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In order to obtain optimum collector efficiency and to control the electron trajectories in the collector region the design process for depressed collectors has to take into account several interlinked parameters. A flow diagram is presented for a systematic design procedure which integrates the various steps. A library of computer codes has been developed to assist in different stages of design. Among the key elements is a code that generates contours of effective potential and helps to define the regions of accessibility for electrons. These contours and the lines of magnetic flux can be drawn against the backdrop of the collector geometry. The different control parameters can be varied and their effect on the contours studied interactively. The search area for optimum parameters is thus considerably reduced. Additional codes make it possible to evaluate each design in terms of collector efficiency, division of beam current between different collectors, and the heat dissipation density profile. Using these codes, a design has been developed for a two-stage depressed collector for a 1-mW, 110-GHz gyrotron yielding a collector efficiency of 68%. Index Terms - Computer graphics, design automation software, depressed collectors, gyrotrons.

The use of a diamond RF switch for super-high-power microwave/millimeter wave generation has been evaluated. An X-band chemical vapor deposition (CVD) diamond window package was theoretically analyzed, designed, and built. Thirty-eight percent of an injected microwave signal with a frequency of 11.424 GHz was reflected from a 100-[micro]m thick, 22-mm-diameter CVD diamond window when activated by a 160-mJ, 266-nm Nd: YAG laser. The details of the CVD diamond window design and experimental results are presented. The results have high application potential for building super-high-power microwave systems. Index Terms--Diamond switch, microwave amplifier, RF compression, RF switch.

Many tokamak devices require a preionization system in order to start up both the discharge cleaning and the main plasma discharge. A stray vertical magnetic field profile of 100 G at the center of the plasma column has been measured on the NOVILLO tokamak by magnetic probes especially made to study the breakdown process. A new vertical field profile, superimposed to the stray field component parallel to the equilibrium field, is created by two additional compensation poloidal coils. Such arrangement of the vertical field profiles, with a 1/R-dependence, has been found optimum to achieve the plasma breakdown without any preionization system. Index Terms--Assisted start-up, magnetic confinement, preionization, stray magnetic fields, tokamak engineering.

Laser-induced fluorescence (LIF) was used to measure the mean and variance of the velocity distribution of xenon ions emitted from two hollow cathode assemblies operating at low power. High-energy ions detected in plume-mode and spot-mode operation are consistent with the potential-hill model of high-energy ion production. The distributions of velocities were modeled, yielding temperatures on the order of a few electronvolts in plume mode and 1 eV in spot mode. The LIF of neutral xenon atoms in the plumes indicated thermal temperatures significantly less than the temperatures associated with the ion velocity distributions. Index Terms--Electric propulsion, hollow cathodes, ion propulsion, laser-induced fluorescence, plasma diagnostics.

One of the key issues in pulsed power generation and its application is switch technology. The switches are divided into two categories of opening and closing switches. The most popular closing switches to date are spark-gap switches. The closing switches have a very limited lifetime due to arc corrosion resulting from the pinch effect and localized arc heating [Hare et al. (1975)]. Electrode phenomena in high-current carrying switch and transient phases in intense arcing modes are presented in papers by Prucker and Christiansen (1977) and Lehr et al. (1987). To extend the switch lifetime by minimizing electrode-vaporization, rotary arc gap-switches are investigated in papers by Kumar and Pramanik (1995), Desaulniers-Soucy and Meunier (1995), Gulie and Sloot (1975), and Yasko (1969). In those switches, rotation of the arc can be achieved by providing a rotating magnetic field from outside or by a self-induced magnetic field. Extensive experimental and theoretical investigations on arc characteristics and arc velocities in ring-type spark gaps are reported in papers by Gulie and Sloot (1975) and Yasko (1969). This paper investigates a rotating arc gap (RAG) switch driven by self-induced magnetic field. The physical dimensions, rotating arc speed, arc voltage drop, and its experimental setup are described in detail. Index Terms--Arc back, arc dimension, arc front, crowbar, rotary-arc gap switch, rotating arc speed.

A radial multichannel pseudospark switch for pulsed power applications with pulsed currents of more than 100 kA, hold-off voltages up to 30 kV, and pulse durations [full-width at half maximum (FWHM)] of several microseconds is presented. Investigations of the electrode phenomena show that the discharge current is nearly homogeneously distributed over a large electrode area of at least 10 [cm.sup.2]. This is a prerequisite for low electrode erosion. Long lifetime operation has been proven in tests in a device for producing shock waves for material processing and in a pinch plasma X-ray source. In the shock wave generator, peak currents up to 60 kA occur and in the plasma focus device peak currents up to 130 kA have been used. In both cases, after 60 000 shots with the water spark and even [10.sup.6] shots in the plasma focus device, the electrodes show negligible erosion. Index Terms--Cathode spots, plasma focus, pseudospark switch, water spark.

An experimental investigation into the rate of voltage collapse in plasma closing switches has been undertaken. A transmission line test switch, incorporating a radial/conical topology to minimize wave reflections, has been used for this investigation. The results described in this paper were obtained from experiments using hydrogen, helium, nitrogen, and [SFS.sub.6]. The results show that there is a distinct change in breakdown behavior as the pressure increases. At the lowest tested pressures, there is a rapid voltage collapse phase. This phase reduces at intermediate pressures before recovering at higher pressures. This transition from low-pressure behavior to high-pressure behavior occurred in all the tested gases, although the transition pressure values were dependent on the gas type. Index Terms--Dielectric breakdown, gas discharges, plasma closing switches, transmission lines, UWB radar.

A thyratron is a very common switching device in many high-power pulse systems. However, it is difficult to diagnose switching characteristics of the thyratron during its operation. A new diagnostic factor of the thyratron has been developed from a convection heat transfer theory. The new diagnostic factor is obtained from a dependence of a thyratron heater current on a thyratron internal gas pressure. We have verified theoretical proprieties of the diagnostic factor and confirmed experimentally that the assumption used in the theory is applicable to typical thyratrons. Using the diagnostic factor, we have designed and constructed an autoranging and autodiagnostic device for the thyratron. Index Terms--Discharge, gas, high power, switch, thyratron.