RECENT RESULTS OF VACUUM-ARC SWITCHED MULTI-MEGAWATT INVERTER TESTS A. So Gilmour, Jr. and Douglas A. Hopkins STATE UNIVERSITY OF NEW YORK AT BUFFALO ABSTRACT Previous studies I have shown the possibility of using a vacuum arc switch (VAS) in high power force commutated or series commutated inverter circuits. One such application, discussed in this paper, is in the development of a i0,000 volt, multi-megawatt series capacitor inverter circuit. Initial testing has been performed on a i0 kHz series resonant L-C circuit using a VAS. Single pulse tests at 3000A peak and 5000 V have been very successful. Extensive energy loss studies of the various cir- cuit components result in predicted inverter efficlen- ties in excess of 95 percent. The tests show that these high efficiencies will be achievable if high quality circuit components are used. The inductors must be fabricated from Litz wire. Low loss materials must be used for the capacitors and distributed connections must be made to capacitor elements. The configuration and composition of the electrodes in the VAS must be such as to minimize the switch voltage drop. INTRODUCTION In the development of high frequency, multi-mega- watt inverters for airborne applications, fast reliable, light-weight switching devices are required. Because thyristors are expected to be relatively heavy 2 a pro- gram for the development of vacuum arc switches is be- ing supported at the State University of New York at Buffalo by the Air Force Aeropropulsion Laboratory. The results of preliminary tests on these devices and their application to inverter circuits have been des- cribed. 1 This paper presents the results of recent detailed energy loss measurements on vacuum arc swit- ches and on other inverter circuit components. The following paragraphs contain discussions con- coming vacuum arc switches and the serles-capacitor inverter circuit. Then, the components selected for use in the circuit are described. Finally, the details of the energy loss study are presented along with inl- tial results of tests of the inverter circuit. BASIC DESCRIPTION OF VACUUM ARC SWITCH Vacuum arc switches(VAS) have been described in detail elsewhereo3, 4 A brief description is given here to orient the reader who is not familiar with these de- vices. The basic configuration of a VAS capable of being turned off as well as on is shown in Figure I. A vac- uum-arc discharge between the cathode and anode is initiated by the use of an igniter electrode. The ig- niter is separated from the cathode by an insulator on which the metallic vapor from the arc can deposit form- * Supported by the Air Force Aeropropulsion Laboratory, Air Force Systems Command, Wright Patterson Air Force Base, Ohio 45433. a conductive thin film. To ignite the arc, a current pulse is passed through this film causing a portion of it to vaporize. The resulting plasma burst quickly fills the interelectrode space allowing the main arc current to pass between the anode and cathode with a rise time on the order of one microsecond. During the ensuing discharge the metallic film is regenerated, pre- paring the system for the next ignition pulse. CATHOOE'k~ COIL ,~////9If I[////~' EXPtODED VIEW OF CATHOOEIGNIT~RASSEM~LY Fig. i. Configuration of vacuum arc switch For arc interruption to occur, the electrodes must be of a coaxial geometry. The cathode is a small electrode placed on the axis and the anode is an annu- i.~ surrounding the cathode. The arc is extinguished by applying a coaxial magnetic field to the device in ~such a way that the field lines are essentially per- pendicular to the paths of the electron current from the cathode to the anode. The effect of the field is to increase the voltage drop across the arc and there- by decrease the discharge current. The arc is extin- guished when the current is reduced to a value where the average lifetime of the arc becomes very short compared to the duration of the magnetic field pulse. When conduction ceases, metallic vapor is no longer emitted by the cathode. The vapor in the interelec- trode space rapidly condenses and the switch returns to the high-vacuum state and remains off when the mag- netic field is removed. In some switch applications a significant voltage may exist between the field toll and the anode and this can lead to malfunctions resulting form arcing between the anode and the field coil. For these applications, IB7-I