The dynamics of long-conduction-time (r,-1 ,z+) plasma opening switches (PQS) is studied using magnetohydrodynamic (MHD) theory, including the Hall term. Plasma switches with initial electron densities of 't,= 1014-10'6 cmm3 are modeled; these densities are appropriate to recent experiments carried out at the Naval Research Laboratory using the Hawk generator (800 kA, 1.2 ,u). The conduction times obtained from the simulation studies are in the range rC=0.4-2.0 w. The POS plasma is strongly redistributed by the penetrating magnetic field. As the field penetrates, it pushes the plasma both axially and radially (i.e., toward the anode and cathode). In the higher-density regime (n,> lOI cmM3), Hall effects do not play a significant role. The magnetic field acts as a snowplow, sweeping up and compressing the plasma as it propagates through the POS plasma. In the lower-density regime (n,< 1015 cmv3), Hall effects become important in two ways: the conduction time is less than that expected from ideal MHD, and the POS plasma becomes unstable as the magnetic field penetrates, leading to finger-like density structures. The instability is the unmagnetized ion Rayleigh-Taylor instability and is driven by the magnetic force accelerating the plasma. The structuring of the plasma further decreases the conduction time and causes the penetrating magnetic field to have a relatively broad front in comparison to EMHD simulations (i.e., Vi=O). The simulation results are consistent with experimental data for conduction currents 300-800 kA.

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This paper presents data and analyses from which emerges a physical picture of microsecond-conduction-time plasma opening switch operation. During conduction, a broad current channel penetrates axially through the plasma, moving it toward the load. Opening occurs when the current channel reaches the load end of the plasma, far from the load. During conduction, the axial line density in the interelectrode region is reduced from its value with no current conduction as a result of radial hydrodynamic forces associated with the current channel. A factor of 20 reduction is observed at opening in a small, localized region between the electrodes. When open, the switch plasma behaves like a section of magnetically insulated transmission line with an effective gap of 2 to 3 mm. Increasing the magnetic field in this gap by 50% results in an improvement of 50% in the peak load voltage and load current rise time, to 1.2 MV and 20 nsec, respectively. An erosion opening mechanism explains the inferred gap growth rate using the reduced line density at opening. Improved switch performance results when the maximum gap size is increased by using a rising load impedance.

Plasma opening switch (POS) experiments performed on the Hawk generator [Commisso et al., Phys. Fluids B 4, 2368 (1992)] (750 kA, 1.2 μs) determine the dependence of the conduction current and conduction time on plasma density, electrode dimensions, and current rise rate. The experiments indicate that for a range of parameters, conduction is controlled by magnetohydrodynamic (MHD) distortion of the plasma, resulting in a low density region where opening can occur, possibly by erosion. The MHD distortion corresponds to an axial translation of the plasma center-of-mass by half the initial plasma length, leading to a simple scaling relation between the conduction current and time, and the injected plasma density and POS electrode dimensions that is applicable to a large number of POS experiments. For smaller currents and conduction times, the Hawk data suggest a non-MHD conduction limit that may correspond to electromagnetohydrodynamic (EMH) field penetration through the POS plasma.

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...