Research Issues in Developing Compact Pulsed Power ... - IEEE Xplore

Research Issues in Developing Compact Pulsed Power for High Peak Power Applications on Mobile Platforms JOHN A. GAUDET, MEMBER, IEEE, ROBERT J. BARKER, FELLOW, IEEE, C. JERALD BUCHENAUER, MEMBER, IEEE, CHRISTOS CHRISTODOULOU, FELLOW, IEEE, JAMES DICKENS, SENIOR MEMBER, IEEE, MARTIN A. GUNDERSEN, FELLOW, IEEE, RAVINDA P. JOSHI, SENIOR MEMBER, IEEE, HERMANN G. KROMPHOLZ, SENIOR MEMBER, IEEE, JUERGEN F. KOLB, MEMBER, IEEE, ANDRÁS KUTHI, MOUNIR LAROUSSI, SENIOR MEMBER, IEEE, ANDREAS NEUBER, SENIOR MEMBER, IEEE, WILLIAM NUNNALLY, EDL SCHAMILOGLU, FELLOW, IEEE, KARL H. SCHOENBACH, FELLOW, IEEE, J. SCOTT TYO, MEMBER, IEEE, AND ROBERT J. VIDMAR, MEMBER, IEEE Invited Paper

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 Manuscript received February 4, 2004; revised March 25, 2004. J. A. Gaudet, C. J. Buchenauer, C. Christodoulou, E. Schamiloglu, and J. S. Tyo are with the Electrical and Computer Engineering Department, University of New Mexico, Albuquerque, NM 87131-1356 USA (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). R. J. Barker is with the Air Force Office of Scientific Research, Arlington, VA 22203-1954 USA (e-mail: [email protected]). J. Dickens, H. G. Krompholz, and A. Neuber are with the Center of Pulsed Power and Power Electronics, Texas Tech University, Lubbock TX 74909-3102 USA (e-mail: [email protected]; [email protected]; [email protected]). M. A. Gundersen and A. Kuthi are with the University of Southern California, Los Angeles CA 90089-0271 USA (e-mail: [email protected]; [email protected]). R. P. Joshi, J. F. Kolb, M. Laroussi, and K. H. Schoenbach are with the Department of Electrical and Computer Engineering, Old Dominion University, Norfolk VA 23529-0246 USA (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). W. Nunnally is with the Electrical Engineering Department, University of Missouri, Columbia, MO 65211 USA, on leave from the Lawrence Livermore National Laboratory, Livermore, CA 94550-9234 USA (e-mail: [email protected]). R.J. Vidmar is with the Physics Department, University of Nevada, Reno, NV 89506 USA (e-mail: [email protected]). Digital Object Identifier 10.1109/JPROC.2004.829006

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. Keywords—Blumlein, compact pulsed power, electrical breakdown, fast switches, modulators, pseudospark switch.

I. INTRODUCTION The Multidisciplinary University Research Initiative (MURI) program on Compact, Portable, Pulsed Power (CP ) began in June 2001, as a three-year program with an option for the U.S. Department of Defense (DoD) to extend it for two additional years. As stated in the call for white papers and proposals from the DoD, “Future military systems could derive great benefit from the availability of compact, lightweight sources of pulsed electrical power for operation aboard aircraft, small ships, and/or airliftable ground vehicles” [1]. The primary objective of this MURI is to address the need of the DoD for practical and portable, compact pulsed power systems for directed energy by conducting

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Fig. 1. load.

Typical pulsed power system for a high-power microwave

fundamental research on the essential components inherent to the operation of a capacitive storage-based pulsed power system. Essential to these studies is a better understanding of the basic electrical properties of solid, liquid, and gaseous dielectrics. The expectation of significant advances in the DoD’s ability to field lighter weight and smaller pulsed power systems in the future will follow from the successes in this MURI. As a prelude to the MURI renewal decision, the DoD conducted a review in the fall of 2003 of the progress to date. These research results were presented in a topical fashion rather than by individual university projects (from six different universities) in order to put them in proper context during the review. This paper uses the same context to establish the status of the MURI and the plans for further studies in this specialized field of pulsed power. Using a specific example of a DoD-stated need, we consider high-power microwave (HPM) systems. They require compact, portable pulsed power in order to operate on mobile platforms [2]. Whereas the emphasis in HPM research in the past has primarily been on the microwave source, considerable attention is now being paid to the pulsed power requirements for portable platforms. Capacitive storage-based pulsed power systems are typically used as HPM source drivers (block 2 of Fig. 1). The requirements on such drivers include maintaining constant impedance (typically of the order of 10–100 ) at 0.1–1.0 MV for time scales on the order of several hundred nanoseconds. The need for maintaining constant impedance is in order to best match the load, which is typically an electron beam diode, a critical component of an HPM source. The CP research team has identified the areas of architecture, switching, increased energy density storage, and thermal management as the technical areas that members will focus on. Section II of this paper will discuss how our CP team is studying system architecture. By architecture, we mean the overall properties of the system that allow it to be compact, lightweight, and efficient. As described elsewhere in this issue, CP consists of energy storage, pulse shaping, transmission, and load matching components. Each of these subelements of the CP system must also be efficient to achieve the overall objective of compactness. We will show in this paper how geometry, materials selection, and modeling tools are important factors in conducting design studies to achieve the best possible outcome. Section III is devoted to the key technology of switching in CP . A critical component of any compact pulsed power

system is the switch. Depending upon the architecture of the CP system, opening and/or closing switches are required. Pulsed power switch technologies use solid-state, gas, and liquid phase media. Each has their own advantages and limitations as a switching medium for high-power CP applications. Our limited efforts in the solid-state area have concentrated on GaAs and SiC semiconductor devices. The latter material shows great promise for extending the power handling, electrical breakdown, and thermal characteristics of small devices for many applications, including HPM. Gas phase switches are a staple of high-power pulsed power. For compact applications, the pseudospark switch offers promising capabilities. Fundamental physics questions regarding measurement techniques in gas switches are being addressed to better understand the complex process of resistive-to-conductive phase change within the switch arc. Finally, liquid switches are also under review, particularly as part of compact pulser systems for laboratory uses. Due to their high dielectric strength, polar liquids are especially attractive as a switching medium for the design of small, low-inductance systems and, therefore, fast, high-power switches. In addition, the ability to flow the liquid in the switch volume allows for easy removal of debris after switch breakdown and facilitates effective thermal management. So far, our research has focused on the use of water and propylene carbonate as polar switching media. Whereas water is readily available, propylene carbonate offers the advantage of a lower freezing point, 55 C ( 67 F) for applications in a cold environment, such as what is faced on aircraft at high altitude. The issue of increased energy density storage is addressed in Section IV. In analyzing what technology is needed to produce a compact pulsed power system, it becomes obvious that the energy density of a given system is one of the dominant size factors. The tradeoff between high dielectric constant and large holdoff voltage has been identified as a major issue in the search for suitable dielectric materials with high energy density. Understanding the breakdown mechanisms of dielectrics has the potential of enabling the engineering of superior materials for use in pulse forming lines as well as advanced capacitor design. Inevitably, compact pulsed power systems will pose a significant challenge to conventional cooling techniques. Virtually every power-handling component (resistor, capacitor, inductor, active electronics, and conductors) has the waste heat removal problem as a detractor, and the role of thermal management is to alleviate this problem. Limitations imposed by mobile platforms further exacerbate the situation. One approach to thermal management under study in this MURI is high-fluence, convective cooling using microchannel heat sinks. Microchannel cooling exploits the high convective heat transfer coefficient in a liquid that flows in small-diameter ducts under turbulent flow conditions. Section V of this paper reviews the progress made in microchannel cooling for compact pulsed power applications. Section VI is devoted to a summary and discussion about how progress in the individual areas discussed in this paper

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will improve the DoD’s capabilities to deliver significant energy in rapid pulses using mobile and relatively compact components for their future requirements in applications, such as in directed energy systems. II. ARCHITECTURE When developing CP systems, a primary factor in identifying directions for component and materials development is the system architecture. In order to minimize the volume and weight of a pulsed power system, efficiency of the system is of prime importance. Thus, any architecture study must start with consideration of efficiency of the system. The system efficiency further requires that the components, each taken individually, also be efficient. The combination of energy storage and energy switching must be optimized for efficiency in the context of reasonable component count for reliability. For example, thin plastic films (used in pulsed forming lines and capacitors) have very large dielectric strengths (electric field limits), which translate into low voltage drops per layer. However, it is difficult to employ very thin films in high-voltage energy storage devices unless many thin layers are employed in series. A CP system is designed to deliver a quantity of energy at a specific voltage and current to a load over the duration of the pulse. In common pulsed power systems, energy is initially delivered from a prime power device, stored in an intermediate storage device, transferred to a pulse forming and voltage–current scaling device and then transferred to a load using multiple switches. From a volume, weight, and complexity point of view, it is always better to store the energy only once in a single storage device that also serves to form the pulse and scale the voltage–current to the parameters required by the load. Furthermore, the pulse-shaping device should also provide a relatively “square” pulse and be matched to the load such that the ratio of delivered pulse energy to stored energy approaches unity. Examples of devices that store, shape, and scale pulse energy in a single unit are the Marx circuit, stacked transmission line circuits, and other voltage vector inversion arrangements (these are discussed in greater detail in the classic paper by Charlie Martin1 ). The most voltage efficient (ratio of load voltage–charge voltage) is the stacked Blumlein transmission line circuit (which will simply be termed a “Blumlein” in this paper2 ) [3]. Note that all of the above circuits require a number of switches that must be closed at high voltages with precision timing. The tradeoff in designing these store, shape, and scale systems is the number of switches versus the allowable charge voltage. Usually, the number of switches required is the ratio of desired load voltage to the charge voltage. Here again, this number is determined by operational considerations. For example, the dc charge voltage is usually chosen based on the 1[Online] Available: http://www.ieee.org/organizations/pubs/proceedings/ 2Note that the U.S. patent was granted posthumously. For a biographical account of this engineer, see R. Burns, The Life and Times of A.D. Blumlein (London, U.K.: IEEE, 2000).

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Fig. 2. Folded Blumlein model showing an average electric field of 970 kV/cm–1.3 MV/cm in the straight section.

allowable voltage on the platform, which is further determined by the voltage level that leads to corona (undesirable arcing). For example, at 1 atm in dry air, this value is on the order of 20–30 kV. To use higher voltages, oil or high-pressure gas insulation must be used, which further increases the weight and complexity of the package. In a repetitive CP system, component efficiency is of even greater importance than in a single-shot system in that energy dissipated in the storage and switching components is additional energy, generated over and over again, that must be supplied by the prime power system, stored by the energy storage system, and removed by an auxiliary cooling system, increasing the overall mass. Therefore, it is imperative that these losses be minimized in order to realize a truly compact pulsed power system. Our approach in improving the architecture of compact pulsed power systems is, first of all, to apply the best available analysis techniques to the obvious geometry-driven issues at hand. For example, as discussed above, the Blumlein approach provides the highest ratio of load voltage to charge ). Further, if the Blumlein is voltage possible ( folded (or stacked), the overall effective length will be re, where is the duced by the amount of folds ( total length and is the number of folds, or bends). Where size is a constraint, and it often is in many applications, the folded Blumlein pulse forming line offers a unique way to provide the required pulsed power in the smallest possible package. However, there is a penalty for such an architecture. This design places difficult constraints on material electrical properties (conductors and insulators in close contact) and on electric field stresses due to the bends, themselves. We have undertaken a numerical study of these features and now report some of the results. A. Blumlein Modeling One component of this research was motivated directly by a test bed in use by Sandia National Laboratories (SNL), Albuquerque, NM [5]. This test bed, a folded Blumlein, was designed with HPM applications in mind and identified several problems. One problem was the existence of waveform disturbances attributed to “folds” in the structure, and another issue identified was associated with prebreakdown processes in the propylene carbonate [6]. (This latter issue is discussed in Section III of this paper.) The first problem was addressed in this MURI through computational modeling. Fig. 2 shows the implementation of the folded Blumlein model [7]. Initial comparisons of the pulse PROCEEDINGS OF THE IEEE, VOL. 92, NO. 7, JULY 2004

Fig. 3.

Effects of folds in a Blumlein transmission line on the voltage waveform at the load.

Fig. 4. Impact of edge effects at shorter pulsewidths. (a) A 750-mil Blumlein model. (b) Simulated output waveform.

characteristics were made for a straight Blumlein and a folded Blumlein, as shown in Fig. 3. In these cases, we used a lumped-parameter load model, which gives more idealistic behavior than a distributed [three-dimensional (3-D)] model. Even with such a more idealized model, one can notice the difference in the pulse duration and shape that is being introduced by use of the folded geometry. The following differences are evident. First, the folded line shows results in a longer pulse than the straight line. We assume that the bending of the line affects the propagation velocity, which becomes lower than in the straight-line case and, consequently, the pulse duration increases. Second, the folded-line pulse shows some nonzero value even before the pulse begins getting formed (before 10 ns in Fig. 3). In the same interval, the straight-line pulse has a true zero value. To emphasize this difference, the inset plots in Fig. 3 show the prepulse behavior on an enlarged scale. The ability to discern features in the pulse shape for the bended line not

apparent in the straight line demonstrates the importance of computer simulation in the design of optimum transmission lines. The simulations also allow us to determine the peak fields generated at various “hot spots” along the bends in the case of the folded Blumlein. A hot spot of nearly 2 MV/cm along one of the bends in the line was identified through our simulation. This is consistent with experimental results and electrostatic simulations conducted by SNL [5]. In their experiments, a number of different assemblies were tested, bulk dielectric breakdown measurements were done on small samples, and electrostatic code (Electro) runs made. The SNL group then estimated that at the edges of the Blumlein the peak field on the conductors would be 1.58 MV/cm. During actual testing of various configurations, breakdown did occur (surface flashover across all edges) at 1.38 MV/cm. The differences in our simulation results can be attributed to differences in the actual assembly compared to our simulation setup.


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Fig. 5. (a) Initial semicircular outward propagating input wave. (b) Input waveform boundary as the wave propagates through the primary transmission line.

Fig. 6. (a) A 750–mil Blumlein model with mismatched impedance at input. (b) Pulse shape improvement using intentional internal reflections.

Since the objective of the MURI program is to address technical issues that stand in the way of making pulsed power compact, it is appropriate to also look at applications that require very small structures. In this regard, compact Blumleins for bioelectric applications are also addressed (see the paper in this Special Issue on bioelectrics) to study the unique electrical features of small structures. The folded Blumlein model described above assumes negligible edge effects and a simple lumped element load. At shorter pulsewidths, of particular interest for the bioelectrics application, input/output edge effects as well as dielectric dispersion must be taken into account. Fig. 4 shows the output waveform for a water Blumlein sized to generate a 1.0-ns pulse to a 10- load. Compared to an idealized transmission line model, there is a 50% increase in the pulsewidth. The pulse rise/fall times are also greater than the input step of 0.5 ns. The disparity 1148

between the simulated waveform and the ideal transmission line model is due to the input and output edge effects. The most profound impact of shorter pulsewidths is seen at the input. Since a spark gap point source is located at the input, an outward propagating semicircular waveform is initially generated at the input seen in Fig. 5(a). As the wave propagates through the primary transmission line, the wave boundary becomes similar to the steady-state transmission assumption, Fig. 5(b). The length at which the semicircular boundary changes to a straight boundary, which can be modeled by a simple parallel plate transmission line, depends on the width of the transmission line plates. Investigation of the injected current at the input suggests that it may be possible to improve the pulse by intentionally including internal reflections in the transmission line. This can be done by allowing an impedance mismatch at the input PROCEEDINGS OF THE IEEE, VOL. 92, NO. 7, JULY 2004

Fig. 7. Pulse shape for . Conductivity: (a) 0.01 S/m and (b) 0.1 S/m.

of the line causing internal reflections. The effect of intentional reflections within the Blumlein is shown in Fig. 6. As can be seen in Fig. 6(b), the pulse shape is improved compared with Fig. 4. More research is required to investigate the possibility of using these reflections within the transmission line to further improve the pulse shape. Lastly, dispersion effects can be important in structures when frequencies in the range of a few gigahertz (that is, pulse characteristics on the nanosecond scale) come into play. In this case, some dielectric media have signif, where is the imaginary part icant dispersion ( of the permittivity). Water is a good example. For short pulsewidths, it is important to ensure that dispersion have a negligible effect on the pulse shape. With finite dispersion, energy will be dissipated as the wave propagates through the transmission lines. As dispersion increases, a greater impact will be seen on the falling edge of the pulse. The pulse shape will be distorted at high dispersion. To demonstrate effects of dispersion, consider a dielectric medium with a relative permittivity (dielectric constant) of 20. A 2.75-in Blumlein length was selected for the simulation. Fig. 7(a) shows the pulse shape for a conductivity of 0.01 S/m, and Fig. 7(b) shows the pulse shape for a conductivity of 0.1 S/m. As seen in these plots, a conductivity of 0.01 S/m has a negligible effect on the pulse output. This result suggests that dispersion must be taken into account for nanosecond pulsewidths in order to ensure proper operation. B. Tapered Transmission Lines With Ferrite Loading Another aspect of architecture in pulsed power systems is the use of tapered transmission lines. An interesting property of such a line is that it performs as a high-pass transformer. From the input to the output of a tapered line, the voltage, current, and impedance transform as an ideal transformer with the transformer ratio determined by the transmission line taper and the transmission line media. Because a tapered line can match a source to a wide range of load impedances for short pulses, its compactness and power limitations have been investigated. A candidate transmission line transformer (TLT) (Fig. 8) with an impedance transformation ratio of about 100 (or

a voltage-current transformation of ten) has been analyzed mm is the thickness [8]. In this specific example, mm is (height) of the exponential transmission line; the maximum width of the transmission line; m is the m maximum length of the transmission line; is the exponential shape factor; mm is the width of exponential transmission line at the high-impedance end ; is the impedance ratio; with and is the distance along the transmission line from the wide low-impedance end. The dielectric material used in this case is a ferrite, in particlular, Fair-Rite 67, and the conductors are aluminum.3 Reflections of power from an exponential taper back to the input is governed by the length and taper rate . Typically an exponential line must be at least one-half wavelength long and have a taper rate of to reduce power reflections to less than 10 dB. A longer line reduces power reflections further. Because the phase velocity decreases with the addition of Fair-Rite 67 by a factor of 21.8, the wavelength decreases by the same factor, and the physical length of the exponential line can be reduced by a factor of 21.8 compared to an air-loaded line. The addition of a ferrite material to a tapered transmission line is a means of reducing its physical size to accommodate use on a mobile platform. The transmission line parameters in this example are as nF m is the capacitance per unit length; follows: nH m is the inductance per unit length; m m is the resistance per unit length; and m is the conductance per unit length. The analA, which implies kW ysis shows that kV. At the high-impedance end of the line, and the current is A and and the voltage is kV. Power deposition is 40 kW into the ferrite (anomalous magnetic and dielectric losses were not included) and , the 22 kW into the conductor. Ferrite operation near 3Ferrites are ceramic materials with a chemical formula of M 1 Fe O , where M is a divalent metal oxide. A material made by the Fair-Rite Products Corporation (http://www.fair-rite.com) is Fair-Rite 67, for which M is based on nickel–zinc oxides and the material is rated for broadband transformers up to 80 MHz with a maximum magnetic flux density of 0.23 T. The relative permittivity and permeability of Fair-Rite 67 are 12 and 40, respectively, which implies a transmission line phase velocity of c=21:8, where c is the speed of light.


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Fig. 8. Exponentially tapered transmission line with uniform height a but exponentially varying width, b(x), where is the exponential taper rate. The complex medium has conductivity of , permeability of , and dielectric constant of ". See the text for complete definition of all parameters.

maximum flux density, exhibits waveform dependent nonlinear and anomalous behavior. Linear operation with re. duced distortion requires a value of much less than For a power of 460 kW, the maximum temperature rise per unit time is 390 K s for the ferrite and 340 K s for the aluminum conductor. A conservative maximum temperature increase of 50 K for the ferrite limits the duration of a waveform to less than 125 ms. For applications that require a pulse duration of 100 ns, the major spectral components are at 10 MHz and higher. An air-filled line operating at 10 MHz or higher would be 30 m long and 1 cm thick, but ferrite loading reduces the length to 1 m with a 1-cm thickness. Power handling of the ferrite-loaded line is 460 kW. For the example, the transmission line has a high-pass cutoff frequency of 5.25 MHz. Fair-Rite 67 has relatively uniform properties to 80 MHz but has increasing loss at higher frequencies. Consequently, a pulse can be synthesized with 10 MHz as its fundamental plus eight harmonics. At low power, the output pulse will droop somewhat due to the finite bandwidth. For high-power operation, at 460 kW, heating in Fair-Rite 67 will increase loss and exacerbate pulse droop. After a few milliseconds, the ferrite at the narrow end of the transmission line heats up, degrades, and eventually fails. The time limitation for use and failure due to heating are, of course, detractors for use in a compact high repetition rate system. However, the application of liquid cooling, addressed in Section V, could mitigate these detractors and make the use of tapered lines in a variety of compact power applications an attractive alternative. C. Analysis Improvements in the Study of Arc Discharges Revolutionary improvements in the design of components for compact pulsed power applications will only be possible when a combination of numerical simulation and experiment 1150

Fig. 9. Switch used to study arc discharges in subnanosecond breakdown studies.

are used together. In this way, many variations in design parameters can be tried at much lower cost. Thus, comparing electromagnetic modeling results with experimental data becomes an important part of the architecture design process. But to successfully make the experimental data/numerical model comparisons, many times the sensor location is not always where the important physics processes are taking place. One such important issue for CP occurs in the study of a fast rise time gas breakdown switch. In this case, the data must be collected and deconvolved from a location downstream of the arc discharge. This is yet another example of how architecture can play a critical role in pulsed power system design. Fig. 9 shows a schematic of such a pressurized gas breakdown switch [9]. The characteristics of such a switch are: 1) the closing process is transient ( 100-ps closing time); 2) the dynamic gap voltage is large; 3) the breakdown plasma channel structure is small; and 4) the temporal parameters of the switch are measured downstream from the arc itself. In this portion of the research, our goal is to create a model for the switch and evaluate the dynamic process by the experPROCEEDINGS OF THE IEEE, VOL. 92, NO. 7, JULY 2004

available and propylene carbonate offers the advantage of a lower freezing point of 55 C ( 67 F) for applications in cold environments. In addition to the gas and liquid switch media, we have examined the theoretical limitations of solid-state switches made of GaAs and SiC and compared them to existing Si-based high-voltage switching components. A. Gas Phase Switches

Fig. 10. Calculated dI=dt of a pressurized SF switch at various pressures. Fast breakdown occurs in two phases. This result was obtained by electromagnetically deconvolving data obtained downstream from the arc itself.

imental data. Finite-element time-domain analysis is used to find a transfer function that would relate the experimentally measured parameters (downstream of the arc) to the arc current itself. The MatLAB PDE toolbox is used to solve the differential equations. Results from the initial analysis are shown in Fig. 10. Here , the time rate of change we present calculations of of current rise, as a function of time in a switch using SF as the dielectric medium at various pressures. We use the half-width of half-maximum time to describe the duration of the initial fast breakdown phase. Our initial results suggest that there appear to be two distinct events during the breakdown process, and the second appears to be pressure dependent. This ability to deconvolve experimental data and relate it to the properties of the arc provides enormous opportunity to understand the complex processes that take place in the intense fields of the discharge. Our future work will address the dynamic channel conductivity. We will invoke the Braginski conductivity model [10] to assess the dynamic channel size. In addition, an absorbing boundary condition on the outward wall of the switch will be employed to increase the clear time of the model. III. SWITCHING This part of the MURI CP research program concentrates on several key thrusts of the compact pulse power project: 1) the investigation of the pseudospark switch as an enabling technology in current applications, and the extension and optimization of this technology to much smaller volume and much higher voltage operation; 2) the use of polar liquids as an attractive switching media due to their high dielectric strength and flow properties for the design of small, low inductance, and, therefore, fast high power switches; and 3) a focus on liquid switch research, namely, water and propylene carbonate as polar switching media, since water is readily

The pseudospark switch [11] offers the prospect of introducing an enabling technology for compact pulsed power that has the potential for reducing size and weight while improving performance.4 This is based on a radically different method for cathode electron emission. The pseudospark, and the optically triggered back-lighted thyratron (BLT), are low-pressure gas phase switches with some operating features similar to thyratrons, but with a very different cathode that enables high current, faster rise, and reduced housekeeping. The electron emission principle differs from traditional thyratrons in that the electrons are produced in sequential phases, including primarily a “hollow cathode” phase (the hollow cathode phase of pseudospark plasma formation and switch operation is described in [13]), followed by a “superemissive” phase (the physical processes responsible for the superemissive cathode for the pseudospark and BLT are discussed in [14]; see also [15]). This physics, which leads to a self-heated cathode producing typically 5000 A from 1 cm area [14], in effect extends thyratron-type switching parameters for current, fast rise, and other aspects, while reducing size and housekeeping, for compact pulsed power. The CP MURI has chosen the pseudospark switch for investigation. The reasons for this include the highly emissive cathode for fast, high peak current, and that it is desirable to study switches for high-voltage pulse generators with potential for lowering housekeeping requirements, while retaining robust characteristics over many pulses. The pseudospark may be compared to a thyratron, in that both operate with a gas pressure that allows space-charge neutralization when a glow plasma is formed. A photograph of a pseudospark switch manufactured by IAP, Incorporated, Waltham, MA, and an Edgerton, Germeshausen, and Grier, Incorporated (EG&G), Salem, MA, thyratron is shown in Fig. 11. A pulse generator based on the pseudospark has been used successfully in collaborative experiments with the Naval Postgraduate School, Monterey, CA, for igniting a pulse detonation engine, and with the University of Cincinnati, Cincinnati, OH, in combustion control. For these experiments, the pseudospark produces fast-rising pulses of approximately 50 ns. Another pseudospark-based pulse generator is under development for longer pulses and lower output impedances to be used in carbon fiber field emission cathode experiments in collaboration with the Naval Postgraduate School for HPM source applications. 4The pseudospark is the subject of a Special Edition of the IEEE TRANSACTIONS ON PLASMA SCIENCE for February 2004 [12].


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Fig. 11. Photograph showing comparison of HY-5 thyratron (left, 4.5-in inside diameter) and BLT 175 (1.75-in inside diameter) pseudospark switch (right). The HY-5 uses an externally heated thermionic cathode and conducts current up to approx. 5 kA for variable pulse lengths. The BLT 175 operates for shorter life and pulse length compared to the HY-5, but has lower inductance, housekeeping power, and conducts higher current ( 10 kA). It employs a self-heated superemissive cathode.

The MURI is, thus, actively developing both short, fast rise pulsed power based on this switch, and higher coulomb transfer applications.5 In order to meet the needs of high-voltage, high-current electron beam generation in diverse applications, we have continued development of a multigap pseudospark switch, ultimately for 200-kV operation. Such a switch, together with innovative TLTs [19] also under development, will be able to provide 500-kV, 10-kA, 500-ns pulses. A high repetition rate rapid capacitor charging power supply has been constructed and tested successfully for repetitively operated pseudospark switching. The switch is operated at 30-kV voltage, 4-kA peak current, delivers 50-ns-wide pulses at 1.5-kHz maximum repetition rate for a burst of 100 pulses. The output of this generator is 80 kV into a 400- load. This generator has been used successfully in several applications including corona ignition, ignition of a pulse detonation engine, and combustion control. Another similar pulse generator under modification will be upgraded to 250-ns-long, 70-kV pulses into a 50- load. In these generators, the pseudospark switches operate at the boundary of the superemissive phase. A coaxial load and capacitor bank structure is being upgraded to be able to test for switch operation in the full superemissive phase, at 30 kV and 10 kA. A second rapid charger power supply is being assembled at present for high repetition rate testing of the pseudospark switches. In addition to these specific applications, we have designed a three-stage Marx generator for investigation of overvolted pseudospark- and BLT-switched voltage multiplication. The generator is under construction. The pseudospark is an attractive candidate for this Marx generator provided the problems of floating housekeeping and trigger synchronization can be solved satisfactorily. The 5Details of various applications of pseudospark pulse generators and their design may be found in papers written for the IEEE Power Modulator Conference and the IEEE Pulsed Power Conference. See, e.g., [16]–[18].

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Fig. 12. Comparison of current–electric field characteristics for propylene carbonate and distilled water between unpolished stainless steel electrodes. Solid lines depict the theoretical current–electric field characteristic calculated from Ohm’s law.

housekeeping circuits for the heater and keep-alive power use a high-efficiency compact 200-kHz class-E power converter [20] and a ferrite high voltage isolation transformer developed at the University of Southern California (USC), Los Angeles. The housekeeping unit has been tested and is operational. Floating, optical fiber isolated trigger circuitry is now under construction. The generator will be charged using a high-power solid-state charging power supply developed by Texas Tech University (TTU), Lubbock, which will provide an electron beam load to the Marx generator as well. B. Switching in Polar Liquids Due to their high dielectric strength, polar liquids are attractive switching media for the design of small, low inductance and, therefore, fast high power switches. The fact that the liquid can be flowed within the switch volume allows for effective thermal management and easy removal of debris after switch breakdown. To date, our research has focused on the use of water or propylene carbonate as the polar switching medium. Whereas water is readily available, propylene carbonate offers the advantage of a lower freezing point of 55 C ( 67 F) for applications in cold environments. We studied the characteristics of electrical breakdown in water and in propylene carbonate with respect to holdoff voltage, breakdown mechanism, rate of current rise, switch recovery, and recovery rate. Switching properties have been investigated as a function of the electrical conductivity of the liquid, surface condition of electrodes, and flow of the liquid. A rectangular voltage pulse of 200-ns duration and amplitudes of up to 40 kV was applied by means of a 50- Blumlein generator. The switch consists of a 1.7-mm-diameter hemispherical that was placed facing a planar electrode at distances from 200 to 400 m. Both electrodes were made of stainless steel. Electrical measurements and optical imaging techniques (photography, PROCEEDINGS OF THE IEEE, VOL. 92, NO. 7, JULY 2004

Fig. 13. Shadowgraphs of the development of inhomogeneities in a water gap during a voltage pulse. The pictures were taken from different applied voltage pulses with an exposure time of 1 and 2 ns. The diagram on top shows typical current and voltage traces for breakdown. The photograph at 230 ns shows the development of a plasma channel (breakdown).

shadowgraphy, and Schlieren imaging) were used to explore switch characteristics and overall performance. The current–voltage characteristics of polar liquids (under high electric stress, as a function of the surface condition of the electrodes and the conductivity of the liquid) have been measured for distilled water (400 k cm), tap water (7 k cm) and propylene carbonate (200–240 k cm) for field levels up to electrical breakdown (Fig. 12). By polishing the electrodes, a breakdown electric field of 1.6 MV/cm was measured for distilled water. For propylene carbonate a breakdown field of 2.2 MV/cm was achieved. However, after only one electrical breakdown, the electrode surface was sufficiently altered such that the breakdown electric field never exceeded 1 MV/cm for water and propylene carbonate on the ensuing switching cycle. The same value was also measured for unpolished or sanded electrodes [21].

In an effort to understand the basic physics of the polar liquid breakdown process, the prebreakdown phase was studied using a variety of diagnostics techniques. Photographs and shadowgraphs were taken with an exposure time of 1–2 ns. As shown in Fig. 13, immediately after a voltage is applied, the shadowgraphs show the growth of treelike structures into the gap that indicate a dramatic change in the index of refraction. The development of these inhomogeneities between the electrodes is likely caused by a local change in conductivity in this region. As confirmed by photographs, these trees are not luminous and can only be observed if the hemispherical electrode is the cathode (i.e, charged negative). A significant drop in voltage or rise in current could not be observed during this phase. If the applied voltage exceeds the breakdown threshold, eventually a single plasma channel develops out of one of these trees,


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Fig. 14.

Recovery rate for static water, laminar flow (0.4 l/min), and turbulent flow (0.6 l/min).

bridging the gap and initiating the main breakdown. If the voltage remains below the threshold value and the gap does not break down, the trees retreat to the cathode after the pulse ends. In most cases, the plasma channel formation initiates in the region of highest field strength at the curved electrode. However, if the curved electrode is the cathode, in about 30% of all cases the plasma formation begins at the planar electrode, i.e., the anode. For water as a switching medium, current measurements during the prebreakdown and the breakdown phase were made using a current viewing resistor of 0.5 connected in a low-inductive geometry in series with the hemispherical electrode. From the current measurements, the current rise time was determined for a 400- m gap. The current rise time seems to be only slightly dependent on the conductivity and is, for water, on the order of 10 A s [22]. Modeling is an important part of our study of liquid breakdown for switching in pulsed power applications. A time-dependent model has been developed for treating the breakdown of water under nanosecond pulsed conditions. In this case, evidence suggests that the traditional mechanisms such as bubble formation and thermal heating no longer apply. Instead, the mechanism of breakdown is dictated by the surface electric field behavior and subsequent injection of electrons at the interface. Thus, the interface layer and electrode conditions are far more important than the bulk liquid properties in determining breakdown and holdoff voltage levels. It is shown that very strong electric field enhancements can occur due to the collective orientation of dipoles near the interface. Under high-voltage conditions, the injected electrons are assumed to have relatively long lifetimes due to continuous replenishment of energy from the external field. The simulations show a breakdown within about 200 ns for an applied pulse of 20 kV for a 200- m water-filled switch [23]. The physical processes following the electrical breakdown of water between planar and semispherical electrodes separated by a submillimeter gap have been studied as well using electrical and optical diagnostics. The expanding 1154

plasma column following breakdown generates shockwaves at first, and at a later stage, a vapor bubble that expands for approximately 200 s, and then decays with a time constant of 1 ms. The bubble decay time determines the dielectric recovery of the switch, as is clear from pulse–probe experiments (Fig. 14). Recent experiments have been focusing on reducing the recovery time and increasing the repetition rate. To reduce the recovery time, we flowed water through the switch volume to remove the vapor bubble on a time scale less than the recovery time in static (still) water. Flowing the water transversely through the rod-pin electrode gap with a flow rate of 0.56 l/min allows us to shorten the recovery time to 700 s [24]. Using axial laminar flow with a flow rate of 0.4 l/min through a nozzle-pin electrode system yields similar results. The recovery rate (the inverse of the recovery time) in this case increased from 1 kHz in static water to 1.4 kHz. Increasing the flow rate only results in an increase in recovery rate as long as the flow is laminar; beyond the transition from laminar to turbulent flow, the holdoff voltage decreases. Improvements of the design of the flowthrough system, such that the transition from laminar into turbulent flow occurs at a higher flow velocity, will lead to additional increases in recovery rate. Further increase in recovery rate can be achieved by operating the switch at a reduced breakdown voltage. Using axial flow and operating the switch at a breakdown voltage of 82% of its full dielectric strength allows us to reach recovery rates of 2 kHz. C. Solid-State Switches Certain classes of solid-state switches are finding increased applications in pulsed power systems. Silicon based devices have limitations associated with fundamental materials properties, such as energy band gap and carrier mobility. In order to investigate novel solid-state materials we have performed simulations of the electrical properties of FET-type switches fabricated from Si, GaAs, 4H–SiC, Wurtzite GaN, and Si MOSFET. Both perfect material properties and the effects of defects have been included. PROCEEDINGS OF THE IEEE, VOL. 92, NO. 7, JULY 2004

We conducted a two-dimensional (2-D) device simulation based on drift and diffusion equations. Because of the lack of parameter values for these materials, it was necessary to first generate a set of parameters by fitting experimental data available in the literature. Device size and doping profiles are kept comparable for JFETs. Deep level defects were considered in simulation setup because they can introduce energy levels near midgap and can act as carrier traps and scattering centers. In the Si MOSFET simulation, a perfect Si material was assumed. Simulation results show that existence of deep level defects in materials reduce device holdoff voltage and current density. In GaAs and wurtzite GaN JFETs, defects capture electrons and then reduce electron mobility in drift regions. In 4H–SiC JFET, implantation-induced deep level defects are along n+/n- and p+/n- junctions, which then cause the depletion region to shrink and lower the hold off voltage. Comparing device performance among these four materials, 4H–SiC JFET has the highest holdoff voltage and smallest temperature rise for a certain current density, while GaAs JFET has the fastest rise time. 4H–SiC is the most promising material for compact pulsed power based on our simulations and the available data. We designed a 4H–SiC VJFET optimized to achieve a high breakdown voltage, low on-state resistance, and fast switching speed. Its performance was predicted by the 2-D simulation tool ATLAS.6 The parameters for physical models of the 4H–SiC VJFET were extracted from the recent literature. The performance of SiC was compared with a GaAs VJFET. The predicted breakdown voltage for this 4H–SiC VJFET with a 50- m drift region is 8 kV at a leakage current of 1 10 A cm . Given the drift region thickness of 50 m, it is observed that the predicted breakdown voltage is 700 V for the GaAs VJFET. This value is about 9% of that for the 4H–SiC device. Due to the intrinsic material properties of GaAs itself, it is impossible for GaAs devices to achieve high breakdown voltages as SiC devices. At a V and V, the SiC VJFET can handle a current density of 185 A cm , while the GaAs VJFET V. Although can handle about 1.5 kA cm at the current handling capability of GaAs VJFETs is eight times greater, with the development of material quality and processing technology, SiC has great potential in improving the current rating. The VJFETs are predicted to have almost the same switching speeds. The rise and fall times are about 2 ns for the SiC VJFET. The rise time of the GaAs VJFET is less than 1 ns, and the fall time is about 1.5 ns. Another avenue of research in solid-state switching being pursued is the development of photoconductive SiC switches for high-power operation [25]. For many years semi-insulating GaAs has been employed as a photoswitch material [26]. This was due to GaAs’s large mobility, large dark resistivity, and the property that GaAs materials can be triggered into conduction through optical seeding of avalanche processes that reduce the optical closure energy by three to 6http://www.silvaco.com

Table 1 Gallium Arsenide (GaAs) and Silicon Carbide (4H–SiC) Properties

four orders of magnitude. However, the filamentary nature of the conduction has been shown to damage the bulk photoconductive material, and high current densities at the contact limit the power-handling capability of these avalanche photoswitches limiting the switch lifetime in high voltage operation to less than 10 shots [27]. Evolving silicon carbide material developments have made semi-insulating SiC an attractive photoconductive switch material. The parameters for 4H–SiC are compared with GaAs in Table 1. Note from the table the large difference in maximum electric field that the materials will support. Also note that the mobility of GaAs at a conduction electric field of 3 kV/cm is 20 times greater than SiC at a conduction electric field of 10 kV/cm. Another factor favoring SiC is that only 16% of the optical energy is necessary to produce the same conduction resistance as in the GaAs switch for a given thickness of switch design (SiC will be 1/6 of that of GaAs so that during conduction, the SiC switch will have a similar conduction voltage but a larger conduction electric field). Conversely, the same optical energy will produce a conduction resistance that is 16% of that in the GaAs switch. The dark resistivity of GaAs is approximately four orders of magnitude lower than that of semi-insulating SiC and, thus, the “off” state power dissipation of a GaAs switch is reduced by the same factor. Another major advantage of the SiC, an indirect band semiconductor, is that the recombination time of SiC is over 400 times greater than that of GaAs. Photoconductively produced carriers recombine in the absence of additional optical energy with the characteristic material recombination time. In order to maintain the initial conduction resistance, additional optical energy must be injected into the switch to replace the carriers lost to recombination. For steady-state operation, the optical power required to maintain conduction in a SiC switch is over 400 times less than that required for the GaAs switch. Further research is required to develop proper packaging in a high-power SiC photoconductive switch design. The major limitation in voltage is presently not the material, but the package; thus, the package is the focus of this MURI’s work (at the University of Missouri, Columbia).


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Fig. 15.

Current at the center of the wire as a function of time.

D. Ultrafast Pulsed Power Switch Characterization Techniques As a final example of switching research underway in the CP MURI, we consider the analysis of ultrafast switches through numerical modeling using transient time-domain techniques. With more and more electromagnetic problems arising in broadband applications, time-domain techniques by integral equations have been widely used in electromagnetic scattering and propagation regime, such as ultrawideband antenna radiation, high-resolution radar scattering, electromagnetic compatibility (EMC). and electromagnetic pulse (EMP) interference. Time-domain integral equation (TDIE) methods are normally used because of their economical cost compared with the frequency-domain counterparts. However, this is still a challenging task for the analysis of electrically large structures. In some circumstances, we can do much better. For example, in certain transient applications (like fast pulsed power switching), only the early time response is of interest. Time-domain methods can be terminated at any point, allowing us to compute solutions only for as long as necessary. Likewise, the transient response generated by short-duration excitations might be prominent only on a small portion of the computation region. It is not economical to integrate over the whole region. In particular, for compact pulsed power switches, there are some aspects of the computational problems faced in fast-closing oil and liquid switches that call for the development of specialized modeling tools. First, the geometry at the feed point is extremely fine, calling for a highly accu1156

rate spatial mesh to be able to capture the variations in the electromagnetic fields. Second, the time step of interest is, in general, very small in the early time (first few hundred picoseconds). These two requirements cause the complexity of traditional numerical techniques like the finite-difference time-domain method to increase unnecessarily; i.e., we have many more spatial and temporal samples than we really need. For that reason, we are developing a multiresolution time-domain method dedicated to treating this type of problem. Most time-domain methods involve the numerical solution of a differential equation. The differential equation requires the field to be evaluated at every point in the volume of interest. Integral equation methods, on the other hand, allow only the unknown currents to be solved for, reducing the number of unknowns exponentially (i.e., from to , where is the characteristic length dimension). Furthermore, in solving for the electromagnetic fields for fast-closing switches, we need high spatial and temporal accuracy at the early time, but lower accuracy at later times. Because of these factors, we are employing a first-of-its-kind wavelet-based integral equation solver to treat these problems. The solver is fully adaptive in both time and space, which means that we can automatically increase or decrease the resolution of our solution to achieve a desired level of accuracy on the fly. We have currently been successful in implementing a version of this algorithm for scattering problems and will be implementing it for radiation problems like the fast-closing switch in the future. See [28] for details of the algorithm. The transient current computed by the adaptive implicit scheme is plotted in Fig. 15. The accuracy is satisfactory PROCEEDINGS OF THE IEEE, VOL. 92, NO. 7, JULY 2004

compared with that obtained from a frequency-domain integral equation method. The scheme is also adaptive in the sense that the resolution is higher at earlier time steps than later time steps. Most importantly, the resolution levels are spatially adjusted to the pulse-like response. To date, we have only applied this new technique to purely electromagnetic problems like the thin wire scatterer. However, the nature of the algorithm makes it ideal for application in pulsed power problems. We are presently extending the algorithms to deal with arbitrary planar current elements. Once we have done that, we will be applying the algorithm to the analysis of the temporal electromagnetic fields near the plasma current channel in fast switches.

(a)

IV. INCREASED ENERGY DENSITY STORAGE In analyzing what technology is required to produce a compact pulsed power system, it becomes obvious that energy density of a given system is one of the dominant size factors. The tradeoff between high dielectric constant and large holdoff voltage has been identified as a major issue in the search for suitable dielectric materials with high energy density. Understanding the breakdown mechanisms of dielectrics has the potential of enabling the engineering of superior materials and advanced capacitor design. Hence, this MURI program has concentrated on the key areas of liquid breakdown, solid breakdown, and capacitor technology for developing compact pulsed power systems. A. Liquid Breakdown Many of the key issues for liquid breakdown (for polar liquids such as water and propylene carbonate) have already been discussed in Section III-B in terms of using liquids in switches. Electrical breakdown in liquids has been studied for several years. It has been reported that breakdown is based on complex interactions of hydrodynamic and electronic phenomena, leading to a rather complex temporal and spatial structure of the development of a conducting channel [29]–[31]. It is generally accepted that the application of a voltage to a gap in a liquid leads to predischarge phenomena, consisting of discrete current spikes and light emission events that form streamer-like structures that propagate stepwise, not continuously. The physical basis of these processes has again been discussed in numerous papers, but a coherent picture for the description is still missing. The use of novel high-speed diagnostics methods, such as oscilloscopes with analog bandwidths in the multigigahertz regime and electrooptical devices, such as subnanosecond gateable microchannel plates (MCPs), provides additional insight into these processes and contributes to the development of more satisfactory models describing the physics of breakdown. To characterize the state of the liquid before breakdown, it is important to have some information on its dc current–voltage characteristics. For polar liquids, such as water, rather complex transport phenomena play a role, beginning with convective charge transport at relatively low fields, transport of ionized molecules, as well as charge injection

(b) Fig. 16. (a) Sketch of setup for dc current with single-sided charging and Keithley 6514 electrometer. (b) Measured V -I characteristics for tip-plane. Three negative needle shots (open symbols) and three positive needle tests (filled symbols).

and transport of “free” electrons at higher field amplitudes approaching breakdown. Furthermore, the transition from dc behavior to breakdown has to be characterized. Some representative results on the dc behavior and breakdown in transformer oil are presented in Fig. 16. The left side depicts the setup for the measurement of the dc voltage–current characteristics. The results are presented as as a function of ) a Fowler–Nordheim plot ( to show the three characteristic regimes. For small voltages, kV ). the behavior is ohmic (linear rise for kV kV ), the For medium voltages ( linear fall characterizes Schottky-barrier emission (which follows the Fowler–Nordheim characteristics). For high kV ), the current is proportional voltages ( to the squared voltage, showing space charge saturation in a resistive medium. The overall behavior is consistent with reasonable assumptions on low-voltage conductivity, expected properties of the Schottky barrier at the transition from metal to oil, and high field mobility. B. Capacitor Technology Discharge capacitors are commonly used for energy storage in pulsed power systems. With increasing demand on compact portable pulse generators, capacitors become critical to reducing the size of generators. Traditionally, polymer capacitors are dominant for high power storage device, but their size is huge due to the low dielectric constant of polymers ( 10). Multilayer ceramic (MLC) capacitors are generating great interest for their high dielectric constant 3000), compact size, capability of integrating, (can be


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and their ability to operate at high temperatures. A high energy density ceramic capacitor results from the use of proper dielectric materials, device geometric design, and fabrication technique. Ceramic materials based on alkaline earth metal titanates have similar crystal structure and high solubility to one another. On a relative scale, dielectric properties can be predicted based on material composition. However, it is well known that properties of ceramic materials, in general, and ceramic materials applied in electronics, in particular, depend to a large extent on both composition and structure of grain boundaries. Therefore, a tiny amount of doping (such as Mn, Cr, and Cu, which stay in grain boundaries) will affect the final properties of the ceramic and, hence, device performance. Usually, identifying a good ceramic formulation needs extensive experimental research and, once found, is mostly proprietary. Fortunately, there are some public formulas that can be used as a basis to obtain enhanced properties, such as large and stable (over temperature and electric field) dielectric constant. Among these public formulations, barium titinate (BaTiO ) and strontium titinate (SrTiO ) are good candidates. Equivalent series resistance (ESR), which causes conduction loss in capacitors, is associated with the imaginary part of the complex dielectric constant. A distributed system design—for example an MLC—can reduce the ESR. High breakdown voltage can be obtained through series stacking. The inductance of the capacitor design also must be considered in the overall packaging. Various investigations show that ceramics with nanocrystalline grain size have enhanced properties compared to those with coarse grain size. Traditional printing technology of MLC capacitors cannot satisfy the requirement of nanograin size due to its high temperature sintering. Other new methods are considered to make dielectric layers, like metal–organic chemical vapor deposition (MOCVD) and sol-gel. MOCVD is a successful technique in semiconductor growth that offers unique combination of composition controllability, high uniformity over large areas, high throughput, and a high degree of conformability over 3-D structures. It also holds promise for dielectric layer growth, although it is totally new for ceramics and it is hard to find appropriate metal organic precursors [32]. A few ceramic layers were obtained by the MOCVD method [33]. Another issue for new capacitor technology is the electrode quality that can cause unexpected failure and loss. For the MLC capacitor, low-temperature cofiring is critical to have good electrodes and electrode–ceramic contact. The next sections will describe MURI investigations in material breakdown for applications to capacitors and compact pulsed power structures. C. Nanocrystalline Ceramic Dielectrics In an effort to develop transmission lines with higher energy storage capabilities for compact pulsed power applications, we have undertaken a collaborative approach to developing and studying ceramic dielectrics. The electrical breakdown strength (BDS) of TiO -based materials has 1158

been investigated for high energy density applications. At the University of Missouri, Rolla (UMR), under the auspices of National Science Foundation funding. The results of research to date show that dense titania ceramics with nanocrystalline grain size ( 200 nm) exhibit significantly higher BDS as compared to ceramics made using coarse grain materials. Processing–microstructure–property relationships in TiO systems are found to play a role with respect to increasing the BDS. In our MURI research, a pulsed power system has been assembled to perform BDS studies of the ceramic materials produced at UMR. Electromagnetic simulations in support of this work will also presented. The long-term aim of this research is to enable the fabrication of large sizes of high energy density ceramics for use in pulsed power systems. High dielectric constant ceramics are being developed for a wide variety of applications. Although the dielectric constant of electronic ceramics typically ranges from a low value of 2.2 for pure SiO up to 30 000 for relaxor ferroelectrics, the parameters of interest in some directed energy applica100) and high tions include high dielectric constant ( BDS ( 400 kV cm). The energy density (in joules per cubic meter) of a ceramic under the influence of an applied electric field is given by (1) where is the permittivity of free space, is the dielectric constant of the ceramic, and is the applied electric field strength. Typical candidate materials that are being studied in this regard are TiO and BaTiO [34]. As can be seen from (1), a high permittivity and a high BDS of dielectric materials result in increased stored energy density ( ). TiO was selected for study as the candidate material for its high BDS ( 350 kV cm) and relatively high ( 110) (compare to water, with ). Since the goal of our work is to develop a dielectric with as high an energy storage density as possible, this study is focused on increasing the BDS of the candidate material. BDS is affected by density, grain size, and defect chemistry of the dielectrics. Single-crystal TiO exhibits higher BDS over polycrystalline samples with high grain boundary mobility and residual porosity. Nanosized materials exhibit a high grain boundary area-to-volume ratio and lower concentration of impurities within the grain boundaries. The samples in this work were prepared by cold isostatic pressing of powders and sintering at various temperatures. In previous work [35], it was shown that dense nanocrystalline TiO structures ( 200 nm) exhibited significantly higher BDS than coarse-grained TiO structures ( 10 m). Our aim seeks to expand this work and apply pulsed testing capabilities in order to measure the intrinsic BDS of the material. The BDS properties of nanostructured TiO and coarse-grained TiO were investigated for dc breakdown using a dimpled electrode configuration. The results were higher values as compared to those measured in a set-up using planar electrodes [36], [37]. As can be seen from PROCEEDINGS OF THE IEEE, VOL. 92, NO. 7, JULY 2004

Fig. 19. Single-pulse tests of nanostructured TiO . (a) Prebreakdown at maximum voltage of 12.5 kV. (b) Breakdown event at 13.5 kV. The thickness of the sample was 7.0 mils.

Fig. 17. Weibull distribution of BDS for nanocrystalline- and coarse-grained TiO .

Fig. 20. Weibull plot of electrical breakdown for TiO nanocrystalline samples. The Weibull parameters and and the BDS of the samples are inferred from the straight line properties.

Fig. 18. BDS as a function of dielectric thickness for nanocrystalline- and coarse-grained TiO .

Fig. 17, the nanostructured TiO shows a higher BDS (1096 kV/cm) compared with the coarse-grained TiO (550 kV/cm). (For these Weibull plots, a straight line that crosses zero intersects at the mean value for the BDS, —coarse grained TiO and —nanocrystalline TiO .) In addition, the effect of sample thickness was studied to see if this had any effect on the measured data not conforming better to the Weibull distribution. Fig. 18 shows the results of a study assessing BDS as a function of sample thickness. Due to the strong dependence on thickness, the intrinsic BDS value for nanostructured TiO may be even higher than the characteristic value seen in the previous figure. Because the specialized field of pulsed power is not a prominent commercial venture (far outpaced by the power industry and communications field), pulsed breakdown studies with pulse lengths of microsecond time scales is not

routinely conducted. Therefore, our MURI effort for new high-BDS solid materials has addressed the issue of pulsed breakdown under a variety of electrical conditions. We have conducted a series of preliminary experiments with nanostructured TiO samples obtained from UMR and conducted single pulse testing to breakdown with them. Fig. 19 shows one of the samples just before breakdown [Fig. 19(a)] and at breakdown [Fig. 19(b)] during a voltage ramp-up test using a similar dimpled electrode configuration as the UMR dc tests. It is interesting to note that the breakdown appears to have occurred after the peak of the pulse in this case. A Weibull plot of the failure rate for the three UMR samples is shown in Fig. 20 using test equipment at the University of New Mexico, Albuquerque. The BDS of 728 kV/cm is similar to values obtained by UMR with other samples. We intend to continue exploration of the pulsed breakdown statistics of these, and similar new ceramic materials. V. THERMAL MANAGEMENT In spite of the enormous amount of research devoted to increasing the efficiency of power-handling devices, a fundamental limitation in a power-handling device remains the waste heat with its associated temperature rise


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Fig. 21.

Microchannel heat exchanger design.

and degradation of component properties. Virtually every power-handling component (resistor, capacitor, inductor, active electronics, and conductors) has this detractor and the role of thermal management is to alleviate the effects of waste heat. The current state of the art for dissipating heat from an insulated gate bipolar transistor (IGBT) module is approximately 100 W cm and typifies power dissipation of many components. Systems that strive to be compact and high power cannot exceed this limit without invoking some form of aggressive heat dissipation. There are many candidate approaches, which include advanced heat pipes, forced-air heat exchangers, materials with high thermal conductivities like diamond, mists and sprays that exploit evaporative cooling, thermal–electric coolers, and advanced liquid cooling. The approach to heat dissipation adopted at the University of Nevada, Reno (UNR), is high-pressure liquid microchannel cooling. Microchannel cooling exploits the high convective heat transfer coefficient in a liquid that flows in small-diameter ducts under turbulent flow conditions. A microchannel heatexchanger plate with many parallel ducts embedded within can maintain a low temperature and transport heat that flows into the plate out of the plate via fluid flow. This approach is suitable for mounting discrete devices such as the IGBT. The microchannels can be applied to conductors used in transmission lines. Application of liquid cooling to ferrites and solid dielectrics is, in principle, possible, but many practical considerations must be overcome, such as mechanical stress and electrical breakdown in the media. The approach at UNR is to address a known need first, then expand into other applications. The first need was an IGBT switching module with power dissipation of 1 kW cm , a Navy specification for its all-electric ship program. Microchannel cooling was applied to a heat sink for an IGBT module. There were many design tradeoffs, but a candidate design with many desirable features was quantified. The result of that effort was a design (Fig. 21) that would provide 1-kW cm cooling to commercial IGBT dies. The cooling is an order of magnitude above the current state 1160

Table 2 Scaling Laws and Asymptotic Behavior of Arc Switches Closing Across a Voltage Source V With Real Impedance Z

of the art and meets the Navy’s specification. Complete specifications and materials for the microchannel heat exchanger were quantified and presented [38], [39]. As a result of the design work, a potential cooperative effort with USC has emerged—the application of microchannel cooling to their compact pulsers. For their pulsers to operate continuously at maximum repetition rate, heat generation of approximately 300 W cm will occur on some active components. Microchannel liquid cooling could be used to dissipate this heat and not otherwise disrupt the USC compact design. This synergism could be exploited in the future. It would provide a tangible technology demonstration with power dissipation beyond the current state of the art. VI. SUMMARY The pulsed power community has matured since the early 1960s, and many applications today await the availability of compact pulsed power systems. We have described aspects of our basic research program that focuses on a better understanding of existing materials used in pulsed power systems PROCEEDINGS OF THE IEEE, VOL. 92, NO. 7, JULY 2004

and the development of new materials that might one day be used in pulsed power systems, especially those that have unique operating environments with extreme weight and size limitations. We have also shown the importance of electromagnetic modeling in both designing tailored materials, as well as facilitating the interpretation of pulsed power data, and the importance of thermal management in actual compact pulsed power systems. The fact that pulsed power research will continue to be an active area of research is undeniable. As one example of the need for new approaches and better tools for the compact pulsed power engineer, consider the issue of scaling of arc switch operation. The seminal work of J. C. Martin provides one technique as to how to scale the important parameter of rise time. However, consider Table 2. This table summarizes the various scaling laws for arc discharge breakdown that have been reported in the literature. The properties of several arc switch models when closing occurs across a voltage source with a fixed real load impedance are shown. is the time-dependent resistance of the arc switch, is a pressureor density-dependent constant for the switching medium, is the voltage drop across the switch, is the supply voltage, is the switch current, is the maximum switch current , is the real load impedance, is the gap spacing, and is the mean electric field in the gap at the time of switch breakdown. The scaling time is used to convert the differential equation for arc current into the dimensionless , with , and . form The rise time based on maximum rate of change of current is . The model according to Toepler [40], [41] is empirical. The models of Rompe and Weitzel [40], [42] and of Vlastos [40], [43] are appropriate for a fixed arc channel width in which the conductivity depends upon temperature to the first and to the 1.5th power, respectively. The Braginskii [40], [44] model is appropriate when the plasma temperature and conductivity are relatively constant and switch closure is dominated by the shock-wave expansion of the spark channel. The model of Sorensen and Ristic [45] is based upon experimental measurements. Last is the celebrated empirical scaling law of J. C. Martin [46]. It is clear from this table that there are wide differences in the scaling of , since the powers of , , and/or are different in each case. The resolution of these important differences in scaling of arc discharges in switches that are needed for compact pulsed power is one example of what the CP MURI is doing to improve our understanding of the problems associated with this technology. The work described in Section III-D of this paper, and other related efforts, will directly contribute to this understanding. It is only through a concerted effort uniting researchers from many fields of study that challenges, like switch arc discharge scaling, in portable pulsed power research will be successfully met. ACKNOWLEDGMENT The authors would like to thank the Director, Defense Research and Engineering of the U.S. Department of De-

fense and the U.S. Air Force Office of Scientific Research for their support. The authors would also like to thank their colleagues, the faculty, staff, and especially the students that participated in the research programs described in this paper—in particular, J. Qian, Old Dominion University, Norfolk, VA; S. Qiong, M. Berhend, F. Wang, and X. (Kathy) Gu of the University of Southern California, Los Angeles; and Z. Zhou, J. Chen, P. Castro, and M. Roybal of the University of New Mexico, Albuquerque. REFERENCES [1] “U.S. Department of Defense Fiscal Year 2001 Multidisciplinary Research Program of the University Research Initiative (MURI) Competition Announcement,”, Washington, DC, FY2001 MURI Topic 30, 2000. [2] High Power Microwave Sources and Technologies, R. J. Barker and E. Schamiloglu, Eds., IEEE, Piscataway, NJ, 2001. [3] A. D. Blumlein, “Electrical network for forming and shaping electrical waves,” U.S. Patent 2 465 840, 1948. [4] R. Burns, The Life and Times of A. D. Blumlein. London, U.K.: IEEE, 2000. [5] J. A. Alexander, S. Shope, R. Pate, L. Rinehart, J. Jojola, M. Ruebush, W. Crowe, J. Lundstrom, T. Smith, D. Zagar, and K. Prestwich, “Plastic laminate pulsed power development,” presented at the Soc. Automotive Engineers, San Diego, CA, 2000, Paper 00PSC-113. [6] M. Joler, C. Christodoulou, J. Gaudet, E. Schamiloglu, K. Schoenbach, R. Joshi, and M. Laroussi, “Study of high energy storage Blumlein transmission lines as high power microwave drivers,” presented at the Soc. Automotive Engineers, Coral Springs, FL, 2002, Paper 2002–01–3179. [7] M. Joler, C. Christodoulou, E. Schamiloglu, and J. Gaudet, “Modeling of a compact, portable transmission line for pulsed-power applications,” in Proc. 14th IEEE Int. Pulsed Power Conf., 2003, pp. 253–256. [8] R. J. Vidmar, “Use of a tapered transmission line as an ideal transformer,” in Proc. BEAMS 2002 Conf., vol. 650, 2002, pp. 41–44. [9] J. Chen, C. J. Buchenauer, and J. S. Tyo, “Numerical and experimental modeling of subnanosecond plasma closing switches in gases and liquids,” in Proc. 14th IEEE Int. Pulsed Power Conf., 2003, pp. 59–62. [10] S. I. Braginski, “Theory of the development of a spark channel,” Sov. Phys. JETP, vol. 34, pp. 1068–1074, Dec. 1958. [11] K. Frank, E. Boggasch, J. Christiansen, A. Goertler, W. Hartmann, C. Kozlik, G. Kirkman, C. G. Braun, V. Dominic, M. A. Gundersen, H. Riege, and G. Mechtersheimer, “High power pseudospark and BLT switches,” IEEE Trans. Plasma Sci., vol. 16, pp. 317–323, Apr. 1988. [12] IEEE Trans. Plasma Sci. (Special Issue on Pseudospark Physics and Applications), pt. 2, vol. 32, pp. 189–248, Feb. 2004. [13] L. C. Pitchford, N. Ouadoudi, J. P. Baeuf, M. Legentil, V. Puech, J. C. Thomaz, Jr., and M. A. Gundersen, “Triggered breakdown in lowpressure hollow-cathode (pseudospark) discharges,” J. Appl. Phys., vol. 78, pp. 77–89, 1995. [14] W. Hartmann and M. A. Gundersen, “Origin of anomalous emission in superdense glow discharge,” Phys. Rev. Lett., vol. 60, no. 23, pp. 2371–2375, 1988. [15] A. Anders, S. Anders, and M. A. Gundersen, “Model for explosive electron emission in a pseudospark ‘superdense glow’,” Phys. Rev. Lett., vol. 71, no. 3, pp. 364–367, July 1993. [16] F. Wang, A. Kuthi, C. Jiang, and M. Gundersen, “Pseudospark based pulse forming circuit for transient plasma ignition and combustion control systems,” in Proc. IEEE Pulsed Power Conf., 2003, pp. 339–334. [17] A. Kuthi, R. Alde, M. Gundersen, and A. Neuber, “Marx generator using pseudospark switches,” in Proc. IEEE Pulsed Power Conf., 2003, pp. 241–244. [18] A. Kuthi, C. Young, F. Wang, P. Wijetunga, and M. Gundersen, “Rapid charger for high repetition rate pulse generator,” in Proc. IEEE Pulsed Power Conf., 2003, pp. 950–952. [19] I. D. Smith, “A novel voltage multiplication scheme using transmission lines,” in Proc. 15th IEEE Power Modulator Symp., 1982, pp. 223–226.


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[20] J. Ebert and M. Kazimierczuk, “Class E high-efficiency tuned power oscillator,” IEEE J. Solid-State Circuits, vol. SC-16, pp. 62–66, Apr. 1981. [21] J. Kolb, S. Kono, S. Xiao, B. Goan, X. Lu, M. Laroussi, R. P. Joshi, K. H. Schoenbach, and E. Schamiloglu, “Water and propylene carbonate storage and switching medium in pulsed power systems,” in Proc. 14th IEEE Int. Pulsed Power Conf., 2003, pp. 715–718. [22] K. H. Schoenbach, J. Cooper, A. Garner, B. Goan, R. P. Joshi, J. Kolb, S. Katsuki, S. Kono, M. Laroussi, F. Leipold, X. Lu, C. Mallot, J. Qian, and S. Xiao, “Electrical breakdown of submillimeter water gaps,” in Proc. BEAMS 2002 Conf., 2002, pp. 111–114. [23] R. P. Joshi, J. Qian, J. Kolb, and K. H. Schoenbach, “Model analyzes of breakdown in high in high-voltage, water-based switches,” in Proc. 14th IEEE Int. Pulsed Power Conf., 2003, pp. 293–296. [24] S. Xiao, J. Kolb, S. Kono, S. Katsuki, R. P. Joshi, M. Laroussi, and K. H. Schoenbach, “High power water switches: Postbreakdown phenomena and dielectric recovery,” IEEE Trans. Dielect. Elect. Insulation, to be published. [25] W. Nunnally and M. Mazzola, “Opportunities for employing silicon carbide in high power photo-switches,” in Proc. 14th IEEE Int. Pulsed Power Conf., 2003, pp. 823–825. [26] A. Rosen and F. Zutavern, Eds., High-Power Optically Activated Solid-State Switches. Norwood, MA: Artech House, 1994. [27] J. A. Gaudet, W. D. Prather, J. Burger, M. C. Skipper, M. D. Abdalla, A. Mar, M. W. O’Malley, F. J. Zutavern, and G. M. Loubriel, “Progress in gallium arsenide photocon-ductive switch research for high power applications,” in Proc. 25th IEEE Power Modulator Symp. and 2002 High Voltage Workshop, pp. 699–702. [28] Z. Zhou and J. S. Tyo, “Transient analysis of straight thin wire scatterer by multiresolution time-domain integral equation method,” in Antennas and Propagation Soc. Int. Symp., vol. 3, 2003, pp. 575–578. [29] T. J. Lewis, “An overview of electrical processes leading to dielectric breakdown of liquids,” in The Liquid State and Its Electrical Properties, E. E. Kunhardt, L. G. Christophorou, and L. H. Luessen, Eds. New York: Plenum, 1987. [30] A. Beroual, “Electronic and gaseous processes in the prebreakdown phenomena of dielectric liquids,” J. Appl. Phys., vol. 73, pp. 4528–4533, 1993. [31] R. Badent, K. Kist, and A. J. Schwab, “Streamer initiation and propagation in insulating oil in weakly nonuniform fields under impulse conditions,” in Proc. 1996 IEEE Int. Symp. Electrical Insulation, pp. 720–723. [32] Y. Takeshima, K. Shratsuyu, H. Takagi, and Y. Sakabe, “Preparation and dielectric properties of the multilayer capacitor with (Ba, Sr)TiO3 thin layers by metalorganic chemical vapor deposition,” Jpn. J. Appl. Phys., pt. 1, vol. 36, pp. 5870–5873, 1997. [33] M. Schumacher, J. Lindner, P. K. Baumann, F. Shienle, N. Solayappan, V. Joshi, C. A. Araujo, and L. D. McMillan, “MOCVD for complex multicomponent thin films—A leading edge technology for next generation devices,” Mater. Sci. Semicond. Process., vol. 5, pp. 85–91, 2002. [34] B. Gilmore, “Development of high energy density dielectrics for pulse power applications,” Ph.D. dissertation, Univ. Missouri, Rolla, MO, 2001. [35] Y. Ye, S. C. Zhang, W. Huebner, and F. Dogan, “Nanostructured TiO for higher energy density dielectrics,” presented at the Center for Dielectric Studies Fall Meeting, Rolla, MO, 2002. , “Nanostructured TiO for higher energy density dielectrics,” [36] presented at the Center for Dielectric Studies Spring Meeting, State College, PA, 2003. [37] Y. Ye, S. C. Zhang, F. Dogan, E. Schamiloglu, J. Gaudet, P. Castro, M. Roybal, M. Joler, and C. Christodoulou, “Influence of nanocrystalline grain size on the breakdown strength of ceramic dielectrics,” in Proc. 14th IEEE Int. Pulsed Power Conf., 2003, pp. 719–722. [38] R. J. Vidmar, “Application of high-fluence convective cooling to pulsed power components,” in Proc. 14th IEEE Int. Pulsed Power Conf., 2003, pp. 257–260. [39] E. Schamiloglu, K. H. Schoenbach, and R. J. Vidmar, “On the road to compact pulsed power: Adventures in materials, electromagnetic modeling, and thermal management,” in Proc. 14th IEEE Int. Pulsed Power Conf., 2003, pp. 3–8.

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[40] Y. D. Korolev and G. A. Mesyats, Physics of Pulsed Breakdown in Gases. Yekaterinburg, Russia: Ural Div., Russian Acad. Sci., 1998. [41] J. M. Meek and J. D. Craggs, Electrical Breakdown in Gases. Oxford, U.K.: Clarendon, 1953. [42] R. Rompe and W. Weitzel, “Uber das toeplersche funkengesetz,” Z. Phys., vol. 122, pp. 9–12, 1944. [43] A. E. Vlastos, “The resistance of sparks,” J. Appl. Phys., vol. 43, no. 4, pp. 1987–1989, 1972. [44] S. I. Braginskii, “Theory of the development of a spark channel,” Sov. Phys. JETP, vol. 34, p. 1068, 1958. [45] T. P. Sorensen and V. M. Ristic, “Rise time and time-dependent spark-gap resistance in nitrogen and helium,” J. Appl. Phys., vol. 48, no. 1, pp. 114–117, 1977. [46] J. C. Martin, J. C. Martin on Pulsed Power, T. H. Martin, A. H. Guenther, and M. Kristiansen, Eds. New York: Plenum, 1996.

John A. Gaudet (Member, IEEE) received the A.B. degree from the College of the Holy Cross, Worcester, MA, in 1969, the M.S. degree from the University of Notre Dame, South Bend, IN, in 1971, and the Ph.D. from the Air Force Institute of Technology, Wright–Patterson Air Force Base, OH, in 1981. During a 22-year U.S. Air Force career, he conducted research on numerical modeling of electromagnetic pulse, developed space experiments for plasma satellite interactions, and directed pulsed power and high-power microwave (HPM) research. He also taught physics at the Air Force Academy. From 1993 to 2001, he was a Staff Member at the New Mexico Engineering Research Institute, working on both narrow-band and ultrawideband high-power microwave projects for the Air Force Research Laboratory (AFRL). In 2001, he joined the Electrical and Computer Engineering Department at the University of New Mexico (UNM) as Associate Research Professor. He continues to work with AFRL, concentrating on nonlinear effects in circuits caused by RF interference. At UNM, he studies the electrical breakdown characteristics of advanced materials for use in compact pulsed power applications. Dr. Gaudet is a Charter Member of the Directed Energy Professional Society.

Robert J. Barker (Fellow, IEEE) received the B.S. degree in physics from Stevens Institute of Technology, Hoboken, NJ, in 1971 and the Ph.D. degree in applied physics from Stanford University, Stanford, CA, in 1978. He was previously with the U.S. Naval Research Laboratory, Washington, D.C., and the Mission Research Corporation, Washington, D.C., where he worked on improvements to and applications of both the two-dimensional MAGIC and three-dimensional SOS plasma simulation codes. He is currently Program Manager for electroenergetic physics at the U.S. Air Force Office of Scientific Research (AFOSR), Arlington, VA. He coedited Advances in High Power Microwave Sources and Technologies (Piscataway, NJ: IEEE, 2001) (with E. Schamiloglu) and is currently coediting two new books. The first is an Institute of Physics Press book on nonequilibrium air plasmas, coedited with K. Becker, K. Schoenbach, and U. Kogelschatz; the second is an IEEE Press book on microwave vacuum electronics, coedited with N. Luhmann, J. Booske, and G. Nusinovich. He is sole inventor on two patents. His current interests include microwave/millimeter-wave generation, pulsed power, medical/biological effects, electromagnetic/electrothermal launchers, air plasmas, charged particle beam generation and propagation, explosive power generation, and computational physics. Dr. Barker is a member of the American Physical Society. He was elected Fellow of the Air Force Research Laboratory in 1998. He has served in various capacities on the organizing committees of numerous international conferences, including cochairing the IEEE Joint Pulsed Power Plasma Science (PPPS) Conference in 2001.


C. Jerald Buchenauer received the A.B. degree in physics from Franklin & Marshall College, Lancaster, PA, and the Ph.D. degree in physics from Cornell University, Ithaca, NY. Since 1975, he has been a Technical Staff Member, Los Alamos National Laboratory, Los Alamos, NM, where he worked in the controlled thermonuclear research program, optics, and electromagnetic phenomenology. From 1991 to 1998, he was also an IPA at the Air Force Research Laboratory, where he conducted research on time-domain antennas and sensors and pulsed power phenomena. Since 2003, he has also been with the University of New Mexico, Albuquerque, working on the Compact Pulsed Power Multidisciplinary University Research Initiative program. Dr. Buchenauer is a Member of the American Physical Society.

Christos G. Christodoulou (Fellow, IEEE) received the Ph.D. degree in electrical engineering from North Carolina State University, Raleigh, in 1985. From 1985 to 1998, he was a Faculty Member, University of Central Florida. In 1999, he joined the faculty of the Electrical and Computer Engineering Department, University of New Mexico, Albuquerque. He has published over 180 papers in journals and conferences, and is the author of Neural Network Applications in Electromagnetics (Norwood, MA: Artech House, 2001). He was a Guest Editor for a Special Issue on Applications of Neural Networks in Electromagnetics in the Applied Computational Electromagnetics Society Journal. His research interests are in the areas of modeling of electromagnetic systems, machine-learning applications in electromagnetics, smart antennas, and microelectromechanical systems. He is a Member of the International Union of Radio Science (URSI) (Commission B). He served as the General Chair of the IEEE Antennas and Propagation Society/URSI 1999 Symposium, Orlando, FL, and as a Cochair of the IEEE 2000 Symposium on Antennas and Propagation for wireless communications, Waltham, MA. He is an Associate Editor for the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION and the IEEE Antennas and Propagation Magazine.

James Dickens received the Ph.D. degree in electrical engineering from Texas Tech University, Lubbock, in 1995. In 1998, he joined the faculty at Texas Tech University and is currently an Associate Professor. His research interests include explosively driven pulsed power generation, investigation of high-efficiency power processing for space applications, compact pulsed power, and liquid breakdown phenomena. He also has broad research interests in power electronics, aerospace electronics, electric space propulsion, and pulsed power technology.

Martin A. Gundersen (Fellow, IEEE) received the B.A. degree from the University of California, Berkeley, in 1965 and the Ph.D. degree from the University of Southern California (USC), Los Angeles, in 1972. From 1998 to 2003, he was Chair of the Department of Electrical Engineering-Electrophysics and Cochair for Electrical Engineering at USC. He is currently Professor of Electrical Engineering, Physics and Astronomy, and Materials Science at USC. He has also been Visiting Professor at UCLA (1986–1987), Visiting Scientist at MIT (1986–1987 and 1989), Visiting Scientist, C.E.R.N. (1987), Visiting Associate at Caltech (1993–1994), and Visiting Professor of Physics at the Naval Postgraduate School (2003–2004), and has worked a little in movies. He has published approximately 240 technical papers, given approximately 250 invited and contributed presentations, and holds several patents. His main research interests are in applied physics, and include pulsed power science and technology, applied plasma physics, lasers, transient plasma devices, applications of pulsed power to biology, and science and entertainment. Dr. Gundersen is a Fellow of the Optical Society of America. He received the Germeshausen Award of the IEEE Power Modulator Symposium (2000). He was Director, NATO Advanced Research Workshop (on Pseudosparks, 1989), Technical Program Chairman, IEEE Power Modulator Symposium (1990 and 1998) and High Voltage Workshop (2000), and is ExCom Chair for the Power Modulator Conference (2002 and 2004).

Ravindra P. Joshi (Senior Member, IEEE) received the B. Tech. and M. Tech. degrees in electrical engineering from the Indian Institute of Technology in 1983 and 1985, respectively, and the Ph.D. degree in electrical engineering from Arizona State University, Tempe, in 1988. He was a postdoctoral fellow at the Center of Solid State Electronics Research, Arizona State University. In 1989, he joined the Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, VA, as an Assistant Professor. He is currently a Full Professor involved in research broadly encompassing modeling and simulations of charge transport, nonequilibrium phenomena, and bioelectrics. He also has used Monte Carlo methods for simulations of high-field transport in bulk and quantum well semiconductors. He has been a Visiting Scientist at Oak Ridge National Laboratory, Philips Laboratory, Motorola, and NASA Goddard. He is the author of over 80 journal publications and reviews manuscripts for ten journals. Dr. Joshi is serving as a Guest Editor for the IEEE TRANSACTIONS ON PLASMA SCIENCE.

Hermann G. Krompholz (Senior Member, IEEE) received the Ph.D. degree in physics from the Technical University Darmstadt (TU Darmstadt) in 1977. From 1977 to 1982, he was a Research Associate at TU Darmstadt, working on nonequilibrium phenomena in high energy density plasmas. From 1982 to 1985, he was with Texas Tech University, Lubbock, with activities in the areas of diffuse discharge opening switches and spark gap erosion. After a brief stay at the Fraunhofer Institute for Laser Engineering and Technology, he rejoined the faculty at Texas Tech University in 1987, where he is now Professor of Electrical and Computer Engineering/ Physics. He has published about 110 journal papers and conference proceedings papers. His research interests include several aspects of pulsed power physics and technology, with emphasis on the physics of electrical breakdown in gases, liquids, and along surfaces.


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Juergen F. Kolb was born in Neustadt/Aisch, Germany, in 1968. He received the first state examination degree in mathematics and physics from the University of Erlangen, Germany, in 1995. From the same university he received the Dr. rer. nat. degree in physics in 1999. As a Postdoctoral Research Associate at the Technical University Darmstadt, he continued to work on the interaction of heavy ion beams with plasmas. In 2002, he was with the Physical Electronics Research Institute. In 2003, he joined the Center for Bioelectrics, Old Dominion University in Norfolk, VA. His current research interests are microplasma discharges, liquid dielectrics for pulsed power systems, and the effects of pulsed electric fields on cells.

Andreas A. Neuber (Senior Member, IEEE) was born in Aschaffenburg, Germany. He received the Dipl.-Phys. and Ph.D., ME, degrees from the Darmstadt University of Technology in 1990 and 1996, respectively. From 1990 through 1996, he was with the Institute of Energy and Power Plant Technology, Darmstadt University of Technology, in the area of nonlinear laser spectroscopy and chemical reaction kinetics in combustion. In 1996, he joined Texas Tech University, Lubbock, and is currently Associate Professor in Electrical and Computer Engineering. He has published 100 journal and conference proceedings papers. His current research interests are high-power microwaves, unipolar surface flashover physics, and explosive-driven pulsed power. Dr. Neuber has served in various capacities on the organizing committees of numerous international conferences, including Technical Program Cochair of the 2002 Power Modulator Conference and Technical Program Chair of the 2003 IEEE International Pulsed Power Conference. He is a Co-Guest Editor of the 2005 IEEE TRANSACTIONS ON PLASMA SCIENCE Special Issue on Power Modulators and Repetitive Pulsed Power, and he has served as an external readiness review panel member for the U.S. Department of Energy.

András Kuthi received the Ph.D. degree in physics from the Royal Institute of Technology in 1981. From 1983 to 1991, he was a Research Physicist in the Plasma Physics Laboratory, University of California, Los Angeles, working in the field of experimental magnetic fusion research. Since 1991, he has been working in industry, first as a senior physicist with First Point Scientific, Inc., concentrating on pulsed power, plasma centrifuge for material and medical isotope separation, high-energy electron beam generation, pulsed laser concepts for plasma diagnostics, electron beam-based methods for treatment of volatile organic compounds and NO , SO emissions, and neutron sources for geophysical exploration, and later as principal scientist of NOVEM Co., a consulting company serving the semiconductor equipment industry on issues of plasma physics and RF technology. He is currently a Research Associate in the Electrical Engineering–Electrophysics Department, University of Southern California, Los Angeles. His current research interests are in pulsed power generators and gas phase high-voltage high-current fast switching devices.

Mounir Laroussi (Senior Member, IEEE) received the B.S. degree from the University of Technical Sciences of Tunisia in 1979, the M.S. degree from the School of Radio-electricity in 1981, and the Ph.D. degree in electrical engineering from the University of Tennessee, Knoxville in 1988. After a few years of teaching, he was with the Microwave and Plasma Laboratory, University of Tennessee, as a Research Assistant Professor from 1995 to 1998. He joined Old Dominion University (), Norfolk, VA, in 1998 as Research Faculty Member in the Applied ODUResearch Center and now holds an Associate Professor position at the Electrical and Computer Engineering Department and is a researcher at ODU’s Center for Bioelectrics. He has authored or coauthored more than 50 papers and holds three patents. His research interests are in the physical electronics area and particularly in the physics and applications of nonequilibrium gaseous discharges. Dr. Laroussi is a Member of Sigma Xi. He received the 1996 Advanced Technology Award from the Inventors Clubs of America, and the IEEE Millenium Medal in 2000. He has served as a Guest Editor of the IEEE TRANSACTIONS ON PLASMA SCIENCE and is currently serving on the Administrative Committee of the IEEE Nuclear and Plasma Science Society.

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William. C. Nunnally received the Ph.D. degree in electrical engineering from Texas Tech University, Lubbock, in 1974. His dissertation studied the interaction of a solid pellet with a simultaneous laser pulse and high-temperature hydrogen plasma impression. He was with the Los Alamos National Laboratory, working in the areas of magnetic fusion, laser fusion, laser isotope separation, plasma physics, and advanced electronics. In 1985, he joined the faculty of the University of Texas at Arlington as Director of the Applied Physical Electronics Research Center to focus on pulse power systems for space-based directed energy systems. In 1996, he moved to the University of Missouri, Columbia, as Director of Industrial Relations. He is currently a C. W. LaPierre Professor in the Electrical and Computer Engineering Department and is currently on leave of absence from the Lawrence Livermore National Laboratory, Livermore, CA, where he has served as a summer faculty member for a number of years. He has a number of patents, book chapters, IEEE seminars, publications, and awards to his credit. His current research interest are electrooptical signal processing, electromagnetic launchers, high-power photoconductive switches, and compact pulse power systems.


Edl Schamiloglu (Fellow, IEEE) received the B.S. and M.S. degrees from the School of Engineering and Applied Science, Columbia University, New York, in 1979 and 1981, respectively, and the Ph.D. degree in applied physics (minor in mathematics) from Cornell University, Ithaca, NY, in 1988. He was appointed Assistant Professor of Electrical and Computer Engineering at the University of New Mexico (UNM) in 1988. He is currently Professor of Electrical and Computer Engineering and directs the Pulsed Power, Beams, and Microwaves Laboratory at UNM. He lectured at the U.S. Particle Accelerator School, Harvard University, Cambridge, MA, in 1990 and at the Massachusetts Institute of Technology, Cambridge, in 1997. He coedited Advances in High Power Microwave Sources and Technologies (Piscataway, NJ: IEEE, 2001) (with R. J. Barker) and he is coauthoring High Power Microwaves, 2nd Ed. (Bristol, U.K.: Inst. of Physics, 2004) (with J. Benford and J. Swegle). He has authored or coauthored 53 refereed journal papers, 100 reviewed conference papers, and one patent. His research interests are in the physics and technology of charged particle beam generation and propagation, high-power microwave sources, plasma physics and diagnostics, electromagnetic wave propagation, and pulsed power. Dr. Schamiloglu has received the Sandia National Laboratories Research Excellence Award as part of the Delphi/Minerva team in 1991, the UNM School of Engineering Research Excellence Award twice (junior faculty in 1992 and senior faculty in 2001), and the titles of UNM Regents’ Lecturer (1996) and Gardner-Zemke Professor (2000). He is an Associate Editor of the IEEE TRANSACTIONS ON PLASMA SCIENCE and has served on a National Academies Panel on Directed Energy Testing.

Karl H. Schoenbach (Fellow, IEEE) received the Diploma and Dr.rer.nat. degrees in physics in 1966 and 1970, respectively, from the Technische Hochschule Darmstadt (THD). From 1970 to 1978, he was with THD, working in the areas of high-pressure gas discharge physics and dense plasma focus. From 1979 to 1985, he held a faculty position at Texas Tech University, where he was involved in research on fast opening switches, especially electron-beam and laser controlled diffuse discharge opening switches. In 1985, he joined Old Dominion University, Norfolk, VA. He was active in research on photoconductive switches until 1993, and has now concentrated his research efforts on high-pressure glow discharges, glow (streamer) discharges in liquids, and on environmental and medical applications of pulse power technology. Since 2002, he has served as the Principal Investigator of a Multidisciplinary University Research Initiative on “Subcellular Response of Biological Cells to Narrow Band and Wide Band Radio Frequency Radiation.” He is currently the Director of the Center for Bioelectrics, Old Dominion University. He is Coeditor of Low Temperature Plasma Physics (Berlin, Germany: Wiley-VCH, 2001). Prof. Schoenbach received the 2000 High Voltage Award sponsored by the IEEE Dielectric and Electrical Insulation Society. He has chaired a number of workshops and conferences, among them the 1991 IEEE International Conference on Plasma Science, and the First International Symposium on Nonthermal Medial/Biological Treatments Using Electromagnetic Fields and Ionized Gases (ElectroMed) in 1999. He was Guest Editor of the IEEE TrANSACTIONS ON ELECTRON DEVICES (1990), the IEEE TRANSACTIONS ON PLASMA SCIENCE (1999), the IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION (2003), and an Associate Editor of the IEEE TRANSACTIONS ON PLASMA SCIENCE.

J. Scott Tyo received the B.S.E., M.S.E., and Ph.D. degrees in electrical engineering from the University of Pennsylvania, Philadelphia, in 1994, 1996, and 1997, respectively. From 1994 to 2001, he was an officer in the U.S. Air Force, leaving service at the rank of captain. From 1996 to 1999, he was a research engineer in the Directed Energy Directorate of the U.S. Air Force Research Laboratory. From 1999 to 2001, he was a member of the faculty of the Electrical and Computer Engineering Department at the U.S. Naval Postgraduate School. Since 2001, he has been an Assistant Professor in the Electrical and Computer Engineering Department at the University of New Mexico. Since joining UNM, his research has focused on ultrawideband microwave radiating systems as well as microwave and optical remote sensing. Prof. Tyo is a member of the Optical Society of America, the International Union of Radio Science (URSI) (Commissions B and E), SPIE, Tau Beta Pi, and Eta Kappa Nu.

Robert J. Vidmar (Member, IEEE) received the B.S. degree in physics from the University of California, Santa Barbara, in 1969, the M.S. degree in physics from the University of Alaska, Fairbanks, in 1974, and the Ph.D. degree in applied physics from Stanford University, Stanford, CA, in 1981. At the University of Alaska he worked on excitation cross section measurements of atmospheric gases. At Stanford, his research included a theoretical description of pulse propagation in dispersive media with application to plasmas and an analysis of long-delayed radio waves. In 1981, he joined SRI International, where he was a Member of the Technical Staff of the Radio Science and Engineering Division. His research included analytic predictions, experimental measurements, and improvements to systems that measure electromagnetic scattering; investigation of electromagnetic emissions due to the interaction between a target and neutral-particle beam; interference reduction technology; and atmospheric pressure plasma as electromagnetic absorbers and reflectors. In 1998, he joined Xebec Engineering Services, Inc. Since 2001, he has been with the Physics Department, University of Nevada, Reno. His current research interests include high-performance heat transfer systems, tapered transmission lines, experimental and computational modeling of atmospheric pressure plasmas, and acoustic devices.


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Research Issues in Developing Compact Pulsed Power ... - IEEE Xplore

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