
Abstract: Power semiconductor devices based on silicon (Si) are quickly approaching their limits, set by fundamental material properties. In order to address these limitations, new materials for use in devices must be investigated. Wide bandgap materials, such as silicon carbide (SiC) and gallium nitride (GaN) have suitable properties for power electronic applications; however, fabrication of practical devices from these materials may be challenging. SiC technology has matured to point of commercialized devices, whereas GaN requires further research to realize full material potential. This review covers fundamental material properties of GaN as they relate to Si and SiC. This is followed by a discussion of the contemporary issues involved with bulk GaN substrates and their fabrication and a brief overview of how devices are fabricated, both on native GaN substrate material and non-native substrate material. An overview of current device structures, which are being analyzed for use in power switching applications, is then provided; both vertical and lateral device structures are considered. Finally, a brief discussion of prototypes currently employing GaN devices is given.
Authors: Pushpakaran, BN; Subburaj, AS; Bayne, SB; Mookken, J
Abstract: The increased awareness of the significance of solar energy has led to intensified research in the areas of solar energy harvesting. To increase the cost effectiveness of the generation of solar power, silicon carbide (SiC) power devices are playing a major role in the power electronics technology due to its superior material properties compared to Silicon (Si). The photovoltaic (PV) inverter is a major component in the solar energy conversion system whose performance relies on the efficient design of power electronics. In order to obtain maximum power from the solar panels, the power loss in the energy conversion system must be minimized by proper selection of semiconductor devices and thereby minimizing the number of power electronic components used. The necessity to reduce the overall switch mass and volume have led to the development of advanced high-power, high-temperature semiconductor materials such as SiC. The performance improvements are based on superior material properties of SiC, such as: bandgap of 3.26 eV, critical breakdown field of 2-4 MV/cm, thermal conductivity of 4.9 W/(cm K), and a saturated drift velocity of 2 x 10(7) cm/s. The aforementioned properties of SiC highlight the importance of silicon carbide semiconductor technology. Some of the limitations of the technology include higher device cost due to emerging technology, and need for high-temperature packaging techniques. Future research includes methods to reduce manufacturing cost, packaging issues, and also face challenges to increase the performance and reliability of SiC devices. The focus of the paper is to discuss the role of SiC semiconductor based power electronics technology in PV energy conversion system. The comparisons and analysis of various PV inverter system prototypes imply that the application of SiC power semiconductor devices in a PV energy system can help eliminate several issues which are at present due to the material limitations of silicon. (C) 2015 Elsevier Ltd. All rights reserved.
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.
Authors: A. Majzoobi; R. P. Joshi; A. A. Neuber; J. C. Dickens
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7570265
Abstract: Particle-in-cell simulations are performed to analyze the role of secondary electron emission (SEE) on the efficiency, the output power and the leakage currents of 12-cavity, 12-cathode Rising-Sun magnetrons with diffraction output. The simulation results seem to indicate that the role of SEE would be fairly negligible. Small changes are predicted, linked to deviations in the starting trajectories of secondary electrons following their generation and the lower fraction of electrons in clusters with a synchronized rotational velocity. Overall, a peak power output of about 2.48 GW is predicted at a magnetic field of 0.45 T, with efficiencies as high as 75%. Furthermore, deviations in the output power with SEE are predicted to occur at shorter times, but would not be an issue for pulses greater than 25 ns in duration.
Authors: Pushpakaran, BN; Bayne, SB; Ogunniyi, AA
PDF: https://link.springer.com/article/10.1007/s10825-015-0766-1
Abstract: The integration of high power silicon carbide (SiC) Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) in today's power systems drives the demand for deeper understanding of the device switching characteristics by way of device simulation. Applications like motor drive require power MOSFETs to drive highly inductive loads which increase the switching power loss by extending the voltage and current crossover, a situation which gets exacerbated by the presence of parasitic inductance. A 2D model of a 1200 V 4H-SiC vertical DMOSFET half-cell was developed using a commercially available TCAD software package to investigate the electro-thermal switching characteristics using clamped inductive switching circuit for ON state drain current density values up to at an ambient lattice temperature of 300 K. Device physics-based models were included to account for carrier mobility, carrier generation and recombination, impact ionization and lattice heating. In order to analyze the areas of localized lattice heating, the lattice temperature distribution was monitored during simulation. The clamped inductive switching circuit simulations were performed with and without the addition of parasitic electrode inductance to observe the difference in switching energy loss.
Authors: L. Yang; M. He; V. Vittal; J. Zhang
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7152977
Abstract: In this paper, stochastic optimization of economic dispatch (ED) and interruptible load management is investigated using short-term distributional forecast of wind farm generation. Specifically, using the statistical information of wind farm generation extracted from historical data, a Markov chain-based distributional forecast model for wind farm generation is developed in a rigorous optimization framework, in which the diurnal nonstationarity and the seasonality of wind generation are accounted for by constructing multiple finite-state Markov chains for each epoch of 3 h and for each individual month. Based on this distributional forecast model, the joint optimization of ED and interruptible load management is cast as a stochastic optimization problem. Additionally, a robust ED is formulated using an uncertainty set constructed based on the proposed distributional forecast, aiming to minimize the system cost for worst cases. The proposed stochastic ED is compared with four other ED schemes: 1) the robust ED; 2) deterministic ED using the persistence wind generation forecast model; 3) scenario-based stochastic ED; and 4) deterministic ED, in which perfect wind generation forecasts are used. Numerical studies, using the IEEE Reliability Test System-1996 and realistic wind measurement data from an actual wind farm, demonstrate the significant benefits obtained by leveraging the Markov chain-based distributional forecast and the interruptible load management.
Authors: Veliadis, V; Steiner, B; Lawson, K; Bayne, SB; Urciuoli, D; Ha, HC
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7369297
Abstract: A requirement for the commercialization of power SiC transistors is their long term reliable operation under the hard-switching conditions and high temperatures encountered in the field. Normally ON 1200 V vertical-channel implanted-gate SiC junction field effect transistors (JFETs), designed for high-power bidirectional (four quadrant) solid-state-circuit-breaker (SSCB) applications, were repetitively pulse hard switched at 150 degrees C from a 1200 V blocking state to an ON-state current of 115 A, which is in excess of 13 times the JFET's 250-W/cm(2) rated current at 150 degrees C. The JFETs were fabricated in seven photolithographic levels with a single masked ion-implantation forming the p+ gates and guard rings, and with no epitaxial regrowth. The pulsed testing was performed using a low inductance RLC circuit. In this circuit, energy initially stored in a capacitor is discharged in a load resistor through the JFET under test. The JFET hard-switch stressing included over 2.4 million 1200 V/115-A hard-switch events at 150 degrees C and at a repetition rate of 10 Hz. The peak energies and powers dissipated by the JFET at each hard-switch event were 73.2 mJ and 68.2 kW, respectively. The current rise rate was 166 A/mu s and the pulse full width at half maximum (FWHM) was 1.8 mu s. After over 2.4 million hard-switch events at 150 degrees C, the JFET blocking voltage characteristics remained unchanged while the ON-state conduction slightly improved, which indicate reliable operation. An optically triggered SSCB, based on these rugged JFET, is proposed.
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.
Authors: Beeson, S; Dickens, J; Neuber, A
PDF: https://aip.scitation.org/doi/10.1063/1.4914043
Abstract: A 4-port S-band waveguide structure was designed and fabricated such that a signal of any amplitude (less than 1 MW) can be switched from a normally closed state, <0.5 dB insertion loss (IL), to an open state >30 dB IL by initiating plasma in a gas cell situated at the junction of this waveguide and one propagating a megawatt level magnetron pulse. The 90/10 switching time is as low as 20 ns with a delay of similar to 30 ns between the onset of the high power microwave pulse and the initial drop of the signal. Two ports of this device are for the high power triggering pulse while the other two ports are for the triggered signal in a Moreno-like coupler configuration. In order to maintain high isolation, these two sets of waveguides are rotated 90 degrees from each other with a TE111 resonator/plasma cell located at the intersection. This manuscript describes the design and optimization of this structure using COMSOL 4.4 at the design frequency of 2.85 GHz, comparison of simulated scattering parameters with measured cold tests (testing without plasma), and finally the temporal waveforms of this device being used to successfully switch a low power CW signal from 2W to <5 mW on a sub-microsecond timescale. (C) 2015 AIP Publishing LLC.
Authors: Tiskumara, R; Joshi, RP; Mauch, D; Dickens, JC; Neuber, AA
PDF: https://aip.scitation.org/doi/pdf/10.1063/1.4929809
Abstract: A model-based analysis of the steady-state, current-voltage response of semi-insulating 4H-SiC is carried out to probe the internal mechanisms, focusing on electric field driven effects. Relevant physical processes, such as multiple defects, repulsive potential barriers to electron trapping, band-to-trap impact ionization, and field-dependent detrapping, are comprehensively included. Results of our model match the available experimental data fairly well over orders of magnitude variation in the current density. A number of important parameters are also extracted in the process through comparisons with available data. Finally, based on our analysis, the possible presence of holes in the samples can be discounted up to applied fields as high as similar to 275 kV/cm. (C) 2015 AIP Publishing LLC.
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.
Authors: Joshi, RP; Qiu, H
PDF: https://aip.scitation.org/doi/10.1063/1.4929808
Abstract: Nanosecond, high-intensity electric pulses have been reported to open rectifying pores in biological cell membranes. The present goal is to qualitatively understand and analyze the experimental current-voltage (I-V) data. Here, nanopore transport is probed using a numerical method and on the basis of an analytical model. Our results show that geometric asymmetry in the nanopore would not yield asymmetry in the I-V characteristics. However, positive surface charge lining the pore could produce characteristics that compare well with data from patch-clamp measurements, and a value of similar to 0.02 C/m(2) is predicted from the numerical calculations. (C) 2015 AIP Publishing LLC.
Authors: Bilbao, AV; Schrock, JA; Ray, WB; Kelley, MD; Holt, SL; Giesselmann, MG; Bayne, SB
PDF: https://aip.scitation.org/doi/10.1063/1.4927822
Abstract: Obtaining accurate collector to emitter voltage measurements when characterizing high voltage silicon carbide (SiC) devices requires the ability to measure voltages in the range of zero to 10 V while the device is in the on-state and the ability to withstand ultra-high voltages while the device is in the off-state. This paper presents a specialized voltage probe capable of accurately measuring the aforementioned range. A comparison is made between the proposed probe and other commonly used high voltage probe alternatives in relation to high voltage SiC device testing. Testing of the probe was performed to ensure linearity, high accuracy, and high bandwidth. (C) 2015 AIP Publishing LLC.
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.
Authors: D. Mauch; C. Hettler; W. W. Sullivan; A. A. Neuber; J. Dickens
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7151899
Abstract: The power output, forward voltage, conversion efficiency, and spectral characteristics of a 365 nm ultraviolet light-emitting diode (LED) were measured for applications of triggering wide-bandgap photoconductive switches for pulsed power applications. Pulsed currents through the LED ranged from 125 mA up to 2.2 A at widths from 10 μs up to several seconds. Using time-resolved electroluminescence spectroscopy, peak emission was observed to occur at 368.5 nm for short pulses with a red-shift to 371.8 nm for pulses 8 s in duration. A peak light output of 4.1 W was measured for short pulses (<;50 μs) of 2.12 A, corresponding to six times the rated output specification. The LED was used to trigger a high-voltage photoconductive semiconductor switch (PCSS) at voltages up to 6 kV into a high-impedance load. The 365 nm LED is a promising candidate for optical triggering of PCSS devices.
Authors: D. Mauch; W. Sullivan; A. Bullick; A. Neuber; J. Dickens
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7108031
Abstract: Several generations of high power, lateral, linear mode, intrinsically triggered 4H-SiC photoconductive semiconductor switch designs and their performance are presented. These switches were fabricated from high purity semi-insulating 4H-SiC samples measuring 12.7 mm × 12.7 mm × 0.36 mm and were able to block dc electric fields up to 370 kV/cm with leakage currents less than 10 μA without failure. Switching voltages and current s up to 26 kV and 450 A were achieved with these devices and ON-state resistances of 2 Ω were achieved with 1 mJ of 355 nm laser energy (7 ns FWHM). After fewer than 100 high power switching cycles, these devices exhibited cracks near the metal/SiC interface. Experimental and simulation results investigating the root cause of this failure mechanism are also presented. These results strongly suggest that a transient spike in the magnitude of the electric field at the metal/SiC interface during both switch closing and opening is the dominant cause of the observed cracking.
Authors: J. A. Schrock; W. B. Ray II; K. Lawson; A. Bilbao; S. B. Bayne; S. L. Holt; L. Cheng; J. W. Palmour; C. Scozzie
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6897959
Abstract: For SiC DMOSFETs to obtain widespread usage in power electronics their long-term operational ability to handle the stressful transient current and high temperatures common in power electronics needs to be further verified. To determine the long-term reliability of a single 4H-SiC DMOSFET, the effects of extreme high current density were evaluated. The 4H-SiC DMOSFET has an active conducting area of 40 mm2, and is rated for 1200 V and 150 A. The device was electrically stressed by hards-witching transient currents in excess of four times the given rating (>600 A) corresponding to a current density of 1500 A/cm2. Periodically throughout testing, several device characteristics including RDS(on) and VG S(th) were measured. After 500 000 switching cycles, the device showed a 6.77% decrease in RDS (on), and only a 132-mV decreased in VG S(th). Additionally, the dc characteristics of the device were analyzed from 25 to 150 °C and revealed a 200-mV increase in on-state voltage drop at 20 A and a 2-V reduction in VG S(th) at 150 °C. These results show this SiC DMOSFET has robust long-term reliability in high-power applications that are susceptible to pulse over currents, such as pulsed power modulators and hard-switched power electronics.
Authors: Nimmagadda, S; Islam, A; Bayne, SB; Sanchez, J; Caballero, LG
Abstract: Several methods to improve the current wind turbine modeling techniques for power system studies are discussed in this paper. Some of the modeling improvements which are discussed are dynamic initialization of models, differences between wind turbine models developed in electromagnetic transient program and bulk power system simulation software, and techniques to avoid numerical instability with minimum errors. Algorithms to provide automatic initial conditions of all variables for any given load flow condition, wind speeds, and wind farm control strategies are developed in this paper. This approach is an improvement to the traditional method of providing constant initial values for given operating conditions. Several physical components such as harmonic filters, cable impedances, and complex control algorithms are either simplified or excluded in the power system models. The fault response of the power system model under extreme conditions leads to a numerically unstable condition. The instability might be a result of simplification discussed above and does not occur in the real turbine. Additional modeling techniques that can be used to achieve numerical stability, dynamic and fault response close to the actual wind turbine are discussed. These techniques are used to compensate for the simplifications made in power system models. Finally, the issue of nuisance tripping faced very frequently by the Independent System Operators, while working on the studies involving wind power plants, is addressed. Various methods to reduce the deviations and prevent nuisance tripping are implemented in the paper. (C) 2015 AIP Publishing LLC.
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.
Authors: Kota, S; Bayne, SB; Nimmagadda, S
PDF: https://www.sciencedirect.com/science/article/abs/pii/S1364032114007667?via%3Dihub
Abstract: Offshore wind is one of the most fascinating industries in the renewable energy sector and it is experiencing a remarkable growth. Offshore wind energy generation offers an opportunity in the race to decrease the dependence on fossil fuels, reduce green house emissions, increase energy security and create employment opportunities. UK has proven success in offshore wind and has been enjoying the economic benefits of offshore wind since over a decade. Offshore wind energy is an emergent renewable energy industry in the United States. The United States is coping up with the challenges and heading up fast to catch up with the industry. India is still in its infancy stage where the policy frameworks are framed by MNRE government and getting ready with the tools to enter into the offshore market. This paper researches the current situation and trend of offshore wind industries in UK and US, from aspects of policy, grid connections, operation and maintenance and cost reduction and analyses the proper direction and pathways of the industry to India. Therefore this paper highlights the scenario as to how these three countries UK, USA and India, respectively, are enabling offshore wind, to make a vital and sizeable contribution to the low carbon economy. (C) 2014 Elsevier Ltd. All rights reserved.
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.
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.
Authors: Subburaj, AS; Pushpakaran, BN; Bayne, SB
Abstract: Reliable source of energy is a topic of momentous concern in the world due to the uncertainty in conventional energy sources. The situation gets exacerbated with the impact of natural disasters on the transmission grid. The advent of cutting edge energy storage technology has provided a competent solution. Energy storage system is an integral part of a grid since it enhances the stability and performance by disengaging the energy generation source and the load especially when intermittent renewable energy sources are a part of the system. Battery technology should not only be able to demonstrate high performance but must be economically viable for project implementation. Several grid connected renewable energy based battery projects have been implemented for research and development as well as commercial application. The projects discussed in this review are considered based on the availability of information. This review paper will focus on grid connected battery projects powered by wind and solar energy generation sources. (C) 2015 Elsevier Ltd. All rights reserved.
Authors: Majzoobi, A; Joshi, RP; Neuber, AA; Dickens, JC
PDF: https://aip.scitation.org/doi/full/10.1063/1.4932634
Abstract: Particle-in-cell simulations are performed to analyze the efficiency, output power and leakage currents in a 12-Cavity, 12-Cathode rising-sun magnetron with diffraction output (MDO). The central goal is to conduct a parameter study of a rising-sun magnetron that comprehensively incorporates performance enhancing features such as transparent cathodes, axial extraction, the use of endcaps, and cathode extensions. Our optimum results demonstrate peak output power of about 2.1 GW, with efficiencies of similar to 70% and low leakage currents at a magnetic field of 0.45 Tesla, a 400 kV bias with a single endcap, for a range of cathode extensions between 3 and 6 centimeters. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
Authors: Ekinci, H; Kuryatkov, VV; Mauch, DL; Dickens, JC; Nikishin, SA
PDF: https://link.springer.com/content/pdf/10.1007/s11664-015-3658-z.pdf
Abstract: Photoconductive semiconductor switches (PCSS) fabricated on high-purity semi-insulating 4H-SiC substrates (000) are capable of switching high currents in compact packages with long device lifetimes. A heavily doped n-type SiC epitaxial layer of appropriate thickness is required to form low-resistance ohmic contacts with these devices. In addition, to enhance the performance of the PCSSs, the SiC surface between the ohmic contacts must be extremely smooth. We report a chlorine-based, inductively coupled plasma reactive ion-etching process yielding n-type SiC epitaxial layers with the required smoothness. The rate of etching and post-etching surface morphology were dependent on plasma conditions. We found that the surface smoothness of epitaxial layers can be improved by including BCl3 in the argon-chlorine mixture. The optimum etching process yielded very smooth surfaces (similar to 0.3 nm RMS) at a relatively high rate of etching of similar to 220 nm/min. This new fabrication approach significantly reduced the on-state resistance of the PCSS device and improved its durability of operation.
Authors: Millerd, PJ; Paden, RG; Lund, JT; Hara, AK; Stiles, WL; He, M; Wu, Q; Johnson, CD
PDF: https://link.springer.com/content/pdf/10.1007/s00261-014-0271-1.pdf
Abstract: To determine whether radiation doses during computed tomography (CT) colonography (CTC) can be further reduced while maintaining image quality using model-based iterative reconstruction (MBIR). Twenty patients underwent CTC at a standard dose in supine and prone positions and at a reduced dose in the supine position. All other scan parameters (except noise index) were held constant. Acquisitions were reconstructed using 3 algorithms: filtered back projection (FBP), adaptive statistical iterative reconstruction (ASIR), and MBIR. Noise was assessed quantitatively by comparing the SD in Hounsfield units at 5 standard locations. Qualitative assessment was made by 2 experienced radiologists blinded to technique who subjectively scored image quality, noise, and sharpness (from 0 to 4). The standard-dose and reduced-dose CT dose index/dose-length product were 6.7/328 and 2.7 mGy/129 mGy-cm, respectively (60 % reduction). Measured mean noise level increased from the standard to the reduced dose (FBP, from 58.6 to 97.2; ASIR from 35.8 to 60.6; and MBIR from 16.6 to 21.9). MBIR had significantly less noise than ASIR on 2-dimensional images at both standard and reduced doses (P < .01). Radiation dose in CTC using MBIR can be reduced by 60 % while maintaining image quality and reducing image noise.
Authors: K. H. Becker; B. B. Godfrey; E. E. Kunhardt; M. Laroussi; L. D. Ludeking; A. A. Neuber; E. Schamiloglu; A. J. Woods
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7047925
Abstract: This paper summarizes the Dr. Robert J. Barker memorial session at the IEEE International Conference on Plasma Science 2014. Each section summarizes progress in a plasma research area strongly influenced by Dr. Barker's leadership: 1) plasma medicine; 2) atmospheric-pressure plasmas; 3) high-power microwaves; 4) pulsed power; and 5) numerical simulation of plasmas. He had a profound influence on these and other plasma science applications, as well as on numerous individual researchers. He will be missed greatly.
Authors: Lin, S; Beeson, S; Liu, C; Dickens, J; Neuber, A
PDF: https://aip.scitation.org/doi/full/10.1063/1.4917471
Abstract: Self-induced gaseous plasma is evaluated as active opening switch medium for pulsed high power microwave radiation. The self-induced plasma switch is investigated for N-2 and Ar environments under pressure conditions ranging from 25 to 700Torr. A multi-pass TE111 resonator is used to significantly reduce the delay time inherently associated with plasma generation. The plasma forms under the pulsed excitation of a 4MW magnetron inside the central dielectric tube of the resonator, which isolates the inner atmospheric gas from the outer vacuum environment. The path from the power source to the load is designed such that the pulse passes through the plasma twice with a 35 ns delay between these two passes. In the first pass, initial plasma density is generated, while the second affects the transition to a highly reflective state with as much as 30 dB attenuation. Experimental data revealed that virtually zero delay time may be achieved for N-2 at 25 Torr. A two-dimensional fluid model was developed to study the plasma formation times for comparison with experimental data. The delay time predicted from this model agrees well with the experimental values in the lower pressure regime (error < 25%), however, due to filamentary plasma formation at higher pressures, simulated delay times may be underestimated by as much as 50%. (C) 2015 AIP Publishing LLC.
Authors: Nagulapally, D; Joshi, RP; Pradhan, A
PDF: https://aip.scitation.org/doi/pdf/10.1063/1.4905702
Abstract: The Inverse Piezoelectric Effect (IPE) is thought to contribute to possible device failure of GaN High Electron Mobility Transistors (HEMTs). Here we focus on a simulation study to probe the possible mitigation of the IPE by reducing the internal electric fields and related elastic energy through the use of high-k materials. Inclusion of a HfO2 cap layer above the AlGaN barrier particularly with a partial mesa structure is shown to have potential advantages. Simulations reveal even greater reductions in the internal electric fields by using field plates in concert with high-k oxides. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
Authors: Yang, L; He, M; Zhang, JS; Vittal, V
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7081774
Abstract: Wind ramps introduce significant uncertainty into wind power generation. Reliable system operation, however, requires accurate detection and forecast of wind ramps, especially at high penetration levels. In this paper, to deal with the wind ramp dynamics, a support vector machine (SVM)-enhanced Markov model is developed for short-term wind power forecast, based on one key observation from the measurement data that wind ramps often occur with specific patterns. Specifically, using the historical data of the wind turbine power outputs recorded at an actual wind farm, data analytics-based finite-state Markov models are first developed to model the normal fluctuations of wind generation, while taking into account the diurnal nonstationarity and the seasonality of wind generation. Next, the forecast by the SVM is integrated cohesively into the finite-state Markov models. Based on the SVM-enhanced Markov model, both short-term distributional forecasts and point forecasts are then derived. Numerical test results, using real wind generation data traces, demonstrate the significantly improved accuracy of the proposed forecast approach.
Authors: M. G. Giesselmann; T. T. Vollmer; W. J. Carey
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6828765
Abstract: This paper presents a 100-kV high frequency transformer/rectifier package, which is capable of a dual output polarity operation. An H-Bridge inverter drives the primary of the high voltage (HV) transformer at a frequency of 20 kHz. The inverter is driven by a Microchip dsPIC33F digital signal controller using peak current mode control with adaptive slope compensation. The HV-tank has two HV-coax output cables with a grounded shield on each cable. If the center conductor of the coax cable designated as negative output is grounded, positive voltage is obtained from the coax cable designated as positive output and vice versa. This paper provides design details and experimental results from tests of the entire system.
Authors: Andrew S. Fierro; James C. Dickens; Andreas A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6779603
Abstract: The development of a low-temperature plasma in a needle-protrusion to plane gap is investigated utilizing a 3-D particle-in-cell/Monte Carlo collision method implemented to run on single NVIDIA graphics processing unit. In addition to electron collisions, the model includes field detachment, photon tracking, and a drift-diffusion approximation for positive ions. The simulated geometry tracks several million electrons with 15-μm spatial resolution.
Authors: Laity, G; Fierro, A; Dickens, J; Frank, K; Neuber, A
PDF: https://aip.scitation.org/doi/pdf/10.1063/1.4869895
Abstract: We demonstrate a method for determining the dissociation degree of atmospheric pressure air discharges by measuring the self-absorption characteristics of vacuum ultraviolet radiation from O and N atoms in the plasma. The atom densities are determined by modeling the amount of radiation trapping present in the discharge, without the use of typical optical absorption diagnostic techniques which require external sources of probing radiation into the experiment. For an 8.0 mm spark discharge between needle electrodes at atmospheric pressure, typical peak O atom densities of 8.5 x 10(17) cm(-3) and peak N atom densities of 9.9 x 10(17) cm(-3) are observed within the first similar to 1.0 mm of plasma near the anode tip by analyzing the OI and NI transitions in the 130.0-132.0 nm band of the vacuum ultraviolet spectrum. (C) 2014 AIP Publishing LLC.
Authors: He, M; Yang, L; Zhang, JS; Vittal, V
PDF: https://ieeexplore.ieee.org/document/6727513
Abstract: In this paper, short-term forecast of wind farm generation is investigated by applying spatio-temporal analysis to extensive measurement data collected from a large wind farm where multiple classes of wind turbines are installed. Specifically, using the data of the wind turbines' power outputs recorded across two consecutive years, graph-learning based spatio-temporal analysis is carried out to characterize the statistical distribution and quantify the level crossing rate of the wind farm's aggregate power output. Built on these characterizations, finite-state Markov chains are constructed for each epoch of three hours and for each individual month, which accounts for the diurnal non-stationarity and the seasonality of wind farm generation. Short-term distributional forecasts and a point forecast are then derived by using the Markov chains and ramp trend information. The distributional forecast can be utilized to study stochastic unit commitment and economic dispatch problems via a Markovian approach. The developed Markov-chain-based distributional forecasts are compared with existing approaches based on high-order autoregressive models and Markov chains by uniform quantization, and the devised point forecasts are compared with persistence forecasts and high-order autoregressive model-based point forecasts. Numerical test results demonstrate the improved performance of the Markov chains developed by spatio-temporal analysis over existing approaches.
Authors: Nimmagadda, S; Islam, A; Bayne, SB; Walker, RP; Caballero, LG; Camanes, AF
PDF: https://www.sciencedirect.com/science/article/abs/pii/S1364032114003128
Abstract: Due to the increased penetration of renewable energy resources, there has been a lot of activity in the regional transmission organizations such as development of new standards, protocol revisions, new study requirements, changes to modeling procedures etc., in the last five years with a special focus given to wind energy. The key objective of this paper is to identify the impacts and the immediate technological and market related improvements required by the wind industry as a result of such changes in Southwest Power Pool (SPP) and the Electric Reliability Council of Texas (ERCOT). The paper documents the most important activities by following the higher-priority committees, work groups and task forces in both companies along with some of the special projects or initiatives such as sub-synchronous control interaction study, primary frequency response, hub concept and other modeling improvements related to wind energy. The paper provides an analysis of the impact of each change resulting in technology upgrades to wind turbines, modeling improvements by turbine manufacturers and policy/market changes affecting wind farm developers. Finally the paper provides recommendations regarding the requirements and capabilities which the future wind farms and wind turbines need to possess. (C) 2014 Elsevier Ltd. All rights reserved.
Authors: Bayne, S; Lacouture, S; Lawson, K; Giesselmann, M; Scozzie, CJ; O'Brien, H; Ogunniyi, AA
PDF: https://ieeexplore.ieee.org/document/6634860
Abstract: This paper describes the design and implementation of a small-scale pulsed power system specifically intended to evaluate the suitability of experimental silicon and silicon carbide high power Super Gate Turn Off thyristors for high action (500 A(2) s and above) pulsed power applications where energy is extracted from a storage element in a rapid and controlled manner. To this end, six of each type of device was placed in a controlled three phase rectifier circuit which was in turn connected to an aircraft ground power motor-generator set and subjected to testing protocols with varying power levels, while parameters such as offset firing angle were varied. (C) 2014 AIP Publishing LLC.
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.
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.
Authors: Ekinci, H; Kuryatkov, VV; Mauch, DL; Dickens, JC; Nikishin, SA
PDF: https://avs.scitation.org/doi/10.1116/1.4892172
Abstract: Inductively coupled plasma reactive ion etching (ICP-RIE) of n-type SiC epitaxial layers grown on (000 (1) over bar) 4H-SiC semi-insulating substrates has been investigated using chlorine-based plasma. The etch rate and postetching- surface morphology have been studied as functions of the plasma composition, ICP power, RTE power, and process pressure. The authors found that the surface smoothness of the epitaxial layer was increased by introducing BCl3 into Cl-2/Ar plasma. An optimized process has been developed yielding etch rates of similar to 220 nm/min and very smooth surfaces with root mean square roughness of similar to 0.3 nm. The spatial-frequency dependence of the one dimensional power spectral density was interpreted using the surface height function h(x) including a low-frequency range, which exhibits saturation and a high-frequency range, Which exhibits scaling properties. Through this etching process, the effects of s-ubcontact doping on 4H-SiC photoconductive semiconductor switch (PCSS) performance were investigated. A PCSS was fabricated using this etching process with a 1 mu m heavily doped (1.6 x 10(18) cm(-3) n-type) epitaxial layer beneath the device contacts and compared with a PCSS fabricated with a subcontact doped layer created through laser enhanced diffusion (similar to 50 nm depth, 2.0 x 10(18) cm(-3) n-type). The PCSS with the epitaxial layer demonstrated on average a 30% reduction in minimum on-state resistance, and eliminated cracking of the bulk material when switching currents <= 38 A. (C) 2014 American Vacuum Society.
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.
Authors: B. N. Pushpakaran; M. Hinojosa; S. B. Bayne; V. Veliadis; D. Urciuoli; N. El-Hinnawy; P. Borodulin; S. Gupta; C. Scozzie
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6774980
Abstract: Silicon carbide (SiC) depletion mode junction field-effect transistors (JFETs) are well suited for pulsed power applications as an opening switch due to their normally ON (N-ON) nature. To assess the robustness and breakdown energy tolerance of JFETs under pulsed conditions, they must be evaluated for breakdown energy capability before failure. This is very important for circuit breaker applications due to the large voltage spikes induced during the opening of the circuit breaker while it still conducts substantial load current. These voltage spikes can drive the JFET into the breakdown voltage regime and may result in device failure if the energy dissipation is above the tolerance limit. To determine the maximum avalanche energy of the device under repetitive pulsed conditions, a N-ON SiC JFET with a nominal rating of 1200 V/13 A was driven into punchthrough breakdown using an unclamped inductive switching (UIS) circuit. The testing comprised of 4000 repetitive pulses at 25°C case temperature at a fixed gate voltage of -20 V. The drain current was increased after every 1000 pulses to increase the energy dissipated. The JFET was able to withstand 1000 pulses at a maximum energy dissipation value of 1160 mJ before failure. The JFET triode breakdown characteristics were analyzed after every 1000 pulses. The peak UIS energy of 1160 mJ corresponded to an energy density of 16.6 J/cm2 based on their active area.
Authors: Pan, X; Ren, GF; Hoque, MNF; Bayne, S; Zhu, K; Fan, ZY
PDF: https://onlinelibrary.wiley.com/doi/10.1002/admi.201400398
Abstract: Transition metal oxides (TMOs), with their very large pseudocapacitance effect, hold promise for next generation high-energy-density electrochemical supercapacitors (ECs). However, the typical high resistivity of TMOs restricts the reported ECs to work at a low charge-discharge (C-D) rate of 0.1-1 V s(-1). Here, a novel vanadium oxides core/shell nanostructure-based electrode to overcome the resistivity challenge of TMOs for rapid pseudocapacitive EC design is reported. Quasi-metallic V2O3 nanocores are dispersed on graphene sheets for electrical connection of the whole structure, while a naturally formed amorphous VO2 and V2O5 (called as VOx here) thin shell around V2O3 nanocore acts as the active pseudocapacitive material. With such a graphene-bridged V2O3/VOx core-shell composite as electrode material, ECs with a C-D rate as high as 50 V s(-1) is demonstrated. This high rate was attributed to the largely enhanced conductivity of this unique structure and a possibly facile redox mechanism. Such an EC can provide 1000 kW kg(-1) power density at an energy density of 10 Wh kg(-1). At the critical 45 degrees phase angle, these ECs have a measured frequency of 114 Hz. All these indicate the graphene-bridged V2O3/VOx core-shell structure is promising for fast EC development.
Authors: Sterling R. Beeson; James C. Dickens; Andreas A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6923463
Abstract: A global model of high-power microwave (HPM) window breakdown is elucidated. The model provides a practical approach for estimating the maximum microwave power and pulse length that can be transmitted for a given window geometry at varying background gas pressure. Based on recent experimental and modeling progress, the formative and statistical breakdown delay time contributions are included in the model. The provided details are intended to give the reader a starting point in designing an HPM system for which surface breakdown along the output window is a major concern. Spanning five orders of magnitude in power, four microwave bands, three orders of magnitude in pulsewidth, three orders of magnitude in pressure, and three different gas types, the model serves to determine the probability of breakdown for a given set of input parameters with the modest computational effort. Examples of how to use the model are given, and the results are compared with actual systems and measured experimental delay times.
Authors: Fierro, A; Dickens, J; Neuber, A
PDF: https://aip.scitation.org/doi/10.1063/1.4903330
Abstract: A 3-dimensional particle-in-cell/Monte Carlo collision simulation that is fully implemented on a graphics processing unit (GPU) is described and used to determine low-temperature plasma characteristics at high reduced electric field, E/n, in nitrogen gas. Details of implementation on the GPU using the NVIDIA Compute Unified Device Architecture framework are discussed with respect to efficient code execution. The software is capable of tracking around 10 x 10(6) particles with dynamic weighting and a total mesh size larger than 10(8) cells. Verification of the simulation is performed by comparing the electron energy distribution function and plasma transport parameters to known Boltzmann Equation (BE) solvers. Under the assumption of a uniform electric field and neglecting the build-up of positive ion space charge, the simulation agrees well with the BE solvers. The model is utilized to calculate plasma characteristics of a pulsed, parallel plate discharge. A photoionization model provides the simulation with additional electrons after the initial seeded electron density has drifted towards the anode. Comparison of the performance benefits between the GPU-implementation versus a CPU-implementation is considered, and a speed-up factor of 13 for a 3D relaxation Poisson solver is obtained. Furthermore, a factor 60 speed-up is realized for parallelization of the electron processes. (C) 2014 AIP Publishing LLC.
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.
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.
Authors: Ren, GF; Pan, X; Bayne, S; Fan, ZY
PDF: https://www.sciencedirect.com/science/article/abs/pii/S0008622314000396
Abstract: Ultrafast electrochemical supercapacitors (EC) that can work at or above kilohertz (kHz) frequency, 3-4 orders higher than traditional EC, call for a structure,with extremely low equivalent serial resistance (ESR) and a reasonably large surface area. Three-dimensional perpendicularly-oriented graphene (POG) network, grown inside of Ni foam (NF) by microwave plasma chemical vapor deposition, is reported as electrode to fabricate such ultrafast EC. The folded POG inside NF provides a large surface area, while the straight-forward and wide-open porous structure of POG ensures fast ion migration. In conjunction with the intrinsic high electronic conductivity of graphene and Ni, POG/NF electrode based ultrafast EC was demonstrated with a specific cell capacitance of 0.32 mF/cm(2) at 1 kHz, a relaxation time constant of 0.248 ms, and an ESR of 70 m Omega. A charge-discharge rate as high as 500 V/s was also measured, at which the cyclic voltammogram maintained a rectangular shape, corresponding to a single electrode capacitance of 0.83 mF/cm(2). (C) 2014 Elsevier Ltd. All rights reserved.
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.
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.
Authors: K. Lawson; S. B. Bayne; S. Lacouture; L. Cheng; H. O’Brien; A. Ogunniyi; C. Scozzie
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6844851
Abstract: One of the major requirements for adoption of new silicon carbide (SiC) super gate turn-off thyristors (SGTOs) into high-energy applications is to verify the safe operating area and long-term reliability capabilities of these devices. In this letter, we have developed a unique high-energy testing system that can evaluate the performance limitations with respect to lifetime capabilities of the 9 kV, 1 cm $^{2}$ , SGTOs at ultrahigh pulsed current levels from 1 to 3.5 kA. The test system produces square current pulses with a user specified current amplitude and a 100- $\mu $ s pulsewidth at a maximum repetition rate of 1 shot/s (>0.1% duty cycle). A lifetime safe operating area with respect to maximum pulsed current was then established that these 1 cm $^{2}$ , 9 kV, SiC SGTOs can perform reliably without significant degradation at pulsed current levels up to 2.0 kA. At current levels above 2.0 kA shifts in the on-state voltage are observed probably due to device over-heating at such high current levels and having not enough time to fully dissipate the heat between any two shots, which results in the device rapidly deteriorating due to increased on-state losses ultimately leading to premature failure.
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.
Authors: Ryberg, D; Fierro, A; Dickens, J; Neuber, A
PDF: https://aip.scitation.org/doi/pdf/10.1063/1.4897295
Abstract: A system for time-discretized spectroscopic measurements of the vacuum ultraviolet (VUV) emission from spark discharges in the 60-160 nm range has been developed for the study of early plasma-forming phenomena. The system induces a spark discharge in an environment close to atmospheric conditions created using a high speed puff value, but is otherwise kept at high vacuum to allow for the propagation of VUV light. Using a vertical slit placed 1.5 mm from the discharge the emission from a small cross section of the discharge is allowed to pass into the selection chamber consisting of a spherical grating, with 1200 grooves/mm, and an exit slit set to 100 mu m. Following the exit slit is a photomultiplier tube with a sodium salicylate scintillator that is used for the time discretized measurement of the VUV signal with a temporal resolution limit of 10 ns. Results from discharges studied in dry air, Nitrogen, SF6, and Argon indicate the emission of light with wavelengths shorter than 120 nm where the photon energy begins to approach the regime of direct photoionization. (C) 2014 AIP Publishing LLC.
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.
Authors: Sullivan, WW; Mauch, D; Bullick, A; Hettler, C; Neuber, A; Dickens, J
PDF: https://aip.scitation.org/doi/10.1063/1.4794734
Abstract: This paper discusses a compact high voltage curve tracer for high voltage semiconductor device characterization. The system sources up to 3 mA at up to 45 kV in dc conditions. It measures from 328 V to 60 kV with 15 V resolution and from 9.4 pA to 4 mA with 100 fA minimum resolution. Control software for the system is written in Microsoft Visual C# and features real-time measurement control and IV plotting, arc-protection and detection, an electrically isolated universal serial bus interface, and easy data exporting capabilities. The system has survived numerous catastrophic high voltage device-under-test arcing failures with no loss of measurement capability or system damage. Overall sweep times are typically under 2 min, and the curve tracer system was used to characterize the blocking performance of high voltage ceramic capacitors, high voltage silicon carbide photoconductive semiconductor switches, and high voltage coaxial cable. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794734]
Authors: He, M; Murugesan, S; Zhang, JS
PDF: https://ieeexplore.ieee.org/document/6461494
Abstract: In this study, we focus on the stochastic reliability of smart grids with two classes of energy users-traditional energy users and opportunistic energy users (e. g., smart appliances or electric vehicles), and investigate the procurement of energy supply from both conventional generation (base-load and fast-start) and wind generation via multi-timescale scheduling. Specifically, in day-ahead scheduling, with the distributional information of wind generation and demand, we characterize the optimal procurement of the energy supply from base-load generation and the day-ahead price; in real-time scheduling, with the realizations of wind generation and the demand of traditional energy users, we optimize real-time price to manage opportunistic demand so as to achieve system-wise reliability and efficiency. More specifically, we consider two different models for opportunistic energy users: non-persistent and persistent, and characterize the optimal scheduling and pricing decisions for both models by exploiting various computational and optimization tools. Numerical results demonstrate that the proposed scheduling and pricing schemes can effectively manage opportunistic demand and enhance system reliability, thus have the potential to improve the penetration of wind generation.
Authors: Bragg, JWB; Sullivan, WW; Mauch, D; Neuber, AA; Dickens, JC
PDF: https://aip.scitation.org/doi/10.1063/1.4804196
Abstract: An all solid-state, megawatt-class high power microwave system featuring a silicon carbide (SiC) photoconductive semiconductor switch (PCSS) and a ferrimagnetic-based, coaxial nonlinear transmission line (NLTL) is presented. A 1.62 cm(2), 50 kV 4H-SiC PCSS is hard-switched to produce electrical pulses with 7 ns full width-half max (FWHM) pulse widths at 2 ns risetimes in single shot and burst-mode operation. The PCSS resistance drops to sub-ohm when illuminated with approximately 3 mJ of laser energy at 355 nm (tripled Nd:YAG) in a single pulse. Utilizing a fiber optic based optical delivery system, a laser pulse train of four 7 ns (FWHM) signals was generated at 65 MHz repetition frequency. The resulting electrical pulse train from the PCSS closely follows the optical input and is utilized to feed the NLTL generating microwave pulses with a base microwave-frequency of about 2.1 GHz at 65 MHz pulse repetition frequency (prf). Under typical experimental conditions, the NLTL produces sharpened output risetimes of 120 ps and microwave oscillations at 2-4 GHz that are generated due to damped gyromagnetic precession of the ferrimagnetic material's axially pre-biased magnetic moments. The complete system is discussed in detail with its output matched into 50 Omega, and results covering MHz-prf in burst-mode operation as well as frequency agility in single shot operation are discussed. (C) 2013 AIP Publishing LLC.
Authors: Lacouture, S; Lawson, K; Bayne, S; Giesselmann, M; Scozzie, CJ; O'Brien, H; Ogunniyi, AA
PDF: https://aip.scitation.org/doi/10.1063/1.4823525
Abstract: A high energy, modular, completely automated test bed with integrated data acquisition and characterization systems was successfully designed in order to perform both safe operating area as well as very high volume reliability testing on experimental silicon carbide Super Gate Turn Off (SGTO) thyristors. Although the system follows a modular design philosophy, with each functional block acting as a peripheral to a main control module and can be adapted to arbitrary power and pulse width levels, for the specific SGTO devices initially evaluated it was configured to have the device discharge variable current levels of up to 6 kA into a 0.5 Omega resistive load with a relatively square pulse fixed at 100 mu s full width at half maximum delivering energy levels up to 1.8 kJ to the load. (C) 2013 AIP Publishing LLC.
Authors: J. -. B. Bragg; J. C. Dickens; A. A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6359866
Abstract: Ferrimagnetic nonlinear transmission lines (NLTLs) have the potential to fill a high-power microwave niche where compact cost-effective sources are lacking. NLTLs utilize nonlinear ferrimagnetic properties and magnetization dynamics to provide ultrafast pulse rise times (100 ps or less) and microwave signals with peak power ranging from kilowatts to hundreds of megawatts. The frequency of operation has been shown to range from 900 MHz up to 5 GHz depending on geometry and external magnetic fields. NLTLs, theoretically, can be pulsed to tens of kilohertz with little to no variance in microwave signal between shots. This paper covers recent advances in ferrimagnetic-based NLTLs, specifically effects of applied and bias magnetic fields on peak power and frequency, as well as temperature dependence.
Authors: Lynn, CF; Dickens, JC; Neuber, AA
PDF: https://aip.scitation.org/doi/10.1063/1.4826209
Abstract: Virtual cathode oscillators, or vircators, are a type of high power microwave device which operates based on the instability of a virtual cathode, or cloud of electrons, which forms when electron current injected into the drift tube exceeds the space charge limited current within the drift tube. Anode heating by the electron beam during vircator operation ultimately limits achievable pulse lengths, repetition rates, and the duration of burst mode operation. This article discusses a novel cathode design that focuses electrons through holes in the anode, thus significantly reducing anode heating by the electrons emitted from the cathode during the first transit through the A-K gap. Reflexing electrons continue to deposit energy on the anode; however, the discussed minimization of anode heating by main beam electrons has the potential to enable higher repetition rates as well as efficiency and longer diode lifetime. A simulation study of this type of cathode design illustrates possible advantages. (C) 2013 AIP Publishing LLC.
Authors: B. N. Pushpakaran; M. Hinojosa; S. B. Bayne; V. Veliadis; D. Urciuoli; N. El-Hinnawy; P. Borodulin; S. Gupta; C. Scozzie
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6472803
Abstract: Silicon carbide (SiC) unipolar transistors are an efficient choice in the design of high temperature 1200 V switching power supplies and dc-dc converters. To reduce the form factor and increase the power density of the circuit, the switching frequency must be high. This intensifies the negative impact of parasitic inductance and results in high voltage spikes that can drive a switching device into breakdown, followed by rapid destruction. To study the device performance under unclamped inductive switching (UIS) conditions, a normally-ON 1200 V/13-A SiC junction field-effect transistor (JFET) is driven into punch through breakdown using a single pulse. The testing is performed using an UIS setup, in which energy initially stored in an inductor is discharged through the JFET. The testing comprises of 90 single pulses each at 25°C and 100 °C case temperatures for different gate voltages and drain current values. The peak energy and power dissipated in the JFET are 621 mJ and 16 kW, respectively, at the rated 1200 V blocking voltage and 13-A drain current. The JFET triode breakdown characteristics are unchanged after 180 single-pulse switching events indicating the robust nature of the device under extreme breakdown conditions. In addition, the 621 mJ peak UIS energy and its corresponding 8871 mJ/cm2 density dissipated in the JFET are the highest reported for any SiC power device.
Authors: Bragg, JWB; Dickens, JC; Neuber, AA
PDF: https://aip.scitation.org/doi/abs/10.1063/1.4792214?journalCode=jap
Abstract: The growing need for solid-state high power microwave sources has renewed interest in nonlinear transmission lines (NLTLs). This article focuses specifically on ferrimagnetic-based NLTLs in a coaxial geometry. Achieved peak powers exceed 30 MW at 30 kV incident voltage with rf power reaching 4.8 MW peak and pulse lengths ranging from 1-5 ns. The presented NLTL operates in S-band with the capability to tune the center frequency of oscillation over the entire 2-4 GHz band and bandwidths of approximately 30%, placing the NLTL into the ultra-wideband-mesoband category of microwave sources. Several nonlinear materials were tested and the relationship between NLTL performance and material parameters is discussed. In particular, the importance of the material's ferromagnetic resonance linewidth and its relationship to microwave generation is highlighted. For a specific nonlinear material, it is shown that an optimum relation between incident pulse magnitude and static bias magnitude exists. By varying the nonlinear material's bias magnetic field, active delay control was demonstrated. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4792214]
Authors: He, M; Vittal, V; Zhang, JS
PDF: https://ieeexplore.ieee.org/document/6475215
Abstract: A data mining approach using ensemble decision trees (DTs) learning is proposed for online dynamic security assessment (DSA), with the objective of mitigating the impact of possibly missing PMU data. Specifically, multiple small DTs are first trained offline using a random subspace method. In particular, the developed random subspace method exploits the hierarchy of wide-area monitoring system (WAMS), the locational information of attributes, and the availability of PMU measurements, so as to improve the overall robustness of the ensemble to missing data. Then, the performance of the trained small DTs is re-checked by using new cases in near real-time. In online DSA, viable small DTs are identified in case of missing PMU data, and a boosting algorithm is employed to quantify the voting weights of viable small DTs. The security classification decision for online DSA is obtained via a weighted voting of viable small DTs. A case study using the IEEE 39-bus system demonstrates the effectiveness of the proposed approach.
Authors: Beeson, S; Dickens, J; Neuber, A
PDF: https://aip.scitation.org/doi/abs/10.1063/1.4822343?journalCode=php
Abstract: Microwave transmission and reflection characteristics of pulsed radio frequency field generated plasmas are elucidated for air, N-2, and He environments under pressure conditions ranging from 10 to 600 torr. The pulsed, low temperature plasma is generated along the atmospheric side of the dielectric boundary between the source (under vacuum) and the radiating environment with a thickness on the order of 5 mm and a cross sectional area just smaller than that of the waveguide. Utilizing custom multi-standard waveguide couplers and a continuous low power probing source, the scattering parameters were measured before, during, and after the high power microwave pulse with emphasis on the latter. From these scattering parameters, temporal electron density estimations (specifically the longitudinal integral of the density) were calculated using a 1D plane wave-excited model for analysis of the relaxation processes associated. These relaxation characteristics ultimately determine the maximum repetition rate for many pulsed electric field applications and thus are applicable to a much larger scope in the plasma community than just those related to high power microwaves. This manuscript discusses the diagnostic setup for acquiring the power measurements along with a detailed description of the kinematic and chemical behavior of the plasma as it decays down to its undisturbed state under various gas type and pressure conditions. (C) 2013 AIP Publishing LLC.
Authors: Veliadis, V; Steiner, B; Lawson, K; Bayne, SB; Urciuoli, D; Ha, HC; El-Hinnawy, N; Gupta, S; Borodulin, P; Howell, RS; Scozzie, C
PDF: https://ieeexplore.ieee.org/document/6459530
Abstract: A requirement for the commercialization of power SiC transistors is their long-term reliable operation under hard switching conditions and high temperatures encountered in the field. Normally ON 1200-V vertical-channel implanted-gate SiC JFETs, designed for high-power bidirectional (four-quadrant) solid-state circuit breaker applications, were repetitively pulsed hard switched at 150 degrees C from a 1200-V blocking state to an ON-state current of 115 A, which is in excess of 13 times the JFET's 250-W/cm(2) rated current at 150 degrees C. The JFETs were fabricated in seven photolithographic levels with a single masked ion implantation forming the p(+) gates and guard rings and with no epitaxial regrowth. The pulsed testing was performed using a low-inductance RLC circuit. In this circuit, the energy initially stored in a capacitor is discharged in a load resistor through the JFET under test. The JFET hard switch stressing included over 2.4 million 1200-V/115-A hard switch events at 150 degrees C and at a repetition rate of 10 Hz. The peak energies and powers dissipated by the JFET at each hard switch event were 73.2 mJ and 68.2 kW, respectively. The current rise rate was 166 A/mu s, and the pulse FWHM was 1.8 mu s. After over 2.4 million hard switch events at 150 degrees C, the JFET blocking voltage characteristics remained unchanged while the ON-state current conduction slightly improved, which indicate reliable operation.
Authors: He, M; Zhang, JS; Vittal, V
PDF: https://ieeexplore.ieee.org/document/6547746
Abstract: Online dynamic security assessment (DSA) is examined in a data-mining framework by taking into account the operating condition (OC) variations and possible topology changes of power systems during the operating horizon. Specifically, a robust scheme is proposed based on adaptive ensemble decision tree (DT) learning. In offline training, a boosting algorithm is employed to build a classification model as a weighted voting of multiple unpruned small-height DTs. Then, the small-height DTs are periodically updated by incorporating new training cases that account for OC variations or the possible changes of system topology; the voting weights of the small-height DTs are also updated accordingly. In online DSA, the updated classification model is used to map the real-time measurements of the present OC to security classification decisions. The proposed scheme is first illustrated on the IEEE 39-bus test system, and then applied to a regional grid of the Western Electricity Coordinating Council (WECC) system. The results of case studies, using a variety of realized OCs, illustrate the effectiveness of the proposed scheme in dealing with OC variation and system topology change.
Authors: Laity, G; Fierro, A; Dickens, J; Neuber, A; Frank, K
PDF: https://aip.scitation.org/doi/10.1063/1.4804369
Abstract: We demonstrate a method for determining the dissociation density of N and H atoms present in a developing low temperature plasma, based on the emission and self-absorption of vacuum ultraviolet radiation produced from the plasma. Spark plasmas are produced via pulsed discharge in N-2/H-2 mixtures at atmospheric pressure, where information on the dissociated densities of the constituent gas molecules is desired without employing invasive diagnostic techniques. By analyzing the self-absorption line profile of 121.5 nm Lyman-alpha H radiation emitted within the first similar to 1.0 mm of plasma near the anode tip, a peak dissociated H atom concentration of 5.6 x 10(17) cm(-3) was observed similar to 100 ns into spark formation, with an estimated electron density of 2.65 x 10(18) cm(-3) determined from Stark broadening. Similarly, simultaneous line fitting of the N 120.0/124.3 nm emission profiles revealed a peak dissociated N atom concentration of 3.8 x 10(17) cm(-3) during the same discharge period. (C) 2013 AIP Publishing LLC.
Authors: Elsayed, MA; Neuber, AA; Dickens, JC; Walter, JW; Kristiansen, M; Altgilbers, LL
PDF: https://aip.scitation.org/doi/10.1063/1.3681443
Abstract: The increased popularity of high power microwave systems and the various sources to drive them is the motivation behind the work to be presented. A stand-alone, self-contained explosively driven high power microwave pulsed power system has been designed, built, and tested at Texas Tech University's Center for Pulsed Power and Power Electronics. The system integrates four different sub-units that are composed of a battery driven prime power source utilizing capacitive energy storage, a dual stage helical flux compression generator as the main energy amplification device, an integrated power conditioning system with inductive energy storage including a fast opening electro-explosive switch, and a triode reflex geometry virtual cathode oscillator as the microwave radiating source. This system has displayed a measured electrical source power level of over 5 GW and peak radiated microwaves of about 200 MW. It is contained within a 15 cm diameter housing and measures 2 m in length, giving a housing volume of slightly less than 39 1. The system and its sub-components have been extensively studied, both as integrated and individual units, to further expand on components behavior and operation physics. This report will serve as a detailed design overview of each of the four subcomponents and provide detailed analysis of the overall system performance and benchmarks. (C) 2012 American Institute of Physics. [doi:10.1063/1.3681443]
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]
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]
Authors: Lawson, K; Alvarez, G; Bayne, SB; Veliadis, V; Ha, HC; Urciuoli, D; El-Hinnawy, N; Borodulin, P; Scozzie, C
PDF: https://ieeexplore.ieee.org/document/6087998
Abstract: A requirement for the commercialization of power SiC transistors is their long-term reliable operation under the hard-switching conditions encountered in the field. Normally ON 1200-V vertical-channel implanted-gate SiC JFETs, designed for high-power bidirectional (four-quadrant) solid-state-circuit-breaker applications, were repetitively hard switched from a 150-V blocking state to an ON-state current in excess of eight times the JFET's 250-W/cm(2) rated current. The JFETs were fabricated in seven photolithographic levels with a single masked ion implantation forming the p(+) gates and guard rings and no epitaxial regrowth. The hard-switch testing was performed using an RLC circuit capable of currents in excess of 200 A with a rise time of 150 A/mu s. In this circuit, energy initially stored in the capacitor is discharged to the resistor through the JFET under test. The JFET hard-switch stressing included 1000 shots at each temperature of 25 degrees C, 50 degrees C, 100 degrees C, and 150 degrees C and at each repetition rate of 1, 5, 10, and 100 Hz for a total of 16 000 shots. Peak energies and powers dissipated by the JFET were 7.5 mJ and 9 kW, respectively. JFET conduction and blocking-voltage characteristics remain unchanged after 16 000 pulsed hard-switching events, which is indicative of reliable operation and excellent JFET suitability for nondegrading repeated bidirectional high surge-current fault isolation.
Authors: J. -. B. Bragg; J. Dickens; A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6148287
Abstract: In pulsed power systems, coaxial based nonlinear transmission lines (NLTLs) loaded with ferrimagnetic materials act as pulse sharpeners or high power microwave sources. Microwave generation comes by way of nonlinearities present in the ferrimagnetic material as well as excitation of damped gyromagnetic precession at large incident power levels. Ferrimagnetic properties highly depend on operating temperature; therefore, there exists a need to understand operational performance of ferrite loaded NLTLs under different temperature environments. Ferrite samples are chilled or heated to temperatures between -20°C to 150 °C, providing a wide range of possible operating temperatures. The Curie temperature of the tested samples is approximately 120 °C; therefore, this study allows observation of precession performance in possible operating temperatures as well as a brief look at the consequences of exceeding the Curie temperature. The design, testing, and results for an NLTL measuring 0.3 m in length with ferrite inner and outer diameters of 3 mm and 6 mm, respectively, are detailed. Results reveal precessional performance, both peak power and frequency of oscillations, versus temperature.
Authors: Curtis F. Lynn; Andreas A. Neuber; John W. Walter; James C. Dickens; Magne Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6329448
Abstract: Many high-power electron devices utilize cold-cathode diodes to generate intense electron beams. These cold cathodes have the advantage of being capable of supplying several kiloamperes of current spread over a large cross section without the need for auxiliary components such as a heater supply. However, they suffer from many known problems such as nonuniform emission that can result in small areas of high current density on the anode and, thus, excessive anode heating. As a consequence, outgassing and vaporization of bulk material frequently leads to premature impedance collapse. Hence, minimizing nonuniform anode heating due to beam nonuniformity is paramount. As previously demonstrated, the use of a CsI-coated carbon velvet cathode improved beam uniformity, reduced outgassing, and mitigated early impedance collapse. To quantify the uniformity, temporal and spatially resolved images of the cathode plasma were taken for a CsI-coated carbon fiber cathode, operated at an average current density of ~150 A/cm2 under various conditions, i.e., without a field shaping ring, before and after discharge cleaning, and with a field shaping ring. All cathodes were operated in a sealed tube with a small integrated sputter ion pump to restore vacuum levels to 10-9 torr levels between subsequent shots.
Authors: John W. Walter; Curtis F. Lynn; James C. Dickens; Magne Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6179553
Abstract: The high-power-microwave (HPM) sources currently under development typically require constant pumping to maintain the high vacuum levels required for operation. This pumping is often done with either a cryo- or turbopumping system, either of which would be difficult to deploy in a compact portable system. A compact sealed-tube virtual cathode oscillator (vircator) source has been developed at Texas Tech University (TTU) that does not require a bulky external vacuum pump for operation. This device has a base vacuum pressure in the low range compared to the majority of laboratory HPM sources having vacuum levels in the - range. The reduced amount of trapped gasses in the sealed-tube ultrahigh-vacuum environment has the potential to greatly impact device performance. The TTU sealed-tube vircator is useful as a testbed for studying HPM source optimization under UHV conditions. Measured operational characteristics of the tube utilizing a carbon fiber cathode and a nickel anode are presented, along with radiated microwave measurements.
Authors: D. V. Reale; J. Mankowksi; S. Holt; J. Walter; J. Dickens; N. R. Gonsalves
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6179550
Abstract: Mobile pulsed ring down arrays are of great interest due to their ability to deliver high peak power electromagnetic pulses. Global positioning system (GPS) devices are used as a means to provide position information to individual array elements. An array of solid state radiating structures was built and tested. The array test results are compared to a Monte Carlo array simulation that take into account source jitter, timing error, and distance error between array elements due to GPS measurements.
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.
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.
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.
Authors: C. James; C. Hettler; J. Dickens
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5643138
Abstract: A high-power vertical photoconductive switch was fabricated from a high-purity semi-insulating 4H-SiC wafer. The device was fabricated from an as-grown wafer with resistivity >; 109 Ω · cm and had a dark resistance of greater than 6 × 109 Ω. The switch was operated at 15 kV/cm and achieved a peak photocurrent of 14 A into a 25-Ω load. Optimization of the excitation wavelength and switch geometry using an optical parametric oscillator was studied in order to decrease the laser requirements for optical triggering. This has led to a decrease in ON-state resistance of almost two orders of magnitude for similar excitation energy levels at visible wavelengths. This work forms the basis for developing very compact high-voltage photoconductive switches.
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.
Authors: S. R. Beeson; P. J. Ford; J. Foster; H. G. Krompholz; A. A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5772013
Abstract: Open-shutter and intensified charge-coupled device images of high-power microwave breakdown were taken in an effort to characterize the pressure dependence of plasma development. These images were taken with a Nikon D200 and Andor iStar DH734-25U-03, respectively. With the pressures increasing from 200 mtorr to 155 torr, the plasma changes from a diffuse discharge encompassing a large volume to a multichannel structure following the electric field lines.
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.
Authors: G. Laity; A. Neuber; A. Fierro; J. Dickens; L. Hatfield
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5976080
Abstract: There is a growing demand for understanding the physics of surface flashover, as it relates to the breakdown of electric fields on high power systems in the aerospace community. Specifically, the quantitative role of vacuum ultraviolet (VUV) radiation which is self-produced during the initial nanoseconds of surface flashover is virtually unknown. An experiment was constructed which allows detailed electrical and optical measurements of VUV emission during the timescales in which streamers are propagating before the transition into spark discharge. Repeated surface flashover events are generated using a solid-state high voltage pulser, with breakdown recorded in a number of gases at atmospheric pressure. Streamers are photographed using fast optical imaging with 3 ns resolution. Fast voltage and current diagnostics revealed a number of distinct stages of streamer development ranging from the onset of cathode directed streamers to the sharp current rise during final voltage collapse. The emission of VUV radiation is discussed in context to the observed streamer and electrical characteristics.
Authors: J. Foster; S. Beeson; M. Thomas; J. Krile; H. Krompholz; A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5976082
Abstract: High power microwave (HPM) dielectric surface flashover can be rapidly induced by providing breakdown initiating electrons in the high field region. An experimental setup utilizing a 2.85 GHz HPM source to produce a 4.5 MW, 3 μs pulse is used for studying HPM surface flashover in various atmospheric conditions. If flashover is to occur rapidly in an HPM system, it is desirable to provide a readily available source of electrons while keeping insertion loss at a minimum. The experimental results presented in this paper utilize a continuous UV source (up to 0.3 mW/cm2) to provide photo-emitted seed electrons from the dielectric surface. Similarly, electrons were provided through the process of field emission by using metallic points deposited on the surface. Initial experiments utilizing 0.2 mm2 aluminum points with a spatial density of 25/cm2 have increased the apparent effective electric field by a factor of ~1.5 while keeping the insertion loss low (<;0.01 dB). The field enhancements have sharply reduced the delay time for surface flashover. For an environment consisting of air at 2.07×104 Pa (155 Torr), for instance, the delay time is reduced from 455 ns to 101 ns. Two radioactive sources were also used in an attempt to provide seed electrons in the high field regions. Presented in this paper is a comparison of various field-enhancing geometries and how they relate to flashover development along with an analysis of time resolved imaging and an explanation of experimental results with radioactive materials.
Authors: G. R. Laity; A. S. Fierro; L. L. Hatfield; J. C. Dickens; A. A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5776698
Abstract: The quantitative role of self-produced vacuum-ultraviolet (VUV) light on photoionization-dominated gas discharges is currently an area of interest in the aerospace community. In this paper, we present the images of the VUV spectroscopic analysis of a pulsed atmospheric flashover, where the spatial content of emission relative to electrode geometry has been preserved. The observed spatial profile of emission is dependent on radiating species in the range of 120-125 nm and is discussed in relation to the physics of nanosecond discharges.
Authors: A. A. Neuber; B. Novac
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5976078
Abstract: It is our pleasure to present to the professional reader this "third" Special Issue on Power Modulators and Repetitive Pulsed Power of the IEEE Transactions on Dielectrics and Electrical Insulation (previous issues were published in 2007 and 2009), and the fifth such Special Issue published by an IEEE Transactions (1991-IEEE TED; 2005-IEEE TPS). Most of the papers that appear in this issue are based on contributions to the 2010 International Power Modulator and High Voltage Conference Paper/Presentation, which was held in Atlanta, GA, from May 23rd to 27th, 2011. Others heeded the "Call for Papers" that attracted many contributions in the general field of interest to this Special Issue.
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.
Authors: K. Lawson; S. B. Bayne
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5976105
Abstract: This research was completed to study the effects of extreme transient conditions on Silicon Carbide MOSFET devices. Two different transient conditions that are common in power converters were studied in this paper. The first is effects of voltage rise time, or dV/dt, on these devices. The second is the effects of current pulses with short pulse width and high peak currents. Both of these tests were conducted at temperatures of 150 °C to determine the performance of these devices in high stress environments. For both of these experiments, testing apparatus had to be designed and built to create these specific conditions.
Authors: E. J. Matthews; M. Kristiansen; A. A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5395624
Abstract: An evaluation of the energy density and efficiency of high-voltage capacitors, from various manufacturers, at voltages above their rated level is presented. Characteristics such as decreasing capacitance, decreasing efficiency, and increasing energy density are described. Data are taken from eight capacitors; six of which are composed of varying nonlinear ceramic materials as a dielectric, and the remaining two are wound capacitors, which were chosen to exhibit their linear characteristics. Rapid (1-100 ms) charging and discharging, similar to that of a generator operation at a repetition rate of more than 10 Hz with an ~100-ns pulse, were the conditions simulated in this particular test setup.
Authors: H. Rahaman; J. W. Nam; Sang H. Nam; K. Frank
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5497195
Abstract: It is our interest to design and develop a high-power pulser system employing an electrical discharge in a spark gap. The type of discharge generates short electric pulses capable of both the characteristics of a fast rise time and a high repetition rate. Such a switch pulser has wide applications in the industry. To generate the electrical discharge in the spark gap in an efficient manner is very important for the switching operation. The regime of the operation utilizes the residual plasma in the interpulse recovery of the spark gap to increase the repetition rate. Therefore, the mechanisms of the discharge in a controlled manner following the electrical breakdown of the spark gap, such as the plasma generation in the spark channel and the dielectric recovery process as well as the residual plasma in the post-spark-discharge period, are of great importance. The understanding of the type of the spark-gap discharge for the switching at a high repetition rate beyond 1 MHz has been the primary goal. The typical voltage and current ratings are below 1 kV and several amperes, respectively. In particular, this paper addresses the optimal set of several parameters, such as the spark-gap geometry, electrode gap distance, gas pressure, and gas type, in conjunction with a unique circuit scheme to drive the discharge in the controlled fashion.
Authors: A. Young; A. Neuber; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5471092
Abstract: A novel modeling and simulation method for flux-trapping flux-compression generators (FT-FCGs) is presented, which utilizes PSpice circuit-simulation software to solve complex differential equations derived from circuit analysis. The primary motivation for the model development is the desire for a technique to rapidly design and prototype FT-FCGs for use as drivers in high-power microwave sources. The derivation of FT-FCG equations will be given, both in the ideal (lossless) and nonideal cases. For the nonideal case, three flux conservation coefficients are added to the equations to account for intrinsic flux loss in the circuit. Time-varying inductance curves are calculated using zero-dimensional models found in literature and adapted to fit this model. A simple FT-FCG design is used as an example to show the steps taken to complete a simulation. The same design was also fabricated and tested for comparison with predicted results from the model. A comparison of the waveforms acquired through simulation and experiment was found to result in good agreement for a given set of values for the flux conservation coefficients. A discussion of the derived equations, both lossless and nonideal, is given, as well as a discussion on the investigation of the impact of the three flux constants on the circuit. Analysis is offered on the results of this investigation, and conclusions are given on the effectiveness of this model to predict FT-FCG behavior.
Authors: Parson, J; Dickens, J; Walter, J; Neuber, AA
PDF: https://aip.scitation.org/doi/10.1063/1.3499245
Abstract: This paper presents a study on energy deposition and electromagnetic compatibility of match-type electroexplosive devices (EEDs), which recently have found more usage in pulsed power environments with high electromagnetic interference (EMI) background. The sensitivity of these devices makes them dangerous to intended and unintended radiation produced by devices commonly used in pulsed power environments. Match-type EEDs have been found to be susceptible to such low levels of energy (7-8 mJ) that safe operation of these EEDs is vital when in use near devices that produce high levels of pulsed EMI. The scope of this paper is to provide an investigation that incorporates results of similar studies to provide detonation characteristics of these EEDs. The three topics included in this study are sensitivity testing, modeling of the thermodynamic heat propagation, and electromagnetic compatibility from pulsed electromagnetic radiation. The thermodynamic joule heating of the primary explosive has been modeled by a solution to the 1D heat equation. A simple pulsed generator, Marx generator with an inductive load, was used for the electromagnetic compatibility assessment of the coupled field between the pulse generator and shorted EED. The results of the electromagnetic compatibility assessment relate the resistive, inductive, and capacitive components of the pulse generator to the area of the shorted EED. (C) 2010 American Institute of Physics. [doi:10.1063/1.3499245]
Authors: D. W. Bolyard; A. A. Neuber; J. T. Krile; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5415669
Abstract: Pressure simulations have been performed for several experimental explosively driven ferroelectric generators, with 2.54-cm-diameter PZT EC-64 discs as the ferroelectric material, using the hydrodynamic code system CTH, developed by Sandia National Laboratories. An empirical relationship was found between the results of the pressure simulations and the output voltages of the experimental generators, and an algorithm was generated to convert the simulated pressure into open-circuit voltage. This empirical algorithm has been applied to simulations of different experimental ferroelectric generators, and the results show a good correlation when compared to the corresponding experimental open-circuit output voltages. The experimentally achieved output voltages normalized for a thickness range from 14 to 34 kV/cm.
Authors: David W. Bolyard; Andreas A. Neuber; John T. Krile; Magne Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5415669
Abstract: Pressure simulations have been performed for several experimental explosively driven ferroelectric generators, with 2.54-cm-diameter PZT EC-64 discs as the ferroelectric material, using the hydrodynamic code system CTH, developed by Sandia National Laboratories. An empirical relationship was found between the results of the pressure simulations and the output voltages of the experimental generators, and an algorithm was generated to convert the simulated pressure into open-circuit voltage. This empirical algorithm has been applied to simulations of different experimental ferroelectric generators, and the results show a good correlation when compared to the corresponding experimental open-circuit output voltages. The experimentally achieved output voltages normalized for a thickness range from 14 to 34 kV/cm.
Authors: Truman G. Rogers; Andreas A. Neuber; Klaus Frank; George R. Laity; James C. Dickens
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5559489
Abstract: There is a growing interest in the physics of surface flashover between the interface of atmosphere and vacuum in some high-power systems. More specifically, the quantitative role of vacuum ultraviolet (VUV) radiation for the photoionization leading to a streamer development during the initial stages of a breakdown is unknown. This paper describes an experimental setup used to measure the VUV radiation emitted from atmospheric flashover as well as time-resolved imaging of the flashover event. A pulser providing the voltage to the gap was designed with special considerations in mind, including long lifetime, low noise, and high reproducibility. This enabled the study of the flashover in various background gases with an emphasis on spectroscopic measurements. The calculated spectra are compared with the measured spectra, and it is found that atomic oxygen and nitrogen are responsible for most of the VUV production in an air breakdown at atmospheric pressure in the wavelength range of 115-180 nm. Time-resolved spectroscopy reveals that the VUV radiation is emitted during the initial stages while the streamers are developing.
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.
Authors: Thomas G. Engel; Magne Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5175368
Abstract: The mechanisms and predictors of insulator degradation and erosion by pulsed high-current surface discharges are presented and discussed. Erosion and degradation depend on the insulator material, the electrode material, the ambient gas, and the presence of UV stabilizers in, or on the surface of, the insulator. Insulator degradation is the result of material decomposition into conductive metal or carbon species and is measured by a decrease in the surface breakdown voltage. Insulator erosion is measured by the material's mass loss. The performance of a large group of ceramic, polymeric, and elastomeric materials tested with graphite and molybdenum electrodes is presented in this investigation. The insulators are exposed to repetitive 300-kA 20-mus-long surface discharges. Tests are performed in atmospheric air and pure nitrogen. Various methods to rank insulators in terms of holdoff voltage degradation, mass erosion, and holdoff voltage conditioning (HVC) using the material's thermochemical properties are presented and discussed. HVC is characterized by an initial increase in surface holdoff voltage. The ranking method developed by the authors characterizes the insulator according to the holdoff degradation resistance (HDR), mass vaporization coefficient (MVC), and HVC figures of merit calculated by the material's thermochemical properties. The investigation also shows the relationship between the HDR, MVC, and HVC figures of merit.
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.
Authors: C. B. Davis; A. A. Neuber; A. Young; J. Walter; J. C. Dickens; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5313897
Abstract: This paper presents a nonexplosive pulsed-power system that replicates the output current waveform of a flux compression generator (FCG). The primary purpose of this system is to efficiently test the power conditioning components of an explosively driven HPM system, while drastically reducing the time between tests which are inherent with explosive experiments. The power conditioning system (PCS) of the HPM system includes an energy-storage inductor, an electroexplosive opening switch (fuse), and a peaking gap and serves to match the FCG output characteristics with the HPM diode load requirements. A secondary purpose of the nonexplosive test bed is to provide data points which could be directly compared with those from explosively driven experiments. For this reason, a reflex-triode virtual cathode oscillator (vircator) was connected to the output of the nonexplosive system, and the results of which were compared with similar testing done with an FCG and a compact Marx generator. Since the behavior of the fuse is known to play a critical role in the performance of the PCS, a study was performed on the effect of different fuse designs on the overall performance of the PCS. Specifically, the quality of the electrical connection between the fuse wire array and the rest of the system was tested. Fuse design experiments were conducted with the nonexplosive test bed firing into a water resistor dummy load, which showed a 13% increase in peak load voltage and more than an 11% increase in energy transfer for fuses with improved wire-electrode connection strength. Some basic rules about fuse design, as well as conclusions on the performance of the PCS when driving an HPM load, are given.
Authors: John T. Krile; Luke McQuage; Gregory F. Edmiston; John Walter; Andreas A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5291765
Abstract: High-power microwave (HPM)-induced surface flashover is investigated in order to gain a better understanding of this phenomenon and reduce the limitations it imposes on transmitted power levels. This paper builds on previous testing using a magnetron producing 5 MW for 4 mus at 2.85 GHz. Both the previous and current experimental setups are designed to produce a flashover on the high-pressure side of a transmission window without the influence of a triple point. The limitations of the previous experiment included a maximum power of 5 MW and a pulse rise time of 50 ns. The current HPM source is an experimental virtual cathode oscillator (vircator), the output of which has been extensively characterized. The vircator is capable of producing 50-MW peak for 100 ns with an adjustable frequency from 3 to 5 GHz and a rise time of < 4 ns. The dominant modes of the vircator and magnetron are the circular TE11 and rectangular TE10 modes, respectively, with the major electric field component in both setups normal to the direction of propagation, yielding comparable field geometries at the transmission window. The experimental setup permits the study of factors, including gas pressure, composition, temperature, and air speed. Diagnostic equipment allows the analysis of power levels and flashover luminosity with subnanosecond resolution. Additional experimental results, including a detailed analysis of the flashover delay times under various conditions, are compared with data from literature and previous testing. A trend of increasing delay time with pressure is clearly observable, and Eeff/p versus p * r data fall within what has been previously observed in literature primarily for HPM volume breakdown.
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.
Authors: J. E. Chaparro; W. Justis; H. G. Krompholz; L. L. Hatfield; A. A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4663141
Abstract: With a RADAN 303-A pulser (a rise time of 150 ps and a maximum voltage of 150 kV into matched load), fast breakdown in argon and air is investigated. An oil-filled coaxial transmission line is coupled with a lens to a biconical section and a radial millimeter-size gap operated at subatmospheric pressure. Diagnostics include capacitive voltage dividers which allow the determination of voltage across and current through the gap with a temporal resolution defined by the digitizer (20 Gs/s, 6 GHz) used. A scintillator-photomultiplier combination with different metal absorber foils and a temporal resolution of 2 ns is used as X-ray detector to obtain a rough energy spectrum of the X-rays and electrons in the range of 10-150 keV. Discharges are characterized by runaway electrons over much of the pressure range, with a strong excitation and ionization layer at the cathode surface, and ldquofree-fallrdquo conditions with negligible gaseous ionization for the rest of the gap. High-energy electrons (> 60 keV) are observed up to atmospheric pressure. Time-to-breakdown curves versus pressure have been measured for different applied voltage rise times. They resemble Paschen curves with a steep increase toward low pressure and a slow increase toward high pressure. The major experimental findings and particularly the time-to-breakdown curves are confirmed using simple force-equation modeling. Monte Carlo calculations simulating collisional ionizations and developing electron avalanches in three dimensions have been used to verify and explain the experimental results.
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.
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.
Authors: John T. Krile; Andreas A. Neuber; Hermann G. Krompholz
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4463641
Abstract: Undesirable surface flashover of high voltage support structures can severely limit the compactness of open air high voltage systems. Only recently, increased effort has been invested in characterizing and quantifying the physical processes involved in surface flashover occurring under atmospheric conditions and under the influence of UV illumination. In this paper, a UV flash lamp and a solid-state UV source, with its much faster turn-off time, were utilized in conjunction with a high temporal resolution testing apparatus. The UV pulse, excitation voltage, discharge current, and flashover self-luminosity were measured with high temporal precision. We relate recent experiments to our experimental findings of surface flashover under atmospheric conditions gained over the past five years. A simple model that describes the observed behavior will be presented. In addition, a more advanced Monte Carlo-type code for electron collision dynamics will be utilized to further analyze the role of UV in surface flashover under atmospheric conditions.
Authors: Elsayed, M; Kristiansen, M; Neuber, A
PDF: https://aip.scitation.org/doi/10.1063/1.3046280
Abstract: Flux compression generators (FCGs) are some of the most attractive sources of single-use compact pulsed power available today due to their high energy density output and mobility. Driving FCGs requires some seed energy, which is typically provided by applying a high seed current, usually in the kiloampere range for midsized helical FCGs. This initial current is supplied by a high-current seed source that is capable of driving an inductive load. High-current seed sources have typically been comprised of discharging large capacitors using spark gaps and overvoltage triggering mechanisms to provide the prime power for FCGs. This paper will discuss a recent design of a self-contained (battery powered with full charge time less than 40 s), single-use compact seed source (CSS) using solid-state components for the switching scheme. The CSS developed is a system (0.005 m(3) volume and weighing 3.9 kg) capable of delivering over 360 J (similar to 12 kA) into a 5.20 mu H load with a trigger energy of microjoules at the TTL triggering level. The newly designed solid-state switching scheme of the CSS incorporates off-the-shelf high-voltage semiconductor components that minimize system cost and size as necessary for a single-use application. A detailed evaluation of the CSS is presented primarily focusing on the switching mechanics and experimental characterization of the solid-state components used in the system.
Authors: J. Krile; A. Neuber; R. Vela
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4512494
Abstract: A recent upgrade of Sandia National Laboratories' Z-machine has exposed a possible failure mode in the 5 MV laser-triggered gas switches (LTGS). During the closure of the cascade section of the switch, the surface flashover (SF) inside the dielectric switch housing occurred sporadically, affecting subsequent closing timing and damaging the switch housing. A small-scale experiment has been constructed to mimic conditions within the LTGS and to examine the survivability of various materials exposed to high-current SFs.
Authors: T. A. Holt; A. J. Young; M. A. Elsayed; J. W. Walter; A. A. Neuber; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4599011
Abstract: A recently developed self-contained compact single-shot high-power microwave (HPM) system was tested and characterized. The explosive-driven system utilizes a reflex triode virtual cathode oscillator (vircator) as the HPM source. An open-shutter image acquired with a digital single-lens reflex camera during operation was used to show plasma development extending beyond the anode-cathode gap of the vircator. The plasma's self-emission is due to ionized material eroded and desorbed from both the cathode and the anode.
Authors: G. F. Edmiston; J. T. Krile; A. A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4598990
Abstract: Dielectric window flashover is a severe pulse-shortening phenomenon limiting the power levels radiated in high power microwave (HPM) systems. This type of flashover develops in regions under high field stress coinciding with the dielectric interfaces separating the vacuum and atmospheric pressure sections of a microwave system. The formation of plasma at the exit aperture of a transmitting system can have several detrimental effects, including premature termination of the radiated pulse and/or the reflection of potentially damaging levels of radiation back toward the microwave source. Experimental studies of HPM surface flashover have been conducted under a variety of conditions in the S-band at power levels up to 5 MW with the aim of quantifying the relative impact of parameters such as gas pressure, type, and window geometry. One particular geometry variant designed with grooves perpendicular to the major electric field component at the window surface exhibited superior flashover suppression characteristics when compared with smooth window geometries. Images of HPM surface flashover evolution on this corrugated dielectric window geometry are presented.
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.
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.
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.
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.
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
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.
Authors: G. Edmiston; A. Neuber; L. McQuage; J. Krile; H. Krompholz; J. Dickens
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4286507
Abstract: One of the major limiting factors for the transmission of high power microwave (HPM) radiation is the interface between dielectric-vacuum or even more severely between dielectric-air if HPM is to be radiated into the atmosphere. Surface flashover phenomena which occur at these transitions severely limit the power levels which can be transmitted. It is of major technological importance to predict surface flashover events for a given window geometry, material and power level. When considering an aircraft based high power microwave platform, the effects on flashover formation due to variances in the operational environment corresponding to altitudes from sea level to 50,000 feet (760 to 90 Torr; 1 Torr=133.3 Pa) are of primary interest. The test setup is carefully designed to study the influence of each atmospheric variable without the influence of high field enhancement or electron injecting metallic electrodes. Experimental data of flashover delay times across different materials, such as polycarbonate, Teflonreg, and high density polyethylene as a function of background pressure and gas type, air, N2, argon are discussed. An empirical relationship between flashover field amplitude and delay time is given.
Authors: Kim Morales; John Krile; Andreas Neuber; Hermann Krompholz
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4286506
Abstract: Dielectric surface flashover along insulators in atmospheric conditions has only been empirically characterized over the years. The underlying physics involved in atmospheric flashover has yet to be understood, where parameters such as background gas, humidity, surface roughness, and temporal characteristics of the applied voltage play a role. Understanding the fundamental physical mechanisms and the extent to which these parameters influence the discharge behavior is vital to characterizing and modeling surface flashover for various structures and conditions. A solid state high voltage pulser with an adjustable pulse width of ~500 ns at FWHM and amplitudes in excess of 30 kV was developed to replicate the non-standard temporal shape of the transient voltage observed inside a rebar enforced building during a lightning strike. Based on experimental results, the phenomenology of pulsed unipolar surface flashover is discussed, with the emphasis on the influence of external parameters (applied voltage pulse shape/risetime, environment, UV illumination, humidity, etc.) on spatial and temporal discharge channel behavior.
Authors: A. A. Neuber; G. F. Edmiston; J. T. Krile; H. Krompholz; J. C. Dickens; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4033091
Abstract: The major limiting factor in the transmission of narrowband high-power microwaves (HPM) has been the interface between vacuum-vacuum or even more severely between vacuum-air if HPM are to be radiated into the atmosphere. Extensive studies have identified the physical mechanisms associated with vacuum/dielectric flashover, as opposed to the mechanisms associated with dielectric/air flashover, which are not as well known. Due to the high electron collision frequencies (in the terahertz range) with the background gas molecules, established mitigation methods and concepts of vacuum/dielectric flashover will have to be re-evaluated. The primarily limiting factors of HPM transmission through a dielectric/air interface are presented based on recent experiments at 2.85 GHz. The physics of the involved mechanisms and their practical ramifications are discussed. The potential of surface roughness/geometry for flashover mitigation is addressed as well
Authors: J. T. Krile; R. Vela; A. A. Neuber; H. G. Krompholz
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4343168
Abstract: A recently upgraded laser-triggered gas switch at Sandia National Laboratories has developed a failure mode that results in the breakdown spark tracking to the inside of the containment envelope. These breakdowns along the surface, or surface flashovers, degrade the performance of the overall switch, causing the switch to prefire in the successive shot. In the following, experimental results of pulsed surface flashover across different dielectric materials in SF6, primarily at atmospheric pressure, as well as flashover and volume breakdown in at pressures from 1.3 to 365.4 kPa are presented. In addition to fast voltage and current monitoring of the breakdown event, an increased emphasis was put on imaging the event as well as gathering optical emission spectra (~200-700 nm) from it. As much as possible, the small-scale experiments were designed to reproduce, at least partly, the conditions as they are found in the large 5-MV switch. An effort was made to determine what changes could be made to reduce the occurrence of surface flashovers, in addition to some broadly applicable conclusions on surface flashovers in an SF6 environment.
Authors: J. T. Krile; R. Vela; A. A. Neuber; H. G. Krompholz
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4287051
Abstract: The Z-machine, which is located at Sandia National Laboratories, is currently undergoing refurbishment to increase the output drive current. Due to increased switching voltage requirements, some switch failure modes have been identified with the laser-triggered gas switch design, including envelope surface flashover. In order to improve the performance and lifetime of these switches, a basic understanding of the underlying physics of the failure mechanisms is required. A small-scale experimental setup has been constructed to approximate conditions within the switch. The possible impact of the SF6 volume spark between the switch electrodes on the envelope surface flashover is investigated. Measured optical spectra of the SF6 volume spark over a wide pressure range, from rough vacuum to 40 psig overpressure, are analyzed regarding their potential to contribute to switch failure.
Authors: M. Butcher; A. A. Neuber; M. D. Cevallos; J. C. Dickens; H. Krompholz
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1621332
Abstract: With a fast coaxial test setup using high speed electrical and optical diagnostics, prebreakdown current pulses and shadowgraphy images are measured for direct current (dc) breakdown in Univolt 61 transformer oil. Also, dc currents across the gap are measured using a high sensitivity electrometer. The conduction and breakdown mechanisms in transformer oil as function of applied hydrostatic pressures are quantified. Together, this information provides data on the development of current flow in the system. We have identified three stages in the conduction process prior to breakdown for highly nonuniform fields. Stage 1 is characterized by a resistive current at low fields. Increasing the applied electric field lowers the effective barrier at the metal/dielectric interface allowing a "tunneling" mechanism to begin, leading to the rapid rise in the injection current observed in stage 2. In stage 3, at high fields, the current reaches space charge saturation with an apparent mobility of 3/spl middot/10/sup -3/ cm/sup 2//V/spl middot/s prior to breakdown. The processes of final breakdown show a distinct polarity dependence. A strong pressure dependence of the breakdown voltage is recorded for negative needle/plane breakdown; a 50% reduction in breakdown voltage is observed when the hydrostatic pressure is lowered from atmospheric pressure to hundreds of mtorr. Positive needle discharges show a reduction of only about 10% in breakdown voltage for the reduced pressure case. Weak pressure dependence indicates the breakdown mechanism does not have a strong gaseous component. We will discuss possible links between conduction current and dc breakdown.
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.
Authors: G. Edmiston; J. Krile; A. Neuber; J. Dickens; H. Krompholz
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1710041
Abstract: The major limiting factor in the transmission of high-power microwave (HPM) has been the interface between dielectric-vacuum or, even more severely, between dielectric-air, if HPM is to be radiated into the atmosphere. Extensive studies have identified the physical mechanisms associated with vacuum-dielectric flashover, as opposed to the mechanisms associated with air-dielectric flashover, which are not as well known. Surface-flashover tests involving high field enhancement due to the presence of a triple point have shown that volume breakdown threshold (dielectric removed) is approximately 50% higher than the flashover threshold with a dielectric interface over the 90-760 torr range. In order to quantify the role of field enhancement in the flashover process independent of electron injection from metallic surfaces, the effects of the triple point are minimized by carefully choosing the geometry, and in some cases, the triple point is "removed" from the flashover location. Experimental results were presented, including the impact of gas pressure and the presence of UV illumination, along with temperature analysis of the developing discharge plasma and temporally resolved images of the flashover formation. These results are compared with literature data for volume breakdown in air, with discussion on the similarities and differences between the data
Authors: H.G. Krompholz; L.L. Hatfield; A.A. Neuber; K.P. Kohl; J.E. Chaparro; Han-Yong Ryu
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1643325
Abstract: Volume breakdown and surface flashover in quasi-homogeneous applied fields in 10-5 to 600 torr argon and dry air are investigated, using voltage pulses with 150 ps risetime, <1ns duration, and up to 150 kV amplitude into a matched load. The test system consists of a transmission line, a transition to a biconical section, and a test gap, with gap distances of about 1mm. The arrangement on the other side of the gap is symmetrical. Diagnostics include fast capacitive voltage dividers, for determination of voltage waveforms in the gap, and conduction current waveforms through the gap. X-ray diagnostics use a scintillator-photomultiplier combination with different absorber foils yielding coarse spectral resolution. Optical diagnostics include use of a streak camera to get information on the discharge channel geometry and dynamics, and temporally resolved measurements with photomultipliers. Breakdown delay times are on the order of 100-400 ps, with minima occurring in the range of several 10torr. X-ray emission extends to pressures >100 torr, indicating the role of runaway electrons during breakdown. Maximum X-ray emission coincides with shortest breakdown delay times at several 10 torr. Simple modeling using the average force equation and cross sections for momentum transfer and ionization supports the experimental results
Authors: K.P. Morales; J.T. Krile; A.A. Neuber; H.G. Krompholz
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1667739
Abstract: Dielectric flashover along insulators in vacuum has been comprehensively researched in the past. Less studied, but of similar importance, is surface flashover at atmospheric pressures and the impact of an atypical electrode geometry, humidity, and ultraviolet (UV) illumination. Previous research has shown distinct discharge behavior in air and nitrogen environments for an electrode geometry in which the applied electric field lines curve above the dielectric surface. It was concluded that the discharge development path, whether along the electric field lines or the surface of the dielectric, is related to the oxygen content in the atmospheric background. It is believed that this dependence is due to the discharge's production of UV radiation in an oxygen rich environment. Thus, experiments were conducted in a nitrogen environment employing UV surface illumination in order to observe the affects on the flashover spark behavior. From the experimental data, it can be ascertained that UV illumination and intensity play a significant role in the discharge development path. Based on these results an explanation of the physical mechanisms primarily involved in unipolar surface flashover will be presented. Additional experiments regarding the effects of humidity on the discharge behavior will be discussed as well
Authors: Krile, J; Edmiston, G; Morales, K; Neuber, A; Krompholz, H; Kristiansen, M
PDF: https://ieeexplore.ieee.org/document/4084225
Abstract: Mechanisms in vacuum surface flashover caused by rf (f < 10 GHz) or unipolar voltages are virtually identical. Similarities between rf (representing high-power microwave window breakdown on the high-pressure side) and unipolar surface flashover are expected in an atmospheric environment as well. Two separate experimental setups were utilized to investigate both unipolar flashover and rf window flashover under atmospheric conditions while controlling excitation, temperature, pressure, humidity, and type of gas present, all under a similar electric field-surface geometry. The local electric field at the flashover initiating points has been numerically calculated in detail for all test geometries. For both rf and unipolar pulsed excitation, the flashover dynamics are changed by the application of UV light to the dielectric surface. A UV prepulse has a distinct impact on the arc's path and a tendency to decrease the hold-off electric field. The effect of humidity on the hold-off electric field for both pulsed unipolar and rf excitations, along with temporally resolved emission spectroscopy of the flashover event, is discussed.
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
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.
Authors: Giesselmann, M; McHale, B; Crawford, M
PDF: https://ieeexplore.ieee.org/document/1398070
Abstract: This paper describes work toward transient energy extraction from high-frequency alternators to drive electromagnetic launchers through intelligent triggering of thyristors, or silicon controlled rectifiers (SCR). The work was done using a three-phase, 75 kVA level motor-generator set with a dc machine acting as the load. This system serves as a scale model to develop control techniques for full-scale, multimegajoule applications. With the use of a microcontroller and advanced sensors, the system is able to maintain synchronization of the SCR converter with the generator voltage despite dynamically changing frequency and voltage distortion.
Authors: Cevallos, MD; Butcher, M; Dickens, J; Neuber, A; Krompholz, H
PDF: https://ieeexplore.ieee.org/document/1420543
Abstract: The breakdown physics of transformer oil is investigated using high speed electrical and optical diagnostics. Experiments are done in self-breakdown mode utilizing a needle/plane geometry. Shadowgraphy combined with high-speed electrical diagnostics are aimed at measuring streamer expansion as a function of external pressure. Assuming a breakdown mechanism for negative needle based on bubble formation with subsequent carrier amplification in the gas phase implies a pressure dependence, which is observed in the experiments, i.e. the expansion velocity decreases with increasing pressure.
Authors: J. Krile; A. Neuber; J. Dickens; H. Krompholz
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1420431
Abstract: Using a gated intensified digital charge coupled device (ICCD) camera, the development of flashovers across a dielectric surface has been imaged in various gasses at atmospheric pressures. The arc displayed a strong tendency to develop close to the surface, as opposed to following the electric field line leading away from the surface, when oxygen is present in the environment. These findings along with spectroscopy data help to yield a better understanding of the processes involved in surface flashover.
Authors: Qian, J; Joshi, RP; Kolb, J; Schoenbach, KH; Dickens, J; Neuber, A; Butcher, M; Cevallos, M; Krompholz, H; Schamiloglu, E; Gaudet, J
PDF: https://aip.scitation.org/doi/10.1063/1.1921338
Abstract: An electrical breakdown model for liquids in response to a submicrosecond (similar to 100 ns) voltage pulse is presented, and quantitative evaluations carried out. It is proposed that breakdown is initiated by field emission at the interface of pre-existing microbubbles. Impact ionization within the microbubble gas then contributes to plasma development, with cathode injection having a delayed and secondary role. Continuous field emission at the streamer tip contributes to filament growth and propagation. This model can adequately explain almost all of the experimentally observed features, including dendritic structures and fluctuations in the prebreakdown current. Two-dimensional, time-dependent simulations have been carried out based on a continuum model for water, though the results are quite general. Monte Carlo simulations provide the relevant transport parameters for our model. Our quantitative predictions match the available data quite well, including the breakdown delay times and observed optical emission. (C) 2005 American Institute of Physics.
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.
Authors: Chen, XP; Dickens, J; Hatfield, LL; Choi, EH; Kristiansen, M
PDF: https://aip.scitation.org/doi/abs/10.1063/1.1743309
Abstract: In high-power microwave diode design, the space-charge-limited current is important because of its relation to the diode impedance, and the formation of the virtual cathode. Although the Langmuir-Blodgett law, as a numerical solution, is helpful, a simple functional expression would be more convenient for practical research. In this paper, a physical approximation has been introduced to analyze the nonlinear Poisson's equation in a one-dimensional (1-D) cylindrical vacuum diode. With the help of this physical approximation, a solution for the space-charge-limited current for 1-D cylindrical diodes has been investigated and developed. In addition, a comparison between our approximate result and the Langmuir-Blodgett numerical solution shows that the physical approximation method is valid in nonlinear differential equation analyses. This physical approximation can be used to analyze similar nonlinear differential equations. Also, a correction for the space-charge-limited current in a two-dimensional cylindrical diode is obtained within a limitation. (C) 2004 American Institute of Physics.
Authors: J. T. Krile; A. A. Neuber; J. C. Dickens; H. G. Krompholz
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1347231
Abstract: Surface flashover is a major consideration in a wide variety of high-voltage applications, and yet has not been studied in great detail for atmospheric conditions, with modern diagnostic tools. Environmental conditions to be considered include pressure, humidity, and gas present in the volume surrounding the dielectric. In order to gain knowledge into the underlying process involved in dielectric surface flashover, a setup has been created to produce and closely monitor the flashover event. Within the setup parameters such as geometry, material, and temporal characteristics of the applied voltage can be altered. Current, voltage, luminosity, and optical emission spectra are measured with nanosecond to subnanosecond resolution. Spatially and temporally resolved light emission data is also gathered along the arc channel. Our fast imaging data show a distinct trend for the spark in air to closely follow the surface even if an electrical field with a strong normal component is present. This tendency is lacking in the presence of gases such as nitrogen, where the spark follows more closely the electric field lines and develops away from the surface. Further, the breakdown voltage in all measured gases decreases with increasing humidity, in some cases as much as 50% with an increase from 10% relative humidity to 90% relative humidity.
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.
Authors: A. A. Neuber; J. C. Dickens
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1306688
Abstract: Magnetic flux compression generators offer the largest pulsed power output per unit size or weight when compared with other more conventional systems. They have found widespread use as pulsed power sources for hydrodynamics programs and high magnetic field research at national laboratories or in commercial applications, including exploration for oil and minerals and mine detection. Also, due to their nature as a true one-time-use device with superior energy density, a large portion of applications is defense related. A variety of basic magnetic flux compression generator designs have been developed and tested during the past four decades. All of them rely on the explosive-driven deformation of a system of conductors having an initial, preferably large, inductance. The most successful basic design is the helical flux compression generator, which is capable of producing a high-energy output into large impedance loads, just as it is needed for a practical pulsed power source. This paper will review the advances and state of the art of primarily helical magnetic flux compression generators mainly developed as pulsed power sources and will offer new insights gained as a result of a recently completed five-year AFOSR/DoD Multidisciplinary University Research Initiative program that studied the basic physics and engineering aspects of helical flux compression generators.
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.
Authors: E. Schamiloglu; R.J. Barker; M. Gundersen; A.A. Neuber
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1306675
Abstract: In this introduction to the Special Issue on pulsed power and its applications, background information is provided for the nonspecialist to better understand the many challenges in designing pulsed power systems, and the wide diversity of applications that are now emerging. The approach to providing a tutorial on pulsed power technology is to make available to the reader the paper written by J. C. Martin which appeared in a Special Section of the Proceedings of the IEEE on pulsed power technology in June 1992. That paper is supplemented in this introduction with additional information that complements many of the invited papers composing this Special Issue.
Authors: J.C. Hernandez; A.A. Neuber; J.C. Dickens; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1347243
Abstract: Helical magnetic flux compression generators (MFCGs) are the most promising energy sources with respect to their current amplification and compactness. They are able of producing high current pulses required in many pulsed power applications with at least one order of magnitude higher energy density than capacitive storage with similar discharge characteristics. However, the main concern with MFCGs is their intrinsic flux loss that limits severely their performance and which is not yet well understood. In general, all flux losses have a differing degree of impact, depending on the generator's volume, current and energy amplification, size of the driven load, and angular frequency of armature-helix contact point. Although several computer models have been developed in the open literature, none of them truly quantify, starting from basic physics principles, the ohmic and intrinsic flux losses in helical MFCGs. This paper describes a novel method that provides a separate calculation of intrinsic flux losses (flux that is left behind in the conductors and lost for compression) and ohmic losses, being especially easy to implement and fast to calculate. We also provide a second method that uses a simple flux quantification, making a mathematical connection between the intrinsic flux losses, quantified by the first method, and the intrinsic flux losses observed in the generators. This second method can also be used to a priori estimate the MFCG performance. Further, we will show experimental and calculated data and discuss the physical efficiency limits and scaling of generator performance at small sizes.
Authors: J.A. Gaudet; R.J. Barker; C.J. Buchenauer; C. Christodoulou; J. Dickens; M.A. Gundersen; R.P. Joshi; H.G. Krompholz; J.F. Kolb; A. Kuthi; M. Laroussi; A. Neuber; W. Nunnally; E. Schamiloglu; K.H. Schoenbach; J.S. Tyo; R.J. Vidmar
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1306684
Abstract: Pulsed power is a technology that is suited to drive electrical loads requiring very large power pulses in short bursts (high-peak power). Certain applications require technology that can be deployed in small spaces under stressful environments, e.g., on a ship, vehicle, or aircraft. In 2001, the U.S. Department of Defense (DoD) launched a long-range (five-year) Multidisciplinary University Research Initiative (MURI) to study fundamental issues for compact pulsed power. This research program is endeavoring to: 1) introduce new materials for use in pulsed power systems; 2) examine alternative topologies for compact pulse generation; 3) study pulsed power switches, including pseudospark switches; and 4) investigate the basic physics related to the generation of pulsed power, such as the behavior of liquid dielectrics under intense electric field conditions. Furthermore, the integration of all of these building blocks is impacted by system architecture (how things are put together). This paper reviews the advances put forth to date by the researchers in this program and will assess the potential impact for future development of compact pulsed power systems.
Authors: Talantsev, EF; Shkuratov, SI; Dickens, JC; Kristiansen, M
PDF: https://aip.scitation.org/doi/abs/10.1063/1.1527719?journalCode=rsi
Abstract: It is demonstrated that it is feasible to produce pulsed power using an autonomous completely explosive system that harnesses two physical phenomena successively: the transverse shock wave demagnetization of Nd2Fe14B high-energy hard ferromagnets and magnetic cumulation. (C) 2003 American Institute of Physics.
Authors: Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M
PDF: https://aip.scitation.org/doi/10.1063/1.1558968
Abstract: Experimental and digital simulation studies of the generation of seed currents by an ultracompact (8.66-8.75 cm(3) in volume) ferromagnetic explosive-driven generator of primary power (FMG) loaded on the coaxial single-turn seeding coil of a magnetocumulative generator (MCG) have been performed. The operation of the FMG is based on transverse shock wave demagnetization of Nd2Fe14B high-energy hard ferromagnets. The FMG is capable of producing in the coaxial seeding coil of MCG a seed current with peak amplitude I(t)(max)=3.0 kA and full width at half maximum of 60 mus. The methodology was developed for digital simulation of the seeding processes in the combined FMG/MCG system. (C) 2003 American Institute of Physics.
Authors: Giesselmann, M; McHale, B; Crawford, M
PDF: https://ieeexplore.ieee.org/abstract/document/1179842
Abstract: This paper describes the use of methods and techniques that have been developed for high-performance control of relatively small and slow turning industrial machines and extend these techniques to the more demanding regime of ac machines used for kinetic energy storage and fast ac servos for military applications. In particular,this paper describes a technique for fast monitoring of the output voltage of ac generators. To accomplish fast monitoring, the (sinusoidal) ac output voltage is converted to a dc quantity that represents the instantaneous amplitude. This is achieved through the use of a rotational transformation. This transformation, also called vector rotation, can be used for very fast observation of the momentary amplitudes of all electrical machine quantities, such that averaging of the ac value (to determine 66 momentary average rms amplitude) is not necessary. The procedure is implemented by tightly integrating a digital motion control coprocessor into the memory map of a 16-bit microcontroller.
Authors: Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M; Baird, J
PDF: https://aip.scitation.org/doi/10.1063/1.1554486
Abstract: A study of a magnetic phase state of Nd2Fe14B high-energy hard ferromagnets subjected to longitudinal-shock-wave compression (where the shock wave propagates along magnetization vector M) has been performed. The results of the investigation show that longitudinal-shock-wave compression of Nd2Fe14B at 28-38 GPa causes a magnetic phase transition terminated by practically complete demagnetization of Nd2Fe14B. Due to this phase transition all electromagnetic energy stored in Nd2Fe14B is released and can be transformed into pulsed power. Explosive-driven autonomous sources of primary power utilizing this effect are capable of producing high-current pulses [current amplitude of 1.0 kA, full width at half maximum (FWHM) of 165 mus] and high-voltage pulses (peak voltage of 13.4 kV, FWHM of 8.2 mus). (C) 2003 American Institute of Physics.
Authors: Tolomeo, J; Klavins, A; Tenerelli, D; Dickens, J
PDF: https://aip.scitation.org/doi/10.1063/1.1622770
Abstract: Future large space-based telescope systems require precise optical surface quality and wave-front stability. One source of noise for very large precise optical systems is ambient thermal energy which induces statistical fluctuations in the strain energy state of the structure. We broadly model such optical systems as bending energy dominated or membrane in-plane energy dominated and derive analytical expressions for the governing parameters that determine noise magnitude. It is shown that for bending-based systems thermal noise increases as aperture is increased and as bending stiffness is decreased, while for membrane mirror systems it is the in-plane pretension level that determines the noise magnitude. The analysis is extended to numerical finite element techniques to illustrate the effects on very general large damped structures where we address the form of equivalent thermal loading density required in modeling such distributed structures. Calculations show that temporal rms deformation noise on the order of a picometer or less can be expected for apertures up to about 10 m and therefore is probably not significant. For lightweight precision aperture systems greater than 10 m, thermal noise may need to be considered in the design. (C) 2003 American Institute of Physics.
Authors: Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M
Abstract: A new type of compact explosive-driven generator of primary power, which utilizes phenomena of a shock-wave demagnetization of hard ferri- and Mirromagnets, was developed. The shock wave initiated by high explosive, as well as accelerated flyer plate, passes along the hard ferri- or ferromagnetic body, which serves as initial energy carrier. The shock wave demagnetizes the energy-carrying element, reducing the initial magnetic flux Phi(0). In accordance with Faraday's law, this change of magnetic flux DeltaPhi(0) generates an electromotive force in, a coil wound on the energy carrier. Several types of compact generators with energy-carrying element of 10 cm(3) in volume were explored. High-voltage generators that utilize energy of BaFe12O19 hard ferrimagnets are capable of producing pulses of amplitude 5.5 kV with full width at half maximum (FWHM) of 1 mus. The generators that utilize energy of Nd2Fe14B high-energy hard ferromagnets are capable of producing pulses with amplitude more than 10 kV and FWHM about 4 mus. The high-current generators based on Nd2Fe14B produced pulses yielded 826 A and FWHM of 180 mus. The developed generator can be used as the most reliable and effective source of primary power capable of seeding magnitocumulative generators.
Authors: Li, SH; Wunsch, DC; O'Hair, E; Giesselmann, MG
Abstract: The extended Kalman filter (EKF) algorithm has been shown to be advantageous for neural network trainings. However, unlike the backpropagation (BP), many matrix operations are needed for the EKF algorithm and therefore greatly increase the computational complexity. This paper presents a method to do the EKF training on a SIMD parallel machine. We use a multistream decoupled extended Kalman filter (DEKF) training algorithm which can provide efficient use of the parallel resource and more improved trained network weights. From the overall design consideration of the DEKF algorithm and the consideration of maximum usage of the parallel resource, the multistream DEKF training is realized on a MasPar SIMD parallel machine. The performance of the parallel DEKF training algorithm is studied. Comparisons are performed to investigate pattern and batch-form trainings for both EKF and BP training algorithms. (C) 2002 Elsevier Science (USA).
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.
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.
Authors: M. Giesselmann; T. Heeren; A. Neuber; J. Walter; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1003943
Abstract: Explosive flux compression generators generate hundreds of kiloamperes and voltages of a few kilovolts. A power conditioning stage is required since typically voltages in the hundreds of kilovolts range are needed. Inductive energy storage systems with an opening switch provide the necessary voltage gain. In our application, the opening switch has been implemented as an exploding wire fuse. The voltage gain, and hence the performance of the system, is greatly dependent on the opening switch. We utilized high-speed optical imaging (up to 10/sup 7/ pictures/s) to assess the performance of the exploding wire fuse.
Authors: Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M
PDF: https://aip.scitation.org/doi/abs/10.1063/1.1431434
Abstract: A study of the effect of shock waves on the phase state of a hard ferrimagnetic material has been performed. A plane shock wave was passed along the axis of a cylindrical BaFe12O19 hard ferrite magnet. The shock wave demagnetized the cylinder, reducing the magnetic flux. This change in magnetic flux generated an electromotive force (EMF) in a coil wound around the ferrite. The value of the EMF calculated on the assumption that the ferrite was completely demagnetized by the shock wave is in good agreement with the peak EMF value obtained experimentally. (C) 2002 American Institute of Physics.
Authors: S. I. Shkuratov; E. F. Talantsev; L. L. Hatfield; J. C. Dickens; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1178233
Abstract: Four different types of capacitors have been tested to determine the maximum usable high voltage. Ceramic, drop-dipped film, molded-mylar tubulars, and polyester/foil capacitors of different values and different nominal voltages were tested in four modes: the single-shot mode, the repetitive mode, the lifetime dc voltage mode, and the group mode. Experiments have shown that the breakdown voltage for all types of the capacitors tested is ten to seventeen times higher than the nominal voltage. The energy stored in the capacitors for a short time under overstress conditions is from 100 to 250 times higher their normal energy. Data are given for the limitations for single capacitors, and for two, three, and four capacitors connected in parallel.
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.
Authors: Talantsev, EF; Shkuratov, SI; Dickens, JC; Kristiansen, M
PDF: https://www.worldscientific.com/doi/abs/10.1142/S0217984902003956
Abstract: The conductivity of Nd2Fe14B hard ferromagnetic subjected to compression by a longitudinal shock wave (the shock wave propagates along the magnetization vector M) with a pressure of 35 GPa is measured. The results of the experiments show that the conductivity of the longitudinal-shock-wave-compressed Nd2Fe14B is sigma(sw) = (2.83 +/- 0.24)x 10(2) (Omega cm)(-1), which is 22 times lower than the conductivity of Nd2Fe14B under normal conditions.
Authors: Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M; Hernandez, JC
PDF: https://ieeexplore.ieee.org/document/1178194
Abstract: Results of an experimental study of the generation of high-current pulses in a moving magnet system based on an open ferromagnetic circuit design are presented. The magnet was accelerated with the use of a light gas gun. Experimental data are given for the output high current pulses, output voltage, and power delivered in the load for different types of pulse-generating coils. The effect of various pulse-generating windings is given. It has been shown that the Nd2Fe14B hard ferromagnetic projectile (diameter 2.54 cm and height 1.9 cm) moving with a velocity of 320 m/s is capable to produce in the pulse-generating coil a current pulse with amplitude of 1.4 kA and a full-width at half-maximum 80 mus.
Authors: Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M
PDF: https://aip.scitation.org/doi/abs/10.1063/1.1480478
Abstract: The action of transverse shock waves (the shock wave propagates across the magnetization vector M) on the magnetic phase state of a Nd2Fe14B high-energy hard ferromagnetic was investigated experimentally. The design of the ferromagnetic sample, which was made as a hollow cylinder, has made it possible to dramatically reduce the amount of the explosive that initiates a transverse shock wave in Nd2Fe14B to 1.0 g (for Nd2Fe14B samples weighing 67.5 g). The results of the experiment have shown that the transverse shock wave propagating through Nd2Fe14B causes hard ferromagnetic-to-paramagnetic phase transformation terminating by practically complete demagnetization of Nd2Fe14B. Pulse generators based on the transverse shock wave demagnetization of hollow cylindrical Nd2Fe14B samples with diameter of 25.4 mm and length of 19.1 mm are capable of producing high-voltage pulses [peak voltage of 11.3 kV, full width at half maximum (FWHM) of 4.5 mus] and high-current pulses (peak current of 1.93 kA, FWHM of 100 mus, peak power of 27.0 kW). The effect of transverse shock wave demagnetization of high-energy hard ferromagnetic, Nd2Fe14B, was detected. (C) 2002 American Institute of Physics.
Authors: Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M
PDF: https://aip.scitation.org/doi/10.1063/1.1487894
Abstract: A new type of explosive driven high-current pulsed source utilizing a shock wave demagnetization of a Nd2Fe14B hard ferromagnetic energy carrier was developed. The design of the ferromagnetic energy carrier, which was made a hollow cylinder, has made it possible to reduce dramatically to 1 g the amount of the explosive providing a complete demagnetization of Nd2Fe14B energy carrier of weight 64 g. The developed generator is capable of producing high-current [up to 1.9 kA, 100 mus full width at half maximum (FWHM)] and high-power pulses (up to 42 kW, 2.8 mus FWHM). (C) 2002 American Institute of Physics.
Authors: Choi, EH; Ahn, JC; Moon, MW; Jung, Y; Choi, MC; Seo, Y; Cho, G; Uhm, HS; Tachibana, K; Whang, KW; Kristiansen, M
PDF: https://aip.scitation.org/doi/10.1063/1.1518772
Abstract: The correlation between the vacuum ultraviolet (VUV) luminous efficiency and the plasma ion density has been investigated in terms of the xenon mole fraction in the neon filling gas for alternating current plasma display panels. The VUV luminous efficiency and plasma ion density are found to have strong correlation and to be saturated at xenon mole fractions greater than 7% and gas pressures of 400 Torr. (C) 2002 American Institute of Physics.
Authors: Li, SH; Wunsch, DC; O'Hair, E; Giesselmann, MG
Abstract: This paper examines and compares regression and artificial neural network models used for the estimation of wind turbine power curves. First, characteristics of wind turbine power generation are investigated. Then, models for turbine power curve estimation using both regression and neural network methods are presented and compared. The parameter estimates for the regression model and training of the neural network are completed with the windfarm data, and the performances of the two models are studied. The regression model is shown to be function dependent, and the neural network model obtains its power curve estimation through learning. The neural network model is found to possess better performance than the regression model for turbine power curve estimation tinder complicated influence factors.
Authors: A. Neuber; J. Dickens; J. B. Cornette; K. Jamison; E. R. Parkinson; M. Giesselmann; P. Worsey; J. Baird; M. Schmidt; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=940950
Abstract: A variety of basic magnetic flux compression (MFC) generator geometries have been tested during the last three decades. Though size and operating regimes differ widely, it is apparent that the helical flux compression generator is the most promising concept with respect to current amplification and compactness. Though the geometry of the helical generator (dynamically expanding armature in the center of a current carrying helix) seems to be basic, it turns out that the understanding of all involved processes is rather difficult. This fact is apparent from the present lack of a computer model that is solely based on physical principles and manages without heuristic factors. A simple generator was designed to address flux and current losses of the helical generator. The generator's maximum current amplitude is given as a function of the seed current and the resulting "seed-current" spread is compared to the output of state-of-the-art computer models. Temporally resolved current and current time derivative signals are compared as well. The detailed generator geometry is introduced in order to facilitate future computer code bench marking or development. The impact of this research on the present understanding of magnetic flux losses in helical MFC generators is briefly discussed.
Authors: Giesselmann, M; Eccleshall, D
PDF: https://ieeexplore.ieee.org/document/911805
Abstract: A PSpice (R) model for a 4-phase, 4-pole air core compulsator was created. With minor modifications it could also model the performance of other pulse alternators. In addition, models for a railgun and switching elements have been created. The models were used to evaluate the system performance of the compulsator and an attached railgun in various configurations with diode rectifiers and SCR's. The process of self-excitation of the machine was also studied. The paper is focusing on the basic theory behind the models as well as techniques for numerical implementation, In addition generic results using normalized parameters are given.
Authors: Jiang, WH; Kristiansen, M
PDF: https://aip.scitation.org/doi/10.1063/1.1382643
Abstract: A one-dimensional analytical theory is developed to describe the behavior of the virtual cathode oscillator. Theoretical predictions for the microwave frequency and efficiency have been obtained. The results suggested that the feedback of the microwave field to the virtual cathode is very important for the microwave efficiency which, under optimum conditions, can be as high as 30%-40%. In addition, the dependence of the microwave efficiency on the electron-beam energy spread and the electron lifetime was determined. The threshold value of the cavity Q, which is necessary to enhance the microwave field, is also estimated. (C) 2001 American Institute of Physics.
Authors: Li, SH; Wunsch, DC; O'Hair, EA; Giesselmann, MG
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=937208
Abstract: This paper uses data collected at Central and South West Services Fort Davis wind farm to develop a neural network based prediction of power produced by each turbine. The power generated by electric wind turbines changes rapidly because of the continuous fluctuation of wind speed and direction. It is important for the power industry to have the capability to perform this prediction for diagnostic purposes-lower-than-expected wind power may be an early indicator of a need for maintenance. In this paper, characteristics of wind power generation are first evaluated in order to establish the relative importance for the neural network. A four input neural network is developed and its performance is shown to be superior to the single parameter traditional model approach.
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.
Authors: M. Giesselmann; T. Heeren; E. Kristiansen; J. G. Kim; J. C. Dickens; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=901200
Abstract: The pulsed power conditioning system (PPCS) is one of the key enabling technologies for using the energy output of a magnetic flux compression generator (MFCG). This paper shows the results of comprehensive experimental studies of an inductive energy storage system using an exploding wire fuse. The effects of metal oxide varistors (MOVs) for use as pulse-shaping devices are also presented. The experimental results are complemented by a comprehensive evaluation and interpretation of the results using the tools available in the professional version of MathCAD.
Authors: Shkuratov, SI; Kristiansen, M; Dickens, JC; Hatfield, LL; Horrocks, E
PDF: https://ieeexplore.ieee.org/document/901242
Abstract: Three types of resistors have been tested to determine maximum usable power at pulsed high,voltage and pulsed high current. Experiments ere carried out using high-voltage cable generators, spark-gap generators, and thyratron drivers, Pulse durations were varied from 0.7 mus to 21 mus The pulse amplitudes were varied from 1 kV to 35 kV. The peak cm rent reached was 3 kA. Metal him, carbon film, and carbon composition resistors of four different rated powers (0.25 W, 0.5 W, 1 W, and 2 W) have been tested. Data are given for the limiting pulsed power and energy for each type of resistor in nanosecond and microsecond time ranges. The experimental investigation of the threshold loading of the resistors in the high-current pulsed mode and in the high-voltage pulsed mode has shown that the process of destruction of resistors has specific features for each mode. The mechanisms of failure and destruction of resistors under the action of high-voltage and high-current pulses are discussed.
Authors: D. Hemmert; A. A. Neuber; J. Dickens; H. Krompholz; L. L. Hatfield; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=887650
Abstract: Microwave window breakdown in vacuum is investigated for an idealized geometry, where a dielectric slab is located in the center of a rectangular waveguide with its normal parallel to the microwave direction of propagation. An S-band resonant ring with a frequency of 2.85 GHz and a power of 60 MW is used. With field enhancement tips at the edges of the dielectric slab, the threshold power for breakdown is observed to be dependent on the direction of the microwaves; i.e., it is approximately 20% higher for the downstream side of the slab than it is for the upstream side. Simple trajectory calculations of secondary electrons in an RF field show a significant forward motion of electrons parallel to the direction of microwave propagation. Electrons participating in a saturated secondary avalanche on the upstream side are driven into the surface, and electrons on the downstream side are driven off the surface, because of the influence of the microwave magnetic field. In agreement with the standard model of dielectric surface flashover for dc conditions (saturated avalanche and electron-induced outgassing), the corresponding change in the surface charge density is expected to be proportional to the applied breakdown threshold electric field parallel to the surface.
Authors: Neuber, AA; Dickens, JC; Krompholz, H; Schmidt, MFC; Baird, J; Worsey, PN; Kristiansen, M
PDF: https://ieeexplore.ieee.org/document/823591
Abstract: Explosively driven magnetic flux compression (MFC) has been object of research for more than three decades, Actual interest in the basic physical picture of flux compression has been heightened by a newly started Department of Defense (DoD) Multi-University Research Initiative. The emphasis is on helical flux compression generators comprising a hollow cylindrical metal liner filled with high explosives and at least one helical coil surrounding the liner. After the application of a seed current, magnetic flux is trapped and high current is generated by moving, i.e., expanding, the liner explosively along the finding of the helical coil, Several key factors involved in the temporal development can be addresses by optical diagnostics. 1) The uniformity of liner expansion is captured by framing camera photography and supplemented by laser illuminated high spatial and temporal resolution imaging. Also, S-ray flash photography is insensitive to possible image blur by shockwaves coming from the exploding liner, 2) The thermodynamic state of the shocked gas is assessed by spatially and temporally resolved emission spectroscopy, 3) The moving liner-coil contact point is a possible source of high electric Losses and is preferentially monitored also by emission spectroscopy, Since optical access to the region between liner and coil is not always guaranteed, optical fibers can be used to extract light from the generator. The information so gained will give, together with detailed electrical diagnostics, more insight in the physical loss mechanisms involved in MFC.
Authors: S. I. Shkuratov; M. Kristiansen; J. C. Dickens; L. L. Hatfield; R. Martin
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=901196
Abstract: Results are presented of an experimental study of the generation of high-voltage and high-current pulses in generators designed as open and closed ferromagnetic circuits. Experiments were carried out using a light gas gun system. The magnetic projectiles were composed of ferromagnetic disks having 1.27- and 2.54-cm diameters. It has been shown that with velocities of the magnetic projectiles of 200-380 m/s, the peak voltage of the pulses produced by the generators reach several tens of kilovolts, peak current reaches kiloampere, and the energy delivered at the load is a few Joules. Generating modules connected in series will make it possible to produce a high-energy pulse with a peak voltage of a few hundred kilovolts. It has been shown that a closed ferromagnetic circuit generator is capable of generating not only single high-voltage pulses, but also repetitive oscillations. Data are given for the effects on the amplitude of high-voltage pulses caused by the length and velocity of the ferromagnetic projectiles and the design of the generating unit for both high-voltage and high-current modes of pulsed power generation.
Authors: L. C. Farrar; D. P. Haack; S. E. McGrath; J. C. Dickens; E. A. O'Hair; J. A. Fralick
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=842896
Abstract: The effectiveness of the decontamination of biological agents (spores) on surfaces by two thermal plasma systems is reported. Using existing systems, operating at nonoptimum conditions, a steam plasma decontaminated surfaces at a maximum speed of 1.4 mph and a nitrogen plasma decontaminated at a maximum of 2.4 mph.
Authors: Neuber, AA; Butcher, M; Krompholz, H; Hatfield, LL; Kristiansen, M
PDF: Not https://ieeexplore.ieee.org/document/825505
Abstract: Results of high-speed electrical and optical diagnostics are used as a basis to discuss a new surface flashover model. Outgassing, caused by electron stimulated desorption, is found to play a crucial role in the temporal flashover development. Dielectric unipolar surface flashover under vacuum is experimentally characterized buy a three-phase development, which covers a current range from 10(-4) A to 100 A. Phase one comprises a fast (several nanoseconds) buildup of a saturated secondary electron avalanche reaching current levels of 10 to 100 mA. Phase two is associated with a slow current amplification reaching currents in the Ampere level within typically 100 ns, The final phase is characterized by a fast current rise up to the impedance-limited current on the order of 100 A. The development during phase tno and three is described by a zero-dimensional model, where electron-induced outgassing leads to a Townsend-like gas discharge above the sur face. This is supported by time-resolved spectroscopy that reveals the existence of excited atomic hydrogen and ionic carbon before the final phase. The feedback mechanism toward a self-sustained discharge is due to space charge leading to an enhanced field emission from the cathode. A priori unknown model parameters, such as outgassing rate and gas density buildup above the surface, are determined by fitting calculated results to experimental data. The significance of outgassing is also discussed with a view to microwave surface flashover.
Authors: Neuber, A; Butcher, M; Hatfield, LL; Kristiansen, M; Krompholz, H
PDF: https://ieeexplore.ieee.org/document/788752
Abstract: Cryogenic components in high power electrical systems and in power electronics gain more and more importance. The behavior of insulators for cryogenic conditions, however, is virtually unknown. In a fast coaxial setup, dielectric test sample and electrodes in vacuum are cooled to <100 K and flashover is characterized using fast electrical and optical diagnostics. Three consecutive development stages for flashover in self-breakdown mode with a gap distance of 0.5 cm can be distinguished: (1) A fast current rise to mA amplitudes within similar to 2 ns, probably associated with field emission, followed by (2) a slow current rise to similar to 5 to 10 A amplitude with duration of 40 ns to 1 mu s , associated with secondary emission avalanche saturation, and (3) a transition to a rapid gaseous ionization above the sample caused by electron induced outgassing, leading to impedance-limited current amplitudes of less than or equal to 300 A. Phase (1) shows a higher final current at lower temperature, which is probably due to a higher initial velocity of the secondary electrons, the duration of phase (2) is a decreasing function of breakdown voltage and only slightly dependent on temperature, which points to a weak temperature dependence of the outgassing process. Flashover potentials show a slight increase at lower temperature.
Authors: Jiang, WH; Dickens, J; Kristiansen, M
PDF: https://ieeexplore.ieee.org/document/799837
Abstract: The microwave field intensity around the virtual cathode oscillator was enhanced by using a microwave reflector in the output waveguide. The experimental results show that the microwave output power strongly depends on the position and geometry of the microwave reflector. The maximum microwave efficiency obtained was twice as large as that without field enhancement by the microwave reflector.
Authors: W. Jiang; K. Woolverton; J. Dickens; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=799836
Abstract: A new type of virtual cathode oscillator, the coaxial vircator, was studied analytically and experimentally. A one-dimensional analytical model was used to describe the steady-state behavior of the electron beam and the virtual cathode, from which the diode current, the space-charge limited current, the virtual cathode position, and the estimated oscillation frequency were obtained. The experiments were carried out with typical electron-beam parameters of 500 kV, 40 kA, and 30 ns, where pulsed microwaves of 400 MW in peak power and 2 GHz in frequency have been obtained. The energy efficiency from the electron beam to microwaves was /spl sim/2%. This efficiency is expected to be improved by increasing the microwave field strength around the vircator.
Authors: A. Neuber; D. Hemmert; J. Dickens; H. Krompholz; L. L. Hatfield; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=763092
Abstract: Using two gated intensified digital charge-coupled device cameras, one sensitive in the near infrared to ultraviolet region and one in the soft X-ray region, the temporal development of high-power microwave-induced surface flashover across a vacuum/dielectric interface has been imaged. The emission of X-ray radiation from the interface is caused by field emitted electrons accelerated in the high electromagnetic field impacting the solid. This generation of bremsstrahlung terminates at the moment of full flashover development that is indicated by the optical light emission. A rising plasma density above the dielectric surface due to electron induced outgassing triggers this behavior.
Authors: Neuber, A; Hemmert, D; Krompholz, H; Hatfield, L; Kristiansen, M
PDF: https://aip.scitation.org/doi/10.1063/1.370953
Abstract: A simple model of vacuum/dielectric/vacuum interface breakdown initiation caused by high power microwave has been developed. In contrast to already existing models, a spatially varying electron density normal to the interface surface has been introduced. Geometry and parameter ranges have been chosen close to the conditions of previously carried out experiments. Hence, physical mechanisms have become identifiable through a comparison with the already known experimental results. It is revealed that the magnetic field component of the microwave plays an important role. The directional dependence introduced by the magnetic field leads to a 25% higher positive surface charge buildup for breakdown at the interface downstream side as compared to the upstream side. This and the fact that electrons are, in the underlying geometry, generally pulled downstream favors the development of a saturated secondary electron avalanche or a saturated multipactor at the upstream side of the dielectric interface. The previously observed emission of low energy x-ray radiation from the interface is explained by bremsstrahlung generated by impacting electrons having initially a higher energy than the average emission energy. Final breakdown is believed to be triggered by electron induced outgassing or evaporation, generating a considerable gas density above the dielectric surface and eventually leading to a gaseous breakdown. (C) 1999 American Institute of Physics. [S0021-8979(99)09315-9].
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.
Authors: A. Neuber; J. Dickens; D. Hemmert; H. Krompholz; L. L. Hatfield; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=700757
Abstract: Physical mechanisms leading to microwave breakdown on windows are investigated for power levels on the order of 100 MW at 2.85 GHz. The test stand uses a 3-MW magnetron coupled to an S-band traveling wave resonator. Various configurations of dielectric windows are investigated. In a standard pillbox geometry with a pressure of less than 10/sup -6/ Pa, surface discharges on an alumina window and multipactor-like discharges starting at the waveguide edges occur simultaneously. To clarify physical mechanisms, window breakdown with purely tangential electrical microwave fields is investigated for special geometries. Diagnostics include the measurement of incident/reflected power, measurement of local microwave fields, discharge luminosity, and X-ray emission. All quantities are recorded with 0.21-ns resolution. In addition, a framing camera with gating times of 5 ns is used. The breakdown processes for the case with a purely tangential electric field is similar to DC flashover across insulators, and similar methods to increase the flashover field are expected to be applicable.
Authors: Liu, P; Bahadur, S; Verhoeven, JD; Gibson, ED; Kristiansen, M; Donaldson, A
PDF: https://www.sciencedirect.com/science/article/abs/pii/S0043164896073942?via%3Dihub
Abstract: The are erosion behavior of Cu-15%Nb and Cu-15%Cr in situ composites was studied for both low-energy make-and-break contact and a high-energy stationary arcing gap configuration. For low-energy make-and-break contacts, a computerized test set-up was developed, while the high-energy pulsed power stationary arcing tests were performed in the Mark VI facility at Texas Tech University, Lubbock, TX. The study dealt with variation in contact resistance for make-and-break contacts, are erosion at both energy levels, and materials response to are erosion. The surface films formed in the make-and-break operation were analyzed by an X-ray diffraction technique, and the eroded surfaces and are erosion mechanisms were studied by scanning electron microscopy. It was concluded that in low-energy contacts, oxidation was the major cause of deterioration of electrical contacts, while melting was the major failure mode in high-energy contacts. The contact resistance of Cu-15%Nb was much lower than that of Cu-15%Cr. The are erosion resistance of Cu-15%Nb and Cu-15%Cr was higher than that of the commercially used Cu-W composite in stationary are erosion tests. (C) 1997 Elsevier Science S.A.
Authors: Hegeler, F; Krompholz, HG; Hatfield, LL; Kristiansen, M
PDF: https://ttu-ir.tdl.org/handle/2346/20438
Abstract: Surface flashover on insulators under UV irradiation or with a plasma background was investigated with high-speed electrical and optical sensors in order to clarify differences in the breakdown development compared to the pure vacuum case, Results with a plasma background show a more rapid development in the breakdown initiation compared to measurements in vacuum with no plasma, With a magnetic shielding technique using permanent magnets, the duration of an applied voltage pulse can be increased by a factor of 2-3 without causing flashover. UV illumination on the electrodes decreases the flashover voltage (for the de case) or the voltage pulse duration without breakdown (for the pulsed case), whereas UV illumination on the dielectric surface increases the flashover potential.
Authors: Giesselmann, M
Abstract: This paper presents the dynamic model of a 3-phase synchronous machine using the graphical user interface provided by the PSpice program environment. The machine model includes the effects of the damper winding and salient poles. The paper presents results of the dynamic behavior of a salient pole synchronous machine including voltage, current, torque, and power traces. The model is organized in a hierarchical structure and includes custom symbols for the machine several functional modules like reference frame transformation modules. The custom symbols have been created within the Schematics editor of the PSpice program group. Details of the implementation as well as the theoretical equations are presented. The model that is shown in this paper is working on the free evaluation version of the PSpice software, Release 6.2a.
Authors: Palsule, C; Liu, S; Gangopadhyay, S; Holtz, M; Lamp, D; Kristiansen, M
PDF: https://www.sciencedirect.com/science/article/abs/pii/S0927024897000044
Abstract: We have investigated the photovoltage and photocurrent spectra of crystalline silicon/porous silicon heterojunctions. The porous silicon layers were prepared using anodic etching of p-type crystalline silicon at a current density of 25 mA/cm(2). From the spectral dependence of the photovoltage and photocurrent, we suggest that the photovoltaic properties of the junction are dominated by absorption in crystalline silicon only. We have also studied the effect of increase in the thickness of porous silicon layers on these spectra. We find that the open-circuit voltage of the devices increases, but the short-circuit current decreases with an increase in the thickness of the porous silicon layers. We propose a qualitative explanation for this trend, based on the increase in the series and the shunt resistance of these devices. The effect of hydrogen passivation on the junction properties by exposing the devices to hydrogen plasma is also reported.
Authors: ENGEL, TG; WESTER, SL; KRISTIANSEN, M; DONALDSON, AL
PDF: https://ieeexplore.ieee.org/document/364606
Abstract: The mechanisms responsible for the erosion and degradation of electrode and insulator materials in high current are discharge environments are reviewed, The review represents the experimental results obtained from several studies and are from investigations into materials performance in high current closing switches such as spark gaps and surface discharge switches, Parameters of interest that affect electrode and insulator material erosion and degradation include peak current, charge transfer, mass erosion, the surface voltage holdoff recovery, and are velocity, Other parameters that have been shown to affect materials performance include the synergistics produced by certain electrode, insulator, and gas combinations, Models that describe the erosion and degradation processes are presented and compared to the experimental results,
Authors: JIANG, WH; ZINSMEYER, K; LESS, M; SCHOENBACH, KH; KRISTIANSEN, M
PDF: https://ieeexplore.ieee.org/document/514049
Abstract: Results of electron-beam controlled switching experiments with switch samples of quartz crystal and polycrystalline zinc selenide (ZnSe) are presented. For switch samples of both materials, drastic reductions of the switch resistance were induced by the electron beam. The quartz sample showed very fast temporal response (less than 1 ns) with potential applicability for current control. The ZnSe samples, on the other hand, showed longer current transients (on the order of 10 ns) with exponential development of the switch resistance after the electron beam pulse.
Authors: T. G. Engel; J. C. Dickens; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=195742
Abstract: Several polymeric insulator materials commonly used as sidewall insulators in electromagnetic accelerators were subjected to repetitive (from approximately 0.1 to 1 discharge per second), high current (from approximately 100 to 300 kA peak or approximately 100 to 300 kA/cm), transient ( approximately 20 mu s pulse width) surface discharges. The insulator materials tested include the thermosetting polymers G-9, G-10, and G-11 (i.e., fiberglass reinforced melamine and epoxy) and the thermoplastic polymers Lexan (i.e., polycarbonate) and Delrin (i.e., polyacetyl). Empirical scaling relationships are given that relate the amount of insulator and electrode (i.e., molybdenum) mass erosion to the total amount of arc energy transferred. Scaling relationships are also given for the lifetime of the given polymer as a function of the initial discharge current. The lifetime of an insulator material is defined as the number of discharges required to reduce the initial surface holdoff voltage to its half-power level for three consecutive discharges and is a useful parameter when specifying insulator materials to be used in high-power switching devices.<
Authors: LOREE, D; GIESSELMANN, M; KRISTIANSEN, M; LARSON, D
PDF: https://www.osti.gov/biblio/5708171
Abstract: Project Hercules is a project to improve ignitron switches which will then be used on the upgrade of Lawrence Livermore's Nova Laser for their ICF program. The goals of Hercules, which stands for High Energy Research Concerning the Ultimate Lifetime of Experimental Switches, are to lifetime test (up to 10,000 shots) prototype ignitrons or other switches with the required Nova current and coulomb parameters (300 kA, 200 C), recommend design changes, and retest the second generation switches. This report describes the design and construction of the test circuit and necessary diagnostics. The details of the design and construction of a 0.5 MJ electrolytic capacitor bank and a semi-automatic diagnostic/ control system are described. The required test run data include peak current and corresponding tube voltage for every shot, entire current and voltage waveforms every few shots, and ignitor resistance values every few shots. Additionally, the conversion of a 120 kW, 12 kV constant voltage supply to an 8 A constant current supply with the use of six SCRs and a commercial control board will be described. The final results of this project will be lifetime data at high current and high coulomb for and improvements on some of the best of the new generation of pulsed power switches.
Authors: LEHR, M; KORZEKWA, R; KROMPHOLZ, H; KRISTIANSEN, M
PDF: https://aip.scitation.org/doi/10.1063/1.350720
Abstract: The influence of magnetic fields (both dc and pulsed) on dielectric surface breakdown in vacuum and simulated low-earth-orbit conditions has been investigated using pulsed test voltages. Predictions from the saturated secondary electron emission avalanche breakdown model and the experimental results both show magnetic insulation effects (i.e., an increase in flashover voltage) at magnetic-field amplitudes as low as 0.1 T. The most favorable configuration for magnetic insulation is with the magnetic field oriented parallel to the insulator surface and perpendicular to the electric field. An increase in flashover voltage with increasing magnetic field is seen when the vector E x B points away from the surface, while a decrease followed by an increase in flashover voltage is seen for E x B into the surface. The magnitude of the insulation effect depends on the dielectric material, ambient pressure, surface roughness, and the presence of background plasma. Predictions from single-particle computer simulations of the secondary electron avalanche process, using nonuniform fields, point to the importance of conditions at the cathode in producing magnetic insulation effects. It was found that it is sufficient to apply the magnetic field in the cathode region only, and that significant magnetic insulation effects can be observed using small, lightweight permanent magnets. An applied magnetic field will also increase the flashover voltage in a low-density (n(e) almost-equal-to 10(4) cm-3) plasma environment. The dependence of the flashover voltage on electrode separation (gap distance) is observed to remain sublinear with the application of an insulating magnetic field. Prebreakdown luminance measurements are presented which further support the saturated secondary electron emission avalanche model.
Authors: ENGEL, TG; KRISTIANSEN, M; OHAIR, E; MARX, JN
PDF: https://ieeexplore.ieee.org/abstract/document/101089
Abstract: The rates of erosion and voltage holdoff degradation are critical parameters when selecting insulator materials that will be used in pulsed power devices such as spark gaps, surface discharge switches, and electromagnetic launchers. This investigation is concerned with modeling the erosion and holdoff degradation performance of various commercially available polymeric and ceramic insulators. The insulators are tested on a surface discharge switch at approximately 300 kA in atmospheric air. Test diagnostics include the surface voltage holdoff recovery and the eroded mass loss of the insulator and electrode materials used. The ceramic materials which were tested include several types of aluminum and magnesium silicates, several alumina and zirconia composities, and aluminum and silicon nitride. The polymeric insulators include polyvinyl chloride, low and high molecular weight polyethylene, polytetrafluoroethylene, polyamide, acetyl, polyamide-imide, and several types of glass-reinforced epoxies, melamines, and phenolics. The test results indicate that the holdoff degradation resistance and erosion rates can be qualitatively predicated by the use of merit figures which are based upon the thermo-chemical properties of the insulator. The results also show that the holdoff degradation and erosion rates can be improved for some thermoset polymers by a suitable choice of electrode material and/or by the U.V. stabilization of the insulator.
Authors: ENGEL, TG; KRISTIANSEN, M; KROMPHOLZ, H
PDF: https://ieeexplore.ieee.org/document/108440
Abstract: Calculations of the arc channel radius using the arc current as the independent variable are presented and discussed. Previously reported experimental results for the expansion of approximately 300 to approximately 3000 A discharges in 460-torr hydrogen are compared with the theoretical radius calculations presented here. Errors introduced in the calculated channel radius are investigated by first assuming the channel conductivity to vary according to the Spitzer conductivity, and then assuming the channel conductivity to be constant and comparing the numerical results of these two methods with the experimentally measured values of channel radius. It is shown that according to Hugoniot adiabatics, the apparent relationship between the arc channel pressure and the directed energy of the shock front is given by the pressure-velocity relationship of the gas leaving the shock front instead of the more commonly used pressure-velocity relationship for the gas entering the shock front. This result is a direct consequence of the channel boundary being defined by the shock front. The scaling relationship for the channel conductivity as a function of the circuit parameters is also reported.
Authors: GANGOPADHYAY, S; PLEIL, M; BORST, W; YOUNG, C; KRISTIANSEN, M
PDF: https://www.sciencedirect.com/science/article/abs/pii/S0022309305802558
Abstract: Microwave excited electron cyclotron resonance (ECR) plasmas were used to produce five a-SiC:H films of varying relative carbon content. This was achieved by adjusting the relative hydrogen gas/organic liquid source flow rates and resulted in optical gaps between 2.3eV and 3.3eV. A time-correlated single photon counting technique incorporating a picosecond laser was used to acquire the fast photoluminescence decay emitted at several wavelengths for each sample. A lifetime distribution analysis of the photoluminescence decay reveals the presence of up to four distinct components. These resolved decay times range from 80ps to over 20ns. Each component decay has its own emission spectrum. The blue component has the fastest and the red the slowest decay. We present the variation of photoluminescence lifetime distributions with deposition parameters. We also present continuous wave (cw) and absorption photoluminescence spectra.
Authors: LOREE, DL; GIESSELMANN, M; KRISTIANSEN, M; SHULSKI, A
PDF: https://ieeexplore.ieee.org/document/101020
Abstract: The development of high power ignitrons with peak current ratings of up to 1000 kA and simultaneous charge transfer rates of 250-500 C is currently under way in a joint effort between Texas Tech University (TTU), Lawrence Livermore National Laboratory (LLNL) and industry. Research at TTU is concentrated on plasma diagnostics, novel anode designs, electrode placements and trigger schemes. 1,2,3 Electrical measurements as well as optical plasma studies such as high speed framing photography, Mach-Zehnder interferometry and spectroscopy have been performed. The cooperative efforts have lead to the development of a new commercial tube (Richardson Electronics NL-9000). This paper describes plasma diagnostics performed on a demountable ignitron (DIG) which provides optical access to the discharge plasma through four viewports, two of which are on opposite sides on a common optical axis. The latter pair of viewports was used to perform plasma density studies using a Mach-Zehnder interferometer. Time resolved recordings of the interference patterns, either visually or electronically, during changes in plasma behavior (such as current conduction or plasma heating) yield time resolved information about the particle density This technique was applied to the demountable ignitron during high current discharges. The light source was a 2W CW argon laser which was pulsed using a ferroelectric liquid crystal light valve. The resulting fringe patterns were recorded with a mechanical high speed camera. In the paper all experimental details, results and a theoretical evaluation are given. In addition, high speed framing photography was used to study the influence on electrode design and placement on the discharge plasma.
Authors: LOREE, DL; GIESSELMANN, MG; KRISTIANSEN, M; SHULSKI, AP; KIHARA, R
PDF: https://ieeexplore.ieee.org/document/201007
Abstract: The development of high-power ignitrons with peak current ratings of up to 1000 kA and simultaneous charge transfer rates of 250-500 C is currently under way in a joint effort between Texas Tech University (TTU), Lawrence Livermore National Laboratory (LLNL), and industry. Two industrial manufacturers, Richardson Electronics, US, and English Electric Valve, UK, have participated with TTU and LLNL in three workshops to advance the state of the art in high-power ignitrons. Less than three years after the start of the program, the cooperative efforts have led to the development of a new commercial tube (Richardson Electronics NL-9000). High power testing of prototypes of this tube and other unique ignitrons was done by Kihara at LLNL. Research at TTU is concentrated on plasma diagnostics, novel anode designs, electrode placements, and trigger schemes. Electrical measurements as well as optical and microwave plasma studies, such as high-speed framing photography, Mach-Zehnder and microwave interferometry, and spectroscopy have been performed. This paper describes the advances made in high-power ignitron switching capabilities in a comparison study between conventional Size D and Size E tubes, demountable experimental tubes and the new NL-9000 (Richardson Electronics). The paper shows the differences in tube design, and the associated peak current and charge transfer capabilities and lifetime expectancies. The critical design criteria are the anode shape and placement in order to control the plasma and prevent prefires. Tube failure modes and recent studies on alternate ignitor schemes are presented. In addition, results of plasma diagnostics performed on a demountable ignitron with optical access to the discharge plasma are shown. Time resolved images of the interference patterns from an optical Mach-Zehnder interferometer were recorded with a mechanical high-speed camera and evaluated with the help of image processing.
Authors: ENGEL, TG; KRISTIANSEN, M; BAKER, MC; HATFIELD, LL
PDF: https://ieeexplore.ieee.org/document/201000
Abstract: Many pulsed modulator applications require high-power, low-impedance closing switches. While the surface-discharge switch (SDS) is easily integrated into most systems (e.g., using a parallel-plate or coaxial geometry) and can easily be made into a low-impedance switch (e.g., operating in the multi-channel mode) most designers prefer and use other types of switches (e.g., spark gaps, thyratrons, ignitrons, etc.). This is because the SDS suffers from poor voltage holdoff recovery (caused by decomposition of the switching dielectric) and from dielectric punch-through (caused by dielectric erosion). Thus the selection of the switching dielectric is the critical factor which must be considered by the designer if the SDS is to have a long and trouble-free lifetime. This paper reports which dielectric properties are critical to designing a long-life SDS. Theory is correlated with experiment by evaluating the performance of a large group of polymeric and ceramic dielectrics. These dielectrics are tested in a single-channel, self-commutating SDS operating at approximately 35 kV and approximately 300 kA (oscillatory discharge) with a pulse length of approximately 20-mu-s (1/4 period approximately 2-mu-s). The performance of a dielectric is characterized by its shot-to-shot breakdown voltage and by its mass erosion. Theoretically, the voltage holdoff degradation resistance (HDR) and the arc melting/erosion resistance (AMR) of a dielectric can be qualitatively predicted from its formativity and its impulsivity, respectively. The formativity and impulsivity are figures of merit calculated from the known thermophysical properties of the dielectric. The effects produced in dielectric performance by choice of electrode material (e.g., molybdenum, graphite, and copper-tungsten) and discharge repetition rate are also discussed.
Authors: CALICO, SE; CRAWFORD, MT; KRISTIANSEN, M; KROMPHOLZ, H
PDF: https://aip.scitation.org/doi/10.1063/1.1142477
Abstract: The design, construction, and calibration of a probe to measure fast rise, short pulse currents is described. This probe is a 60-degrees section of a Rogowski coil that is designed to behave as a slow wave structure. The device described has nanosecond response and the output is proportional to the excitation current for approximately 70 ns.
Authors: KORZEKWA, R; LEHR, FM; KROMPHOLZ, HG; KRISTIANSEN, M
PDF: https://ieeexplore.ieee.org/document/201008
Abstract: The influence of low-amplitude magnetic fields, in a variety of configurations, on pulsed dielectric surface flashover has been investigated. These variations include dc magnetic fields; pulsed magnetic fields simulating conditions for magnetic self-insulation; and different environments (vacuum, ambient gas, plasma), geometries, dielectric materials, and orientations of the magnetic field. For field amplitudes of 0.3 T, typically a doubling of the flashover voltage is observed, if the E x B drift is away from the surface. For flashover in vacuum, it is sufficient to place permanent magnets in the cathode vicinity to increase the flashover voltage. The observations are consistent with the saturated surface secondary avalanche model and electron-induced gas desorption. The pulse shape of light emission during the prebreakdown phase depends on the orientation and amplitude of the magnetic field and shows the electron trajectories above the surface are altered by magnetic fields.
Authors: KRISTIANSEN, M; GUENTHER, AH; THOMPSON, JE
PDF: https://physicstoday.scitation.org/doi/abs/10.1063/1.881247?journalCode=pto
Authors: LEHR, FM; KRISTIANSEN, M
Authors: KORZEKWA, R; LEHR, FM; KROMPHOLZ, HG; KRISTIANSEN, M
Authors: HATFIELD, LL; BOERWINKLE, ER; LEIKER, GR; KROMPHOLZ, H; KORZEKWA, R; LEHR, M; KRISTIANSEN, M
Authors: DONALDSON, AL; ENGEL, TG; KRISTIANSEN, M
Authors: ADKINS, DL; GIESSELMANN, M; KRISTIANSEN, M
Authors: ENGEL, TG; DONALDSON, AL; KRISTIANSEN, M
Authors: GAHL, JM; ISHIHARA, O; HAGLER, MO; KRISTIANSEN, M
Authors: HATFIELD, LL; LEIKER, GR; KRISTIANSEN, M; COLMENARES, C; HOFER, WW; DICAPUA, MS
Authors: MIEDZINSKI, B; KRISTIANSEN, M
Authors: SCHAEFER, G; GIESSELMANN, M; PASHAIE, B; KRISTIANSEN, M
Authors: AKIYAMA, H; KRISTIANSEN, M; KROMPHOLZ, H; MAAS, B
Authors: KORZEKWA, R; SCHAEFER, G; KRISTIANSEN, M
Authors: AKIYAMA, H; WONG, KL; GAHL, J; KRISTIANSEN, M; HAGLER, M
PDF: https://iopscience.iop.org/article/10.1088/0741-3335/29/1/008/pdf
Authors: WONG, KL; ISHIHARA, O; GAHL, J; HAGLER, M; KRISTIANSEN, M
Authors: WONG, KL; HAGLER, M; KRISTIANSEN, M; ISHIHARA, O; AKIYAMA, H
PDF: https://iopscience.iop.org/article/10.1088/0029-5515/27/1/014/pdf
Authors: IKUTA, K; KRISTIANSEN, M; ROSE, MF
Authors: CURRY, RD; KRISTIANSEN, M; AGARWAL, VK; HATFIELD, LL; LEIKER, GR
Authors: IKUTA, K; KRISTIANSEN, M
PDF: https://inis.iaea.org/search/search.aspx?orig_q=RN:18042010
Authors: DONALDSON, AL; KRISTIANSEN, M; WATSON, A; ZINSMEYER, K; KRISTIANSEN, E; DETHLEFSEN, R
Authors: GAHL, J; ISHIHARA, O; WONG, KL; KRISTIANSEN, M; HAGLER, M
Authors: SCHAEFER, G; SCHOENBACH, KH; KRISTIANSEN, M; STRICKLAND, BE; KORZEKWA, RA; HUTCHESON, GZ
Authors: RANON, PM; KRISTIANSEN, M; LEHR, FM; HATFIELD, LL
Authors: WATSON, A; DONALDSON, AL; IKUTA, K; KRISTIANSEN, M
Authors: KOLARIK, WJ; LANDERS, TL; KRISTIANSEN, M
Authors: SCHOENBACH, KH; SCHAEFER, G; KRISTIANSEN, M; KROMPHOLZ, H; HARJES, HC; SKAGGS, D
Authors: AKIYAMA, H; GAHL, J; RATHBUN, K; KRISTIANSEN, M; HAGLER, M
Authors: GAHL, J; ISHIHARA, O; WONG, K; HAGLER, M; KRISTIANSEN, M
Authors: DONALDSON, AL; HAGLER, MO; KRISTIANSEN, M; HATFIELD, LL; NESS, RM
Authors: H. Akiyama; M. O. Hagler; M. Kristiansen
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4316380
Abstract: The dispersion relations for the compressional Alfvén waves in a two-ion species plasma of deuterium and hydrogen are calculated for a configuration which includes a vacuum layer between the cylindrical plasma and the conducting wall. The presence of the vacuum layer strongly affects the propagation of the compressional Alfvén wave, permitting some branches to propagate and penetrate the plasmacolumn over most frequencies in the ion-cyclotron range. Basic Alfvén-wave propagation and heating experiments in two-ion species consequently should be possible using tokamak and mirror devces with minor radii smaller than the Alfvén wavelength.
Authors: SCHOENBACH, KH; KRISTIANSEN, M; SCHAEFER, G
Authors: DONALDSON, AL; HAGLER, MO; KRISTIANSEN, M; JACKSON, G; HATFIELD, L
Authors: HARJES, CH; SCHOENBACH, KH; SCHAEFER, G; KRISTIANSEN, M; KROMPHOLZ, H; SKAGGS, D
Authors: KROMPHOLZ, H; DOGGETT, J; SCHOENBACH, KH; GAHL, J; HARJES, C; SCHAEFER, G; KRISTIANSEN, M
Authors: JACKSON, G; HATFIELD, L; KRISTIANSEN, M; HAGLER, M; MARX, J; DONALDSON, AL; LEIKER, G; CURRY, R; NESS, R; GORDON, L; JOHNSON, D
Authors: SCHAEFER, G; SCHOENBACH, KH; KROMPHOLZ, H; KRISTIANSEN, M; GUENTHER, AH
Authors: JACKSON, GL; HATFIELD, LL; KRISTIANSEN, M; MARX, J; BOWLING, A
Authors: SCHOENBACH, KH; SCHAEFER, G; KRISTIANSEN, M; HATFIELD, LL; GUENTHER, AH
Authors: MIEDZINSKI, B; KRISTIANSEN, M
Authors: GORDON, LB; KRISTIANSEN, M; HAGLER, MO; KIRBIE, HC; NESS, RM; HATFIELD, LL; MARX, JN
Authors: HARJES, HC; KUNHARDT, EE; KRISTIANSEN, M; HATFIELD, LL; GUENTHER, AH
Authors: TZENG, YH; KUNHARDT, EE; KRISTIANSEN, M; GUENTHER, AH
Authors: COLEMAN, PD; BLACKWELL, BD; KRISTIANSEN, M; HAGLER, MO
Authors: MCDONALD, K; NEWTON, M; KUNHARDT, EE; KRISTIANSEN, M; GUENTHER, AH
Authors: DRUCE, R; KRISTIANSEN, M; HAGLER, MO
Authors: THOMPSON, JE; LIN, J; MIKKELSON, K; KRISTIANSEN, M
Authors: THOMPSON, JE; LIN, J; MIKKELSON, K; KRISTIANSEN, M
Authors: HARJES, HC; SCHONBACH, KH; KRISTIANSEN, M; GUENTHER, AH; HATFIELD, LL
Authors: COLEMAN, PD; BLACKWELL, BD; KRISTIANSEN, M; HAGLER, MO
Authors: CHU, E; DRUCE, R; KRISTIANSEN, M; HAGLER, M; BENGTSON, R
PDF: https://www.researchgate.net/publication/45704079_BEAT_HEATING_IN_PLASMAS_USING_CO2_LASERS
Authors: MIKKELSON, K; KRISTIANSEN, M; LIN, J; THOMPSON, J
Authors: DOLLINGER, RE; KRISTIANSEN, M; HAGLER, MO
PDF: https://ieeexplore.ieee.org/document/4317177?arnumber=4317177
Authors: HATFIELD, LL; HARJES, HC; KRISTIANSEN, M; GUENTHER, AH; SCHONBACK, KH
Authors: COLEMAN, PD; BLACKWELL, BD; BECKERICH, SR; KRISTIANSEN, M; HAGLER, MO
PDF: https://www.sciencedirect.com/science/article/pii/B9781483283739500811
Authors: BURKES, TR; CRAIG, JP; HAGLER, MO; KRISTIANSEN, M; PORTNOY, WM
Authors: KNOX, SO; COLEMAN, PD; KRISTIANSEN, M; HAGLER, MO
Authors: SMITH, DL; KRISTIANSEN, M; HAGLER, MO
Authors: KNOX, SO; KIRBIE, HC; CROSS, RC; KRISTIANSEN, M; HAGLER, MO
Authors: DRUCE, RL; KRISTIANSEN, M; HAGLER, MO
Authors: KRISTIANSEN, M; HAGLER, MO
PDF: https://iopscience.iop.org/article/10.1088/0029-5515/16/6/011
Authors: THOMPSON, JE; KRISTIANSEN, M; HAGLER, MO
Authors: NUNNALLY, WC; KRISTIANSEN, M; HAGLER, MO
Authors: KNOX, SO; PAOLONI, FJ; KRISTIANSEN, M
Authors: NUNNALLY, WC; KRISTIANSEN, M; HAGLER, MO
Authors: MOLEN, GM; KRISTIANSEN, M; HAGLER, MO; BENGTSON, RD
Authors: NUNNALLY, WC; KRISTIANSEN, M; HAGLER, MO
Authors: MOLEN, GM; KRISTIANSEN, M; HAGLER, MO
Authors: HIPP, JE; KRISTIANSEN, M; HAGLER, MO
Authors: G. M. Molen; R. H. Trotter; M. Kristiansen; M. O. Hagler
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4320716
Abstract: The design of an inexpensive CO2 laser and some associated laboratory projects are described. The laser is easily constructed by undergraduate students. The experiments are designed to familiarize students with some characteristics and applications of CO2 lasers.
Authors: CATO, JE; WATSONMU.CN; HAGLER, MO; KRISTIANSEN, M
Authors: CATO, JE; HAGLER, MO; KRISTIANSEN, M
Authors: PORTER, WA; HAGLER, MO; KRISTIANSEN, M
Authors: MOLEN, GM; ROSELAND, LG; KRISTIANSEN, M; HAGLER, MO
Authors: HIPP, JE; KRISTIANSEN, M; HAGLER, MO
Authors: CATO, JE; HIPP, JE; KRISTIANSEN, M; HAGLER, MO
Authors: MELTON, RD; CATO, JE; KRISTIANSEN, M; HAGLER, MO
Authors: KRISTIANSEN, M; DOUGAL, AA
Authors: KRISTIANSEN, M; DOUGAL, AA
Authors: KRISTIANSEN, M; DOUGAL, AA
Authors: KRISTIANSEN, M; MELTON, JG; DOUGAL, AA
Authors: DODGE, NB; KRISTIANSEN, M; DOUGAL, AA
PDF: https://www.osti.gov/biblio/4479442
Abstract: The spatial waveform of the azimuthal electric field Eθ of the Stix coil is investigated. Methods are developed to improve the Eθ waveform by making it more nearly sinusoidal. Varying the spacing between individual coil turns in the coil section, along with the use of a ``squirrel cage'' type Faraday shield, is found to give a more nearly sinusoidal waveform. The effect of a number of Faraday shields on the Eθ waveform is also presented and it is found that certain types of shields, notably the single slit copper cylinder shield, introduce distortions into the accelerating (Eθ) field. The efficiency of these shields in reducing the longitudinal electric field Ez is reported. Several are found to be efficient Ez shields, except for a Faraday shield placed outside the Stix coil. The effect of Faraday shields on the equivalent circuit parameters of the Stix coil is also determined, and the variation of these parameters among the various shields is tabulated.
Authors: M. Kristiansen; L. G. Clark
PDF: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6373270
Abstract: Hamilton's Modified Principle (HMP) is used to solve two nonlinear problems. The amplitude, frequency, and stability in the limit cycle of a simple pentode oscillator are calculated in order to demonstrate the principle. A standard graphical method finds the amplitude of oscillations, and results are compared with experimental data. For the case under consideration, the HMP solution gives better agreement with the experimental results - 6.6% deviation as compared with 12.4% for the graphical solution. An approximate answer is also found for the transient behavior of a nonlinear system where the input is a step function. The result is compared with an analog computer solution and shows good agreement. The solution of a simple relay servo system is indicated in which the amplitude and frequency in the limit cycle along with its stability are obtained by application of the one method of HMP.