Abstract: Barrier discharges, also known as dielectric-barrier discharges or silent discharges, provide a simple technology to establish nonequilibrium plasma conditions in atmospheric-pressure gases. This property has led to a number of industrial applications, including ozone generation, surface modification, pollution control, CO/sub 2/ lasers, excimer lamps, and flat plasma-display panels. Depending on a variety of gas properties, operating parameters, and boundary conditions, the discharge can exhibit pronounced filamentary character, self-organized regular-discharge patterns, or completely diffuse appearance. The literature on these different types of barrier discharges is reviewed, and the underlying physical phenomena are discussed. Relative recent investigations on low-current density diffuse barrier discharges suggest novel applications of fairly "mild" plasmas for sterilization and disinfection purposes and utilizing their selective influence on biological cells.

Abstract: The most important polymer film used in commercial capacitors is biaxially oriented polypropylene. Other materials, such as polyester or paper, are also used for selfhealing metallized capacitors, depending on the application. Capacitors manufactured with polypropylene have the big advantage of being less expensive than other materials, and have a very low equivalent series resistance (ESR), due to the propylene-repeating group, which results in a regular polymer chain. Electrical breakdown behavior will be analyzed in order to improve the electric field stress. Performance versus energy density will be investigated as well. We will give an overview of new proposals in the field of films, considering their reported better dielectric and mechanical performance.

Abstract: This review discusses the present state-of-the-art of passive high-power microwave components for applications in microwave systems for RF plasma generation and heating, plasma diagnostics, plasma and microwave materials processing, spectroscopy, communication, radar ranging and imaging, and for drivers of next generation high-field-gradient electron-positron linear colliders. The paper reports on high-power components for overmoded high-power transmission systems such as smooth-wall waveguides, HE/sub 11/ hybrid mode waveguides and quasi-optical TEM/sub 00/ beam waveguides. These include various types of mode converters, polarizers, cross-section tapers, bends, mode selective filters, pulse compressors, DC-breaks, directional couplers, beam combiners and dividers, vacuum windows, and instruments for mode analysis. Problems of ohmic attenuation and unwanted conversion to parasitic modes are discussed in detail and rules for alignment requirements are given. In the case of waveguide transmission, this review mainly concentrates on circular waveguide components but also deals with rectangular waveguide.

Abstract: The development of a new three-dimensional electron gun and collector design tool is reported. This new simulation code has been designed to address the shortcomings of current beam optics simulation and modeling tools used for vacuum electron devices, ion sources, and charged-particle transport. The design tool specifically targets problem classes including gridded-guns, sheet-beam guns, multibeam devices, and anisotropic collectors, with a focus on improved physics models. The code includes both structured and unstructured grid systems for meshing flexibility. A new method for accurate particle tracking through the mesh is discussed. In the area of particle emission, new models for thermionic beam representation are included that support primary emission and secondary emission. Also discussed are new methods for temperature-limited and space-charge-limited (Child's law) emission, including the Longo-Vaughn formulation. A new secondary emission model is presented that captures true secondaries and the full range rediffused electrons. A description of the MICHELLE code is presented.

Abstract: In the reduced-pressure (/spl les/10 torr) afterglow stemming from discharges in O/sub 2/- containing mixtures such as N/sub 2/-O/sub 2/, the test-reference spores are ultimately inactivated by UV photons through destruction of their genetic material (DNA). To show this, we assume the inactivation to result from a sufficiently large number of successful hits of the DNA strands by UV photons. This implies that the higher the UV intensity, the shorter the time required to reach the lethal dose. Simultaneously, the increased erosion of the spores by the oxygen atoms as time elapses reduces the incident number of photons required to meet the lethal dose. Erosion, as observed by scanning electron microscopy, also increases with the O/sub 2/ percentage in the mixture. Actually, sterilization time is found to be the shortest when the O/sub 2/ percentage in the mixture is set to maximize the UV emission intensity, which occurs at O/sub 2/ percentages typically below 2%, where erosion is low. This proves the predominant role of UV radiation over erosion as far as spore inactivation is concerned. In any case, plasma sterilization always implies some erosion of the test spores, in contrast to what happens with conventional sterilization methods.

Abstract: The design and experimental study of a 35-GHz gyrotron-traveling-wave tube (gyro-TWT) amplifier operating in the circular TE/sub 01/ mode at the fundamental cyclotron harmonic are presented. The interaction circuit in this experiment consisted of a new type of ceramic loading that provided the required loss for stable operation. A saturated peak power of 137 kW was measured at 34.1 GHz, corresponding to a saturated gain of 47.0 dB and an efficiency of 17%, with a -3-dB bandwidth of 1.11 GHz (3.3%). Peak output powers in the range of 102.1 to 148.6 kW with -3-dB bandwidths of 1.26 and 0.94 GHz, respectively, were measured by varying the operating parameters. The gyro-TWT was found to be zero-drive stable at these operating points, demonstrating that ceramic loading is a highly effective means of suppressing spurious oscillations in gyro-TWTs. This type of ceramic loading has the added advantage of being compatible with high average power operation.

Abstract: Minimization of the power required to sustain weakly ionized plasmas can be achieved if the energy of ionizing electrons is high, from tens to thousands of electronvolts. These electrons spend about a half of their energy on ionization cascades, in contrast to low-energy (1-3 eV) electrons in conventional discharges that dissipate most of their energy in nonionizing inelastic collisions. High-energy electrons can be injected into the gas as beams. Alternatively, they can be created in situ by applying a very strong electric field for a short time, with a repetition rate matching the rate of recombination. Analytical calculations show that the power budget in the high-voltage, repetitive pulse mode can be significantly lower than in the dc regime, but still much higher than in the case of electron beam ionization. For each pulse length, there exists an optimum electric field that minimizes the power budget. A detailed modeling of spatio-temporal dynamics of pulsed discharges reveal that voltage displacement into the cathode sheath plays a critical role. Fully coupled modeling of nonlocal kinetics of high- and low-energy electrons, ionization processes, charge particle transport, and electrodynamics was performed for a high-voltage pulse developing with a substantial pre-existing plasma. The kinetic modeling shows formation of a large group of high-energy electrons in high-voltage nanosecond pulse and dramatic increase in ionization rate. New effects are indicated, such as field reversal, "two-cathode" effect, and interpulse ionization.

Abstract: By using a tungsten wire with 75-/spl mu/m diameter, 2 cm apart from a plane cathode, and applying a 600-ns, 120-kV square wave pulse, we were able to obtain a pulsed-water corona discharge (PWC). The effect of these discharges on bacteria was studied using water contaminated with Escherichia coli or Bacillus subtilis, the latter in both the vegetative and spore state. The strongest effect was obtained on E. coli. The concentration of E. coli could be reduced by three orders of magnitude after applying eight corona discharges to the water. The corresponding energy expenditure is 10 J/cm/sup 3/. The decontamination rate had the largest values at the beginning, and decreased considerably after 15 electrical discharges, reaching a constant residual concentration value of 10/sup -4/ of the initial concentration. For B. subtilis in the vegetative state, it took almost 30 discharges to reach the same result, corresponding to an energy expenditure of 40 J/cm/sup 3/. There was no effect on B. subtilis spores. Comparisons with the pulsed-electric field (PEF) method indicate that the decontamination efficiency of the PWC method is slightly higher than that of the PEF method.

Abstract: PC-based arc modeling taking account of the effects of nozzle ablation and the mixing process of PTFE vapor with SF/sub 6/ has been carried out for a full-scale auto-expansion circuit breaker with a moving contact. An equation to calculate the PTFE concentration in the mixture is introduced together with overall mass, momentum, and energy conservation equations for the mixture. The thermodynamic and transport properties of SF/sub 6/-PTFE vapor mixture calculated by the transport theory are given. An empirical relationship between radiation reabsorption factor and the relevant arcing parameters (nozzle size and instantaneous arcing current) has been employed to make predictions. The features of temperature, pressure, PTFE mass concentration, and flow fields during the whole arcing period are discussed. It is shown that the energy brought into the expansion volume by the hot PTFE vapor is mainly responsible for the pressure buildup. The propagation of the pressure wave in the expansion volume is responsible for the pressure peaks in the second loop of arcing. The predicted arc voltage and pressure variation are compared with the test results and with those predicted using the thermodynamic and transport properties of pure SF/sub 6/. Results show that the use of the properties of SF/sub 6/-PTFE vapor mixture substantially improves the agreement between the predicted and measured pressure rise in the expansion volume.

Abstract: The gyrotron traveling-wave tube (gyro-TWT) is a millimeter-wave amplifier based on the electron cyclotron maser instability. It is a device of increasing importance because of its power and bandwidth capabilities. The current paper presents a brief overview of the gyro-TWT research over the past quarter of a century. Advances made by different groups employing various schemes are discussed and achieved performances are surveyed.

Abstract: The development of high-power gyrotrons (118 GHz, 140 GHz) in continuous-wave (CW) operation for heating nuclear fusion plasmas has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. The 140-GHz gyrotron being under development for the installation at the W7-X stellarator now under construction at the IPP Greifswald, Germany, operates in the TE/sub 28,8/ mode and is equipped with a diode type magnetron injection electron gun, an improved beam tunnel, a high mode-purity low-Ohmic loss cavity, an optimized nonlinear up-taper, a highly efficient internal quasi-optical mode converter, a single-stage depressed collector and an edge-cooled, single disk CVD-diamond window. RF measurements at pulse duration of a few milliseconds yielded an RF output power of 1.15 MW at a beam current of 40 A and a beam voltage of 84 kV. Depressed collector operation has been possible up to decelerating voltages of 33 kV without any reduction of the output power. Long pulse operation (10 s at 1 MW) was possible without any signs of a limitation caused by the tube. For this output power the efficiency of the tube could be increased from about 30% without to about 50% with depression voltage. The best performance reached so far has produced an energy per pulse as high as 90 MJ (power 0.64 MW, pulse length 140 s) which is the highest value achieved in gyrotrons operating at this frequency and power level. The pulse-length limitations so far are mainly due to the external system.

Abstract: The feasibility of fabrication of coaxial cavity gyrotrons with an output power up to 2 MW, continuous wave (CW) has been demonstrated and information necessary for a technical design has been obtained. Experiments with a gyrotron equipped with newly designed components-electron gun, cavity, RF output system-have been performed. In short pulses, a maximum radio frequency (RF) output power of 2.2 MW has been reached in stable operation. At the nominal output power of 1.5 MW an efficiency of 30% has been achieved. This has been enhanced to 48% in operation with a single-stage depressed collector. The stability of the coaxial insert has been measured to be within /spl plusmn/0.03 mm under operating conditions. The losses at the coaxial insert have been found to be about 0.1% of the RF output power. Investigations of the microwave stray radiation captured inside the tube have been performed with the following results: (1) the captured stray radiation due to diffraction losses is approximately uniformly distributed inside the mirror box; (2) about 8% of the captured microwave power is radiated through a relief window with 100-mm diameter in the used setup; and (3) the total amount of stray radiation has been found to be about 11% of the RF output power. Parasitic low-frequency oscillations have been successfully suppressed and stable operation has been achieved over a wide parameter range. Fast (/spl sim/0.1-ms) frequency tuning has been demonstrated by applying a rapid variable bias voltage at the coaxial insert. In particular, step frequency tuning by /spl plusmn/2.2 GHz due to switching from the nominal mode at 165 GHz to its azimuthal neighbors has been done and continuous tuning by up to 70 MHz within the bandwidth of the TE/sub 31,17/ mode has been performed.

Abstract: Recent developments in the application of controllable air ionization processes that apply dielectric-barrier discharge devices to generate nonthermal plasmas have led to applications for chemical and biological decontamination in indoor air environments. These include significant reductions in airborne microbials, neutralization of odors, and reductions of specific volatile organic compounds (VOCs). Removal of very fine particulates (PM/sub x/) is also is enhanced by air ionization. The process of air ionization involves the electronically induced formation of small air ions, including reactive oxygen species, such as superoxide O/sub 2//spl middot//sup -/, the diatomic oxygen radical anion, which react rapidly with airborne VOC and PM/sub x/. The physics and chemistry of air ionization, and its utility for contributing to significant improvements in indoor air quality are discussed.

Abstract: High-power millimeter wave sources are a key enabling technology in fusion energy research. The present state of the art of application of these sources to the areas of heating, current generation, and scattering for diagnostic purposes in fusion plasmas is reviewed. The extrapolation of these applications to future devices and the requirements which they place on sources and transmission lines are also discussed.

Abstract: A repetitive X-band relativistic backward-wave oscillator (BWO) driven by a SINUS-881 accelerator is described. Relativistic electron beams with peak current of 5.4 kA and voltage of 610 kV at a repetition rate of 100 Hz were generated by the SINUS-881 and then guided through the corrugated waveguide by an axial magnetic field of 3.0 T produced by a superconducting magnet. An electron collector was used to collect the electron beams in order to mitigate the effect of secondary emission electrons and to prevent ionization and breakdown near the electron beam dump. This BWO produces a microwave pulse power of 1.1 GW at a 100-Hz repetition rate, a frequency of 9.38 GHz, a pulse duration of 23 ns, and a power transforming efficiency of 33%.

Abstract: Two types of nonthermal plasma processes such as pulsed corona discharge and dielectric barrier discharge were investigated for the decomposition of volatile organic compounds and nitric oxide. The performance equation of the plasma reactor was derived with the assumption that the decomposition rate of the pollutant is directly proportional to the concentration of the pollutant and the discharge power. From this model equation and the experimental decomposition data, the apparent decomposition rate constants of various organic compounds and nitric oxide were determined. Alkene and substituted alkene were found to have much larger decomposition rate constants than aromatics and substituted alkane compounds, which implies that the decomposition of the derivatives of aromatics and alkanes requires more energy. To verify the validity of the model derived, the experimental data in the present study and in the literature were compared with the calculation results, and relatively good agreements were achieved between them.

Abstract: Results of experimental investigation and modeling of prolonged plasma production during implosion of cylindrical wire arrays are presented. Results of the radiography of dense cores of imploding wire array and the measurements of internal azimuthal magnetic field in wire array give new experimental evidences of prolonged plasma production phenomenon. This phenomenon is an important property of current-driven implosion of the wire arrays at current rise rates /spl sim/(0.5-1)10/sup 14/ A/s. The prolonged plasma production can determine the current and the density profiles before final stage of a Z pinch compression, and also in the moment of Z pinch stagnation. From this point of view, the requirement that residual uncompressed plasma should not shunt the current at the discharge periphery becomes of the greatest importance. The conditions exist when the prolonged plasma production isn't an obstacle for the achievement of high-power X-ray emission from Z pinch. Presented experimental results on multiwire array implosion can be explained on the basis of prolonged plasma production without referring to multiwire array azimuthal structure.

Abstract: Nonthermal gas discharges at atmospheric pressure, such as dielectric barrier discharges are currently investigated for low-temperature packaging sterilization in order to reach the conditions required for aseptic food packaging. In particular, understanding the basic sterilization mechanisms and the enhancement of the main bacterial reduction pathways are the goals of our investigations. For this purpose, germ reduction experiments were carried out with Bacillus Subtilis and Aspergillus Niger spores using different gas mixtures and plasma conditions with the direct and the indirect influence of barrier discharges. In order to analyze the contribution of UV radiation during plasma germ deactivation, experiments with different excimer UV lamps, also driven by barrier discharges in special UV-emitting gas mixtures, have been carried out. Results of germ reduction experiments using barrier discharges and prospects for atmospheric discharge systems, suitable for industrial packaging sterilization, are presented in this paper.

Abstract: The X-radiation emission from a low energy plasma focus with argon as a filling gas is investigated. Specifically, the attention is paid to determine the system efficiency for argon K-lines and Cu-K/sub /spl alpha// line emission at different filling pressures, and identify the radiation emission region. The highest argon line emission found at 1.5 mbar is about 30 mJ and the corresponding efficiency is 0.0015%. The same pressure is suitable for high Cu-K/sub /spl alpha// emission, which is about 70 mJ in 4/spl pi/ geometry and the system efficiency is 0.003%. The bulk of X-radiation is emitted from the region close to the anode tip, whereas some radiation emission takes place from the formed hot spots along the focus axis. These radiations are found suitable for backlighting in Al (1-1.56 keV) and Ti (2.9-4.96 keV) energy transmission bands.

Abstract: Nitric monoxide (NO) is increasingly being used in medical applications. Currently, a gas cylinder of N/sub 2/ mixed with a high concentration of NO is used. This arrangement is potentially risky due to the possibility of accidental leak of NO from the cylinder. The presence of NO in air leads to the formation of nitric dioxide (NO/sub 2/), which is toxic to the lungs. Therefore, an on-site generation of NO would be very desirable for patients with acute respiratory distress syndrome and other related illnesses. We have recently reported on the production of NO using a pulsed arc discharge. In the present work, the discharge reactor was made simpler and smaller. NO was generated using a pulsed arc discharge in dry air and in mixtures of oxygen and nitrogen. The composition of the gas mixture after treatment with an arc discharge followed by exposure to heated molybdenum was 540 ppm of NO, 48 ppm of NO/sub 2/, and the balance dry air at 0.1 MPa and 300 /spl plusmn/ 3 K. No ozone was detected at the outlet of the system by UV absorption. The density of the brass particles emitted from the electrodes, which had diameters over 0.3 /spl mu/m, was less than 1.39 /spl mu/g/L. A filter could readily capture and thus remove the brass particles.

Abstract: The maximum achievable gain in individual sections of helix-type traveling-wave amplifiers is limited by the requirement that the device be stable with respect to the excitation of spurious modes. The excited modes may be of two types: backward waves, which are absolutely unstable, or forward waves, which are unstable in the presence of reflections and regenerative amplification. Whether a specific device is unstable depends on a number of details that must be computed numerically. We will present a model, used in the traveling-wave tube simulation code CHRISTINE, in which stability is determined including a number of important effects. These are: the placement of severs, their reflection and transmission coefficients, the profile of attenuation along the interaction length, the presence of a driven signal, and the coupling of forward and backward waves due to asymmetries in the helix support structure. Asymmetries result in a stopband near the "/spl pi/" point formed by the coupling of the forward and backward waves. For this case, a model is developed to evaluate the maximum stable length.

Abstract: This article illustrates a variety of direct current and pulsed glow discharges obtained either in ambient or preheated (2000 K) air or nitrogen at atmospheric pressure. The images presented show that stable and diffuse discharges, with electron densities as high as 10/sup 12/ cm/sup -3/, can be obtained at atmospheric pressure with no filamentation or constriction. Spectroscopic measurements confirm that these discharges are of the glow type.

Abstract: Modern high-power gyrotrons typically use a mode converter and launcher to convert the high-order cavity mode to a Gaussian like output mode. Efficiencies of the conversion are usually only 85%-90%. The analysis codes used to design these systems are based on approximate techniques which are not sufficiently accurate to allow design of higher efficiency converters. We have developed an analysis code based on the magnetic field integral equation. This approach will give engineers a tool for design of more efficient converter and launcher systems.

Abstract: A "plasma decon chamber" has been developed at Los Alamos National Laboratory (LANL), Albuquerque, NM, to study the decontamination of chemical and biological warfare agents. This technology is targeted at sensitive electronic equipment for which there is currently no acceptable, nondestructive means of decontamination. Chemical reactivity is provided by a downstream flux of reactive radicals such as atomic oxygen and atomic hydrogen, produced in a capacitively coupled plasma. In addition, the decon chamber provides an environment that accelerates the evaporation of chemical agents from contaminated surfaces by vacuum, heat, and forced convection. Once evaporated, agents and agent byproducts are recirculated directly through the plasma, where they undergo further chemical breakdown. Preliminary studies on actual chemical agents were conducted at the U.S. Army Dugway Proving Ground, Dugway, UT. Exposures were conducted at a system pressure of 30 torr, exposure temperature of 70/spl deg/C, plasma-to-sample standoff distance of 10 cm, and 10% addition of oxygen or hydrogen to a helium balance. This exposure condition was based on optimization studies conducted at LANL on agent simulants. The agents studied were VX and soman (GD) nerve agents and sulfur mustard (HD) blister agent, as well as a thickened simulant. All agents were decontaminated off aluminum substrates to below the detection limit of /spl sim/0.1% of the initial contamination level of approximately 1 mg/cm/sup 2/. For VX, this level of decontamination was achieved in 8-16 min of exposure, while only 2 min were required for the more volatile HD and GD. Evaporation and subsequent gas-phase chemical breakdown in the plasma appears to be the dominant decontamination mechanism for all of the agents. However, an observed difference in the decontamination process between oxygen and hydrogen indicates that chemical reactivity in the liquid phase also plays an important role.

Abstract: A self-consistent large-signal beam-field interaction model for vacuum electronic microwave sources with external cavities is described. The model includes a self-consistent solution of the three-dimensional equations of electron motion and the time-dependent field equations. The RF fields are decomposed into the fields inside the beam region and the fields inside outer resonators. The RF fields inside the beam region are represented as a superposition of local waveguide modes. The RF fields inside resonators are represented as a sum over resonator modes. The various modes are coupled together due to gaps connecting cavities with each other and with the beam region. The numerical implementation of the model requires additional analytical steps to obtain an effective, convergent, and stable numerical solution. The modified version of the code MAGY has been tested by a comparison with known results and also with measured data.

Abstract: The results of a focused program to develop high-power W-band gyro-amplifiers, which culminated in the demonstration of record average output powers from amplifiers in this band, are described. Following an experimental and theoretical study of low-duty prototype amplifiers, two high-average power devices were designed, built, and demonstrated. The first high-average power amplifier achieved 10.1-kW average output power at 33% efficiency in the TE/sub 0.1/ mode at 93.8 GHz. The instantaneous bandwidth was 420 MHz and the saturated gain at the 10.1-kW point was 32 dB. The second high-average power gyroklystron, designed for improved bandwidth, demonstrated 10.2-kW average power at 31% efficiency with 700-MHz instantaneous bandwidth and 33-dB saturated gain. The measured results of the low-duty prototype amplifiers and the high-average power gyroklystrons are described in detail. In addition, theoretically predicted results for a high-average-power W-band gyrotwystron amplifier, which is currently in construction, are presented.

Abstract: The energy deposited by a submicrosecond, /spl sim/1-kA current pulse in a 25-/spl mu/m diameter metal wire prior to its explosion, correlates directly with the expansion rate of the wire after the explosion. Energy deposition by resistive heating is terminated by the formation of plasma around the wire and a collapse of the voltage along the wire, and is evidently facilitated by the desorption of gases from the wire and/or the evaporation of the wire material (or impurities within it) as it heats up. Data presented here implies that the relationship between materials with the lowest resistivities and high exploding wire expansion rates found in earlier work (D. B. Sinars et al. 2000) is a result of the reduced voltage delaying the gas breakdown along such wires. This, in turn, increases the energy deposited resistively in the wire before the current shifts to the surrounding plasma. If gas breakdown does not occur until close to the full vaporization energy is deposited in the wire, the expansion rate will be more rapid than if a small fraction of the vaporization energy is deposited before voltage collapse.

Abstract: This paper presents results of tests of a hybrid subnanosecond modulator with an output resistance of 45 /spl Omega/. The modulator comprises an all-solid-state nanosecond charger, which is equipped with an inductive energy store and a semiconductor opening switch, and a pulse peaker with hydrogen spark gaps. The modulator generates stable pulses-(180 to 200) kV in amplitude and 400 to 700 ps long at a pulse repetition rate of up to 3.5 kHz. An average output power of 1.5 kW was achieved under the pulse burst mode.

Abstract: Capillary discharge experiments were carried out for soft X-ray laser studies. A ceramic capillary, which has an inner diameter of 3 mm and a length of 150 mm, has been used for the end-on X-ray diode observation, and a Pyrex capillary has been used for side-view observation. Spike output has been observed, when operating the device with a predischarge current of 5 to 15 A, a current of 9 to 35 kA with a rise time of 55 ns in an argon gas pressure range from 100 to 800 mtorr. It is found that without a predischarge current, spike output has been hardly observed. Observation of spike output at a low discharge current of 9 kA provides us a possibility for design of a compact soft X-ray laser device. When the predischarge is turned off, the side-view observation of the capillary discharge clearly shows the growth of instabilities during the pinch process. This suggests that the predischarge is indispensable for achieving a uniform plasma, which is required by the soft X-ray lasing in a capillary discharge.