High power microwave

For more than a decade, the Swedish Defence Authorities have, in cooperation with Swedish industry and other countries, studied the effects of high-power microwave (HPM) radiation on electronic systems. Testing at high-field levels has been carried out on military equipment as well as on civil equipment, such as cars, computers, and security systems. From these studies, it is concluded that the distance for HPM sabotage can reach about a kilometer. Experience from system testing has, besides giving information about system susceptibility, also demonstrated the need for a careful pre-analysis of the system under test. This is due to the fact that high-level testing, in most cases, includes only a small fraction of the threat parameter space, such as test frequencies and irradiation angles.

Preliminary experimental results of an axially extracted virtual cathode oscillator (vircator) built on the low impedance pulsed electron beam accelerator AMBICA-600 is reported. The AMBICA-600 pulsed power system mainly comprises of a coaxial waterline charged from single turn Tesla transformer. Waterline has impedance of ~5Ω and generates pulse width of ~60ns. The generated pulsed high power microwave (HPM) emission from axially extracted virtual cathode oscillator has been detected actively as well as passively. The active detection technique utilizes receiving horn antenna, RF cable for signal transport, attenuator and microwave detector diode. The measured duration of HPM pulse (FWHM) varies from 30ns to 50ns on shot-to-shot basis. High power microwave emission of considerable power density has been passively confirmed by the bright glowing of freely standing neon's and compact fluorescent lamps (CFL) placed close to the output window of microwave launching horn. The precise measurement of generated microwave power and radiation frequency band are presently underway.

An experimental test facility has been established for measuring the secondary electron yield (SEY) of materials thought to be suitable for low yield vacuum electronic applications such as collectors in high-power microwave (HPM) tubes. Experiments can be broadly divided into two energy-regimes: a high-energy (1-50 keV) and a low-energy (10 eV-1 keV) regime. Measurements of SEY at high energies are presented for the following materials: copper, titanium, and Poco graphite. Observation of time-dependent SEY behavior in these samples suggests that surface processes play an important role during measurements. In addition, SEY at low energies and as a function of the angle of incidence of primary electrons has been measured for plasma sprayed boron carbide (PSBC). The experimental results presented here are benchmarked with existing SEY data in the literature, empirically and to first principle formulae.

Wideband and ultrawideband (UWB) technologies have achieved notable progress in recent years, aided immeasurably by improvements in solid-state manufacturing, computers, and digitizers. As a result, wideband systems that were difficult or impossible to implement ten years ago are now being built for an increasingly wide variety of applications including transient radar, concealed object detection, mine clearing, pipeline inspections, archeology, geology, electronic effects testing, and communications. In this paper, we discuss current wideband source technology around the world as well as laboratory and test facilities. We also will briefly touch on frequency limitations of anechoic chambers, design of transmission line simulators, frequency regulation requirements for outdoor ranges, and personnel exposure limits.

This article reviews the state-of-the-art in high-power microwave source research. It begins with a discussion of the concepts involved in coherent microwave generation. The main varieties of microwave tubes are classified into three groups, according to the fundamental radiation mechanism involved: Cherenkov, transition, or bremsstrahlung radiation. This is followed by a brief discussion of some of the technical fundamentals of high-power microwave sources, including power supplies and electron guns. Finally, the history and recent developments of both high-peak power and high-average power sources are reviewed in the context of four main areas of application: ͑1͒ plasma resonance heating and current drive; ͑2͒ rf acceleration of charged particles; ͑3͒ radar and communications systems; and ͑4͒ high-peak power sources for weapons-effect simulation and exploratory development.

The physical and chemical properties of wide bandgap semiconductors silicon carbide and diamond make these materials an ideal choice for device fabrication for applications in many different areas, e.g. light emitters, high temperature and high power electronics, high power microwave devices, micro-electromechanical system (MEMS) technology, and substrates. These semiconductors have been recognized for several decades as being suitable for these applications, but until recently the low material quality has not allowed the fabrication of high quality devices. Silicon carbide and diamond based electronics are at different stages of their development. An overview of the status of silicon carbide's and diamond's application for high temperature electronics is presented.

Wideband and ultrawideband (UWB) technologies have achieved notable progress in recent years, aided immeasurably by improvements in solid-state manufacturing, computers, and digitizers. As a result, wideband systems that were difficult or impossible to implement ten years ago are now being built for an increasingly wide variety of applications including transient radar, concealed object detection, mine clearing, pipeline inspections, archeology, geology, electronic effects testing, and communications. In this paper, we discuss current wideband source technology around the world as well as laboratory and test facilities. We also will briefly touch on frequency limitations of anechoic chambers, design of transmission line simulators, frequency regulation requirements for outdoor ranges, and personnel exposure limits.

We report on the high-power performance of the 0.25-m gate Doped-Channel GaN/AlGaN Heterostructure Field Effect Transistors (DC-HFET's). At a drain bias voltage of 18 V and drain bias current of 46 mA, these 100-m wide devices exhibit high gain at 8.4 GHz with a power density reaching 1.73 W/mm. The devices also display high gain at moderate power over a wide range of frequencies. This high gain at high frequency is a result of an optimal doping level in the AlGaN layer that gives rise to a high sheet charge density while maintaining a high-channel electron mobility. These results demonstrate the excellent microwave power capability of the GaN/AlGaN based heterostructure field effect transistors. W E recently reported on the temperature dependence of the cutoff frequency in the Doped-Channel Al Ga N/GaN Heterostructure Field Effect Transistors (DC-HFET's) with 0.25m gate length [1]

In most of applications involving both vacuum and high temperatures, outgassing of structural materials is a critical issue. For instance, this is the case of fusion test devices, where the gas released from the vessel walls contaminates the plasma. Four upper ports in the ITER vacuum vessel are reserved for Electron Cyclotron Heating and Current Drive (EC H&CD) Upper Launchers (UL), which have to provide plasma MHD stabilization by localized deposition of high power microwave beams. The structural material foreseen for the UL is the 316L(N)-IG stainless steel. It has to withstand temperatures in the range 120-150 • C during normal operation and 240 • C during the baking process. One of the preferred manufacturing routes for the UL is the Hot Isostatic Pressing (HIP) which is a very sophisticated method to manufacture structural components of complex geometry with good mechanical properties. The materials for use in the ITER vacuum systems have to comply with the outgassing limits given in the ITER vacuum handbook, but no outgassing data for HIPed stainless steel are available in literature, thus they must be obtained by experimental measurements. In this paper, measurements of partial outgassing rates are shown and discussed for stainless steel prototype samples AISI 316LN (on which the 316L(N)-IG is based) and AISI 317LMN, obtained by powder and solid HIPing method. A variant of the gas throughput method in vacuum systems was used for the measurements which were carried out over periods larger than 8 h and at different temperatures.

One class of high-current relativistic HPM devices, the relativistic magnetron [1], provides a low-impedance (10-50 Ω) load that inherently needs matching to a low-impedance pulsed-power (Gigawatt or greater) supply to operate with maximum efficiency. One of the most common and reliable pulsed-power drivers used to accomplish this is a Marx generator.

This article reviews the state-of-the-art in high-power microwave source research. It begins with a discussion of the concepts involved in coherent microwave generation. The main varieties of microwave tubes are classified into three groups, according to the fundamental radiation mechanism involved: Cherenkov, transition, or bremsstrahlung radiation. This is followed by a brief discussion of some of the technical fundamentals of high-power microwave sources, including power supplies and electron guns. Finally, the history and recent developments of both high-peak power and high-average power sources are reviewed in the context of four main areas of application: ͑1͒ plasma resonance heating and current drive; ͑2͒ rf acceleration of charged particles; ͑3͒ radar and communications systems; and ͑4͒ high-peak power sources for weapons-effect simulation and exploratory development.

Vertical interconnect accesses (VIAs) were fabricated between the source electrode on the front and the ground on the backside of high-power microwave AlGaN/GaN high-electron mobility transistors (HEMTs) on ∼ 400-µm-thick silicon carbide substrates. Through-wafer microholes with an aspect ratio of up to ∼ 8 were drilled using pulsed UV-laser machining and subsequently metallized using electroplating. The successful implementation of the laser-assisted VIA technology into device processing was proven by dc and RF characterization. When biased at 26 V, a saturated output power of 41.6 W with an associated power-added efficiency of 55% at 2 GHz was achieved for a 20-mm AlGaN/GaN HEMT with through-wafer VIAs.

The electrical characteristics and design features of a low inductance, compact, 500 kV, 500 J, 10 Hz repetition rate Marx generator for driving an high-power microwave ͑HPM͒ source are discussed. Benefiting from the large energy density of mica capacitors, four mica capacitors were utilized in parallel per stage, keeping the parasitic inductance per stage low. Including the spark-gap switches, a stage inductance of 55 nH was measured, which translates with 100 nF capacitance per stage to ϳ18.5 ⍀ characteristic Marx impedance. Using solely inductors, ϳ1 mH each, as charging elements instead of resistors enabled charging the Marx within less than 100 ms with little charging losses. The pulse width of the Marx into a matched resistive load is about 200 ns with 50 ns rise time. Repetitive HPM generation with the Marx directly driving a small virtual cathode oscilator ͑Vircator͒ has been verified. The Marx is fitted into a tube with 30 cm diameter and a total length of 0.7 m. We discuss the Marx operation at up to 21 kV charging voltage per stage, with repetition rates of up to 10 Hz in burst mode, primarily into resistive loads. A lumped circuit description of the Marx is also given, closely matching the experimental results. Design and testing of a low cost, all-metal Vircator cathode will also be discussed.

High Power Microwave technology has matured to the point where practical Elec- troMagnetic Pulse (1) weapons are becoming technically feasible, with applications in Electronic Combat, Offensive Counter-Air, Combat Air Support and in Strategic Air Attack. The current development of conventional ElectroMagnetic Pulse devices allows their use in non-nuclear confrontations. This paper proposes a doctrinal foundation for the use of such devices in warhead and bomb applications. † Carlo Kopp is currently working on a PhD in Computer Science. He is also a practising computer industry consultant who has specialised in computer systems, communications and operating systems software. He has prior industry R&D experience both as high speed electronic design engineer and systems integrator. As a Defence Technology Analyst writing for Australian Aviation journal in Canberra, he has produced technical fea- tures on modern military aviation since 1980. His other interests include the strategy, tac...

Record high RF power densities up to 30 W/mm have been recently achieved using AlGaN/GaN Heterostructure Field-Effect Transistors (HFETs). These results exhibit the promise of III-Nitride HFETs for high-power microwave applications. However the output RF power of these devices was found to degrade with the operation time. Gate leakage current in conventional HFETs is a major contributor to device instability and failure. We previously reported on the development of Metal-Oxide-Semiconductor Insulated gate HFETs (MOSHFETs) with gate leakage currents four to six orders of magnitude lower than those of the HFETs. The MOSHFETs has demonstrated a significant performance improvement as compared to conventional HFETs resulting in higher channel currents and RF powers. In this study, we present the first comparative experimental evaluation of the HFET and MOSHFET lifetime at room and elevated temperatures. The room temperature HFET stability lifetime as high as 2.4 years is estimated. We also present the detailed study of the HFETs stabilization and failure mechanisms. The pronounced similarity of the mechanisms of device degradation and current collapse is demonstrated. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The physical and chemical properties of wide bandgap semiconductors silicon carbide and diamond make these materials an ideal choice for device fabrication for applications in many different areas, e.g. light emitters, high temperature and high power electronics, high power microwave devices, micro-electromechanical system (MEMS) technology, and substrates. These semiconductors have been recognized for several decades as being suitable for these applications, but until recently the low material quality has not allowed the fabrication of high quality devices. Silicon carbide and diamond based electronics are at different stages of their development. An overview of the status of silicon carbide's and diamond's application for high temperature electronics is presented. Silicon carbide electronics is advancing from the research stage to commercial production. The most suitable and established SiC polytype for high temperature power electronics is the hexagonal 4H polytype. The main advantages related to material properties are: its wide bandgap, high electric field strength and high thermal conductivity. Almost all different types of electronic devices have been successfully fabricated and characterized. The most promising devices for high temperature applications are pn-diodes, junction field effect transistors and thyristors. MOSFET is another important candidate, but is still under development due to some hidden problems causing low channel mobility. For microwave applications, 4H-SiC is competing with Si and GaAs for frequency below 10 GHz and for systems requiring cooling like power amplifiers. The unavailability of high quality defect and dislocation free SiC substrates has been slowing down the pace of transition from research and development to production of SiC devices, but recently new method for growth of ultrahigh quality SiC, which could promote the development of high power devices, was reported. Diamond is the superior material for high power and high temperature electronics. Fabrication of diamond electronic devices has reached important results, but high temperature data are still scarce. PN-junctions have been formed and investigated up to 400 ∘C. Schottky diodes operating up to 1000 ∘C have been fabricated. BJTs have been fabricated functioning in the dc mode up to 200 ∘C. The largest advance, concerning development of devices for RF application, has been done in fabrication of different types of FETs. For FETs with gate length 0.2 μ m frequencies f T = 24.6 GHz, f max (MAG) = 63 GHz and f max (U) = 80 GHz were reported. Further, capacitors and switches, working up to 450 ∘C and 650 ∘C, respectively, have also been fabricated. Low resistant thermostable resistors have been investigated up to 800 ∘C. Temperature dependence of field emission from diamond films has been measured up to 950 ∘C. However, the diamond based electronics is still regarded to be in its infancy. The prerequisite for a successful application of diamond for the fabrication of electronic devices is availability of wafer diamond, i.e. large area, high quality, inexpensive, diamond single crystal substrates. A step forward in this direction has been made recently. Diamond films grown on multilayer substrate Ir/YSZ/Si(001) having qualities close those of homoepitaxial diamond have been reported recently.

The KALI-1000 pulse power system has been used to generate single-pulse nanosecond-duration high-power microwaves from an axial virtual cathode oscillator. The typical electron beam parameters were 250 kV, 15 kA, and 100 ns, with a current density of a few hundreds of amperes per square centimeter. The shot-to-shot variation of the microwave pulsewidth was studied for various anode-cathode gaps and two different cathode materials (velvet and graphite). Results indicated that the average microwave pulsewidth for a diode with a graphite cathode increased as the anode-cathode gap was increased. The one-way analysis of variance was employed to examine the statistical correlation between the diode voltage, current, perveance, and the microwave pulsewidth for various diode gaps and for two different cathode materials. It was shown that the microwave pulsewidth variations are not statistically correlated with the diode voltage, current, and perveance.

A research activity involving the detailed consideration of novel high voltage transformers (HVTs) for pulsed-power applications has recently begun at Loughborough University (LU). Although the main goal is the demonstration of a compact and lightweight unit employing magnetic self insulation under vacuum conditions, the initial stage of the work is directed towards the development of a conventional air-cored HVT as a main component in a compact power supply for HPM sources. In cooperation with the Swedish Defence Research Agency (FOI), the power supply has been tested with a HPM source of the vircator type.

The generation of ultra-wideband (UWB) pulses is a challenging problem that involves generating pulses with fast rise times on the order of 100 ps and voltages of more than 500 kV. Pulsewidths from 130 ps to a few nanoseconds (ns) are possible. A critical step involves switching high voltages with precision. The use of both gas and oil for the switching medium has been accomplished with varying results. The Air Force Research Laboratory (AFRL) is pursuing both media in the gas-switched H-series of pulsers and in studies of oil switches that promise good performance in compact packages. We are also pursuing solid-state switched systems that have demonstrated the potential for use in compact systems and in transient antenna arrays with steerable beams. This paper reviews recent progress in fast, high voltage switching and UWB transmitter development. These UWB pulsers and antennas have the potential for use in transient radar, target identification, and communications. Index Terms-Brewster angle window, gas switch, high power microwave, impulse radiating antenna, oil switch, photoconductive solid-state switch, radio frequency lens, silicon avalanche shaper, spark gap switch, transient antenna, ultra-wideband. I. BACKGROUND U LTRA-WIDEBAND (UWB) sources and antennas are of interest for a variety of potential applications that range from transient radar systems to communications systems. Predating the recent technology developments of the last several years were a number of important discoveries in the 1980's that led to the first UWB pulsers capable of transmitting high peak field signals through a variety of antennas. The Bournlea pulse generator is a good example of an instrument that is capable of fast rise times and a high repetition rate [1]. Using a CX1599 hydrogen thyratron as its basis, the original Bournlea pulser, made in 1986, could produce a 5-kV pulse with a rise time of 3 ns and a pulse repetition frequency (PRF) Manuscript

A Lower Hybrid Current Drive (LHCD) system would be the prime method to drive current in SST-1. The LHCD system is based upon two TH2103D klystrons of M/s Thomson CSF at 3.7 GHz capable of delivering 500 kW CW microwave power. The klystrons have been commissioned and deliver the microwave power through two standard WR-284 waveguides. It is electrically isolated from the main system using DC breaks. Two high power circulators protect the tube from high reflection. Directional couplers measure forward and reflected power in the system. The output power is deposited on two matched dummy loads during testing. In the event of arcing or any other fault, the high voltage from the klystron is removed in less than 5 ms, with the help of a crowbar system, based on a pressurized rail gap switch, for the protection of the klystron. High power (up to 200 kW), 1000-s tests of two klystrons have been conducted and detailed test results are described in this paper. It also describes the high power microwave system at 3.7 GHz that is being used to test the high power microwave components, being designed and fabricated for use in the LHCD system on SST-1.

Physical mechanisms leading to microwave breakdown on windows are investigated for power levels on the order of 100 MW at 2.85 GHz. The test stand uses a 3-MW magnetron coupled to an S-band traveling wave resonator. Various configurations of dielectric windows are investigated. In a standard pillbox geometry with a pressure of less than 10 06 Pa, surface discharges on an alumina window and multipactor-like discharges starting at the waveguide edges occur simultaneously. To clarify physical mechanisms, window breakdown with purely tangential electrical microwave fields is investigated for special geometries. Diagnostics include the measurement of incident/reflected power, measurement of local microwave fields, discharge luminosity, and x-ray emission. All quantities are recorded with 0.2-1-ns resolution. In addition, a framing camera with gating times of 5 ns is used. The breakdown processes for the case with a purely tangential electric field is similar to dc flashover across insulators, and similar methods to increase the flashover field are expected to be applicable.

One of the major limiting factors for the transmission of high power microwave (HPM) radiation is the interface between dielectric-vacuum or even more severely between dielectric-air if HPM is to be radiated into the atmosphere. Surface flashover phenomena which occur at these transitions severely limit the power levels which can be transmitted. It is of major technological importance to predict surface flashover events for a given window geometry, material and power level. When considering an aircraft based high power microwave platform, the effects on flashover formation due to variances in the operational environment corresponding to altitudes from sea level to 50,000 feet (760 to 90 Torr; 1 Torr=133.3 Pa) are of primary interest. The test setup is carefully designed to study the influence of each atmospheric variable without the influence of high field enhancement or electron injecting metallic electrodes. Experimental data of flashover delay times across different materials, such as polycarbonate, Teflon®, and high density polyethylene as a function of background pressure and gas type, air, N 2 , argon are discussed. An empirical relationship between flashover field amplitude and delay time is given.

A research activity involving the detailed consideration of novel high voltage transformers (HVTs) for pulsed-power applications has recently begun at Loughborough University (LU). Although the main goal is the demonstration of a compact and lightweight unit employing magnetic self insulation under vacuum conditions, the initial stage of the work is directed towards the development of a conventional air-cored HVT as a main component in a compact power supply for HPM sources. In cooperation with the Swedish Defence Research Agency (FOI), the power supply has been tested with a HPM source of the vircator type.

Results of theoretical and experimental studies of a GW-class, large diameter microwave oscillator are presented. The device consists of a large cross-section (overmoded), slow-wave structure with a unique profile of wall radius specifically designed to support surface waves and to provide a strong beam-wave coupling at moderate voltage (500 kV), an internal adjustable microwave reflector, a coaxial microwave extraction section, and a coaxial magnetically insulated field emission electron gun. In preliminary experiments carried out at 8.3 GHz, the power level exceeding 0.5 GW and efficiency of 15% have been measured calorimetrically.

Experiments were carried out to generate High Power Microwaves (HPM) from KALI 5000 pulse power system using an axial and a coaxial virtual cathode oscillator (vircator). The KALI 5000 pulse power system is a Marx generator and Blumlein line based system capable of operation at 1 MV, 50 kA, 100 ns. The typical electron beam parameters were 400 kV, 20 kA, and 100 ns, with a current density of a few hundreds of amperes per square centimeter. It was found that the generated HPM power is almost negligible in an axial vircator configuration due to severe impedance collapse problem. Impedance collapse in a planar diode due to fast cathode and anode plasma expansion and bipolar flow across the diode gap. However, HPM could be generated in a coaxial vircator configuration because of the fact that the electrode plasma expansion velocity of the cylindrical diode is much smaller as compared with the planar diode for the same accelerating gap and diode voltage. Therefore, much higher voltage can be obtained for the cylindrical diodes as compared with the planar diodes for the same accelerating gap. The measured HPM power density and the electric field strength at antenna mouth are (4.3 m distance from the Vircator window) 104.5 kW/m2 and 6.27 kV/m respectively. Frequency of the HPM signal was 2.9 GHz and a time-dependent frequency analysis shows that the frequency remains constant in time for almost entire pulse duration.

High-quality SiO 2 was successfully deposited onto GaN by photo-chemicalvapor deposition (photo-CVD) using a D 2 lamp as the excitation source. The AlGaN/GaN metal-oxide semiconductor, heterostructure field-effect transistors (MOSHFETs) were also fabricated with photo-CVD oxide as the insulating layer. Compared with AlGaN/GaN metal-semiconductor HFETs (MESHFETs) with similar structure, we found that we could reduce the gate-leakage current by more than four orders of magnitude by inserting the photo-CVD oxide layer in between the AlGaN/GaN and the gate metal. With a 2-m gate, it was found that the saturated I ds , maximum g m , and gate-voltage swing (GVS) of the fabricated nitride-based MOSHFET were 512 mA/mm, 90.7 mS/mm, and 6 V, respectively.

Simulation-based-design (SBD) techniques to achieve "first-pass design success" depend on the development of fast, accurate, realistic models that can handle material properties, geometry, and appropriate boundary conditions. This paper describes a new three-dimensional (3-D) electromagnetic and large-signal simulation tool, Cold-Test and Large-Signal Simulator (CTLSS), which has been developed as part of an SBD tool suite for vacuum electron devices.

The results from an investigation into the performance of a fuse opening switch, to be used as a component of a compact power conditioning unit, PCU, in an explosively driven high power microwave system, is presented. A pulse forming network designed to mimic the current action of a flux compression generator is utilized for these experiments. The investigation focused on the effect of altering fuse parameters, such as the diameter of fuse conductors and conductor winding geometry, on voltage and power levels delivered to a resistive load. Also, experiments were conducted with a thin layer of Semicosil, a commercial silicon material used in slower fusing opening switches, applied to fuse conductors, to investigate the possible advantages of using such a coating. Experiments showed that an increased number of conductor wires (with an approximately constant total cross-sectional area) resulted in similar voltages delivered to the resistive load, and that coating fuse conductors with Semicosil had a negligible effect on power delivery. A detailed description of these experiments will be given, along with data and waveforms illustrating the effects of these parameter variations on power levels delivered to the load. I.

Abstract—In the first part of this paper, the status of the 140-GHz continuously operated gyrotrons with an output power of 1 MW for the stellarator Wendelstein 7-X will be described. With the first series tube, an output power of 1000 kW has been achieved in short pulse operation ( ...

The evolution of high power microwave (HPM) sources into practical systems requires the development of compact pulsed power that can be integrated into mobile platforms. One approach to pursuing this objective, developed by researchers at Sandia National Laboratories (Sandia) [1], is to utilize parallel-stacked Blumlein transmission lines energized with a compact Marx generator. Such a configuration would be capable of driving low impedance HPM sources with a long pulse waveform. One of the limitations of this approach is field enhancement-induced breakdown at the edges of the line. Another limitation is percolation of, and subsequent breakdown of the liquid dielectric that is used in the system. This paper describes a research program that, both computationally and experimentally, is studying electrical breakdown in such transmission line configurations for a variety of dielectric materials and substrate geometries. In this collaborative study calculations are being performed using various electromagnetic simulation tools at the University of New Mexico. Liquid breakdown investigations are being performed at Old Dominion University. The goal of this research is to better understand the properties of solid and liquid dielectrics to facilitate the development of compact pulsed power.

Abstract—In the first part of this paper, the status of the 140-GHz continuously operated gyrotrons with an output power of 1 MW for the stellarator Wendelstein 7-X will be described. With the first series tube, an output power of 1000 kW has been achieved in short pulse operation ( ...

Explosive field-emission cathodes as well as plasmaflashover cathodes are important for high-power microwave tubes. These cathodes have the advantage of being lightweight as well as requiring no heater for electron emission. However, this class typically suffers from large amounts of outgassing, nonuniform emission, and very high emittance. In this paper, we review research into the uniformity and emittance of tufted carbon-fiber cathodes that have been coated with cesium iodide (CsI) salt. The CsI cathode is compared to polymer velvet, metal-dielectric, and carbon-slat cathodes. We find the uniformity and emittance are related for all of these cathodes. In general, the more uniform the electron emission, the lower the emittance of the cathode. This article shows that given proper diode design, the tufted carbon-fiber cathode with CsI has some promise as an electron emitter producing a good quality electron beam.

The KALI-1000 pulse power system has been used to generate single-pulse nanosecond-duration high-power microwaves from an axial virtual cathode oscillator. The typical electron beam parameters were 250 kV, 15 kA, and 100 ns, with a current density of a few hundreds of amperes per square centimeter. The shot-to-shot variation of the microwave pulsewidth was studied for various anode-cathode gaps and two different cathode materials (velvet and graphite). Results indicated that the average microwave pulsewidth for a diode with a graphite cathode increased as the anode-cathode gap was increased. The one-way analysis of variance was employed to examine the statistical correlation between the diode voltage, current, perveance, and the microwave pulsewidth for various diode gaps and for two different cathode materials. It was shown that the microwave pulsewidth variations are not statistically correlated with the diode voltage, current, and perveance.

High frequency, high power microwave devices require pencil-shaped electron beams with much higher current densities than can be produced at a cathode surface; this has led to the development of space charge limited convergent beam electron guns. A new approach to simulation of these devices is used in a computer program currently under development. In this program, different physical processes are more tightly coupled for greater accuracy and more rapid convergence.

Traditional uses of the Marx generator have been limited to energy storage and delivery systems, such as charging capacitors or pulse forming lines. However, low energy, high peak power Marx generators are finding applications in Ultra Wideband radar and high power microwave systems. This paper discusses the compact generator as well as an assortment of impulse antennas. Experimental results, including those of the Marx generator and antenna range measurements are presented and discussed.

Carbon nanofiber and carbon nanotube reinforced polymer composites have shown promise due to their enhanced mechanical, electrical, thermal, and dielectric properties. In this study, vapor grown carbon nanofiber reinforced polyethylene composites were exposed to microwave radiation in a conventional resonance cavity microwave oven to explore the possibility of using microwave energy to assist the processing of carbon nanofiber reinforced thermoplastic composites and determine the effect of microwave radiation exposure on material properties. Average temperatures of up to 40 • C above ambient were measured after 5 min of microwave exposure. The effect of high power microwave radiation on the physical integrity of the composites via dynamic mechanical analysis, tensile tests, and dielectric analysis is presented. A drop of 50% in failure strain has been observed for composites with 15 and 20 wt% nanofiber concentrations after exposure to microwave radiation.

The mission of the Army is evolving, which means that the weapon systems required must evolve as well. This will require a new class of munitions with either enhanced lethality or less-than-lethal capability. This requires that we develop new technologies. In order to test these new technologies to ascertain their capabilities, we need suitable test beds. One such test bed that will allow us to evaluate new types of explosive pulsed power devices, power conditioning systems, high power microwave as well as other directed energy sources, and radiators is described in this paper.

We review the research progress in reversed Cherenkov radiation in double-negative metamaterials (DNMs) starting from the first experimental verification of the DNMs reported in 2001, including theories, numerical computation and simulation and experiments. We also discuss the potential applications to particle detectors and highpower microwave or millimeter-wave devices, including the oscillators and amplifiers, and the formidable challenges needed to be resolved before the benefits of using such artificial materials can be harvested.