Density redistribution in a microsecond-conduction-time plasma opening switch

Abstract

The first measurements of the line-integrated electron density in a coaxial microsecond-conductiontime plasma opening switch during switch operation are presented. Current conduction is observed to cause a radial redistribution of the switch plasma, with a large decrease in axial line density over most of the radial extent of the switch. A local reduction in line density of more than an order of magnitude occurs by the time opening begins. It is hypothesized that this density reduction allows the switch to open by an erosion mechanism. Initial numerical modeling efforts have reproduced the principal observed results. PACS numbers: 52.75.Kq, 52.40.Hf, 52.65,+z, 52.70.KzPulsed power generators that use inductive energy storage techniques offer potential benefits for producing TW and higher electrical power pulses [1]. Applications for such generators include inertial confinement fusion and the production of intense x-ray pulses. A plasma opening switch [1] (POS) allows the use of vacuum inductive storage for the generation of such high power pulses. A POS consists of plasma injected between two conductors in vacuum, through which current flows, storing magnetic energy in the circuit. At some point, depending on the details of the POS and the driving current, this conduction phase ends and the switch opens, transferring energy to a load. Over the past decade much attention has been directed toward microsecond-conduction-time POS development [2-7]. This technology has promise for the development of compact, multi-TW, multi-MA generators. In experiments to date switches have conducted MA-level peak currents for -1 |*s before opening in tens of ns [4-6]. Magnetic probe measurements (discussed below) show that large-scale translation of the current-carrying plasma toward the load does not occur, indicating a relatively high (--10 15 -10 16 cm -3 ) plasma density. This high density makes the observed, rapid opening of the switch difficult to explain. We report here the first quantitative, nonperturbing, in situ measurements of the plasma electron density during POS operation. The electron density is measured using heterodyne-phase-detection HeNe interferometry. These measurements indicate that the plasma mass is rarified during the conduction phase. Based on these observations, we propose a mechanism by which current conduction is limited and opening occurs.A schematic of the experiment, on the Hawk generator [8], is shown in Fig. 1. The coaxial switch geometry comprises a 5-cm-radius center-conductor cathode and an array of twelve axial anode rods at a radius of 7.5 cm. A short-circuit load is located 25 cm beyond the switch. Plasma is injected by eighteen flashboards positioned at an 18-cm radius. Each dashboard consists of an array of surface discharges across a carbon-coated insulator. A mask outside the anode rods shields all but the 8-cm-long switch region from the injected plasma. The flashboards are typically pulsed 1 to 2 ^s before the generator is fired. Electrical diagnostics include sets of dB/dt loops at the...

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