P.J. Goodrich

Characterization of a microsecond-conduction-time plasma opening switch

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.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 u...

Plasma opening switch conduction scaling

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.

Intense ion‐beam‐transport experiments using a z‐discharge plasma channel

A z‐discharge plasma channel is used to confine and transport an intense proton beam. A pinch‐reflex ion diode on the NRL Gamble II accelerator focuses a proton beam onto the entrance aperture of a 2.5 cm diam, 1.2 m long z‐discharge transport system. The beam ions are charge and current neutralized in the discharge plasma, and execute betatronlike orbits in the magnetic field of the discharge. Ion beam diagnostics include shadowbox imaging and prompt‐γ radiation measurements from LiF targets. Under appropriate conditions, 95% particle transport and 90% energy transport are observed, with the only energy loss attributed to classical stopping in the channel gas. The transverse phase‐space distribution of the beam measured by the shadowbox is consistent with full charge and current neutralization of the transported beam.

Plasma Opening Switch Experiments On Hawk With An E-beam Diode Load

Successful application of inductive energy storage depends critically on the performance of the opening switch. The new Hawk generator at NRLI is used in plasma opening switch (POS) experiments in the 1-/spl mu/s conduction time regime to study long conduction time POS physics. In this experiment, different POS configurations were used, including various switch to load distances and different cathode center conductor radii. The load was an e-beam diode. Peak load powers of 0.5 TW, with load current risetimes of 20 ns and current transfer efficiencies of 80%, were achieved with a POS conduction time of 0.75 /spl mu/s using a 5 cm diam cathode. Typically, 40 kJ were coupled into the diode, which is 20% of the energy stored in the Hawk capacitance. The data indicate that above a critical load impedance the final switch gap, as determined from magnetic insulation arguments, is fixed to 2.5-3 mm, independent of conduction current and center conductor radius. Above this critical load impedance, current is shunted into the transition section between the switch and the load such that the voltage remains constant. At lower impedance values, the load voltage decreases in proportion to the load impedance. This critical load impedance is then the optimum impedance for maximum load power. Increasing the cathode magnetic field by conducting more current (up to a limit) or by decreasing the cathode center conductor radius at a given current level allows the switch to remain insulated at a higher voltage. Peak load voltages up to 1.7 MV were achieved using a 5 cm diam center conductor, a factor of 2 higher than that obtained with a 10 cm diam center conductor and 2.7 times higher than the erected Marx voltage (640 kV).

Initial results of parallel-plate plasma opening switch experiments on HAWK

Summary form only given. Results of parallel-plate plasma opening switch (POS) experiments on HAWK are presented. The Hawk generator produces a sinusoidal output current of amplitude /spl sim/750 kA and quarter period /spl sim/1.2 its. The POS used is a tri-plate with a single cathode and two anode plates, each 15 cm wide. The plasma is injected from plasma guns through slots in the anodes. This POS performs almost as well as a comparable coaxial POS for short-circuit load shots with conduction times less than 700 ns. The parallel plate geometry provides much better diagnostic access than the standard coaxial POS. Framing camera images of the plasma imply that the current snowplows until it reaches the end of the injected plasma and opens near the cathode. A two-color laser interferometer is used to measure the electron and neutral densities of the plasma in the switch and in the magnetically insulated transmission line (MITL) between the switch and load integrated along chords in the /spl theta/-like direction. The electron density measured in the injected plasma region rises sharply during conduction and falls rapidly near the time of opening consistent with a snowplow traveling through the injection region.

Analysis of NRL wire-guided ion beam transport experiments

Summary form only given. A series of WGT (wire-guided transport) experiments has been fielded on the Gamble II accelerator at NRL (US Naval Research Laboratory). A pinch reflex ion diode focused a ~100-kA, ~1-MeV proton beam onto the 2.54-cm-diameter entrance aperture of a large-radius, 1-m-long transport tube. A thin copper wire positioned along the axis of the transport tube carried currents ranging up to 28 kA, thereby providing the beam-confining azimuthal magnetic field. The beam diagnostics used were a large-radius witness plate at the downstream end of the transport tube to determine the beam radius. Teflon targets positioned at various locations in the path of the beam to measure transport efficiency and time-of-flight effects using the 19F(p,αγ)16O reaction, and a shadowbox positioned at the downstream end of the transport tube to reconstruct the velocity-space distribution of the beam. The results of the three beam diagnostics were in agreement with ion orbit analyses. In particular, an analytic description of the velocity-space distribution of the beam predicted quite accurately the results of the shadowbox diagnostic

Low mass wall-confined discharge for light ion beam transport

Summary form only given. A low-mass version of the wall-confined discharge transport channel employing 250-μm-wall. 2.5-cm-diameter spiral-wound Kapton tubes has been developed. These tubes weigh only 30 g/m and are free-standing. The return current is carried by copper tape: vapor-deposited aluminum could be used in an eventual reactor. Discharges up to 2.5 m in length have been created with the Kapton surviving intact. Electrical monitors have been used to study the breakdown process and the resulting radial current distribution in the channel as a function of length and fill pressure. A uniform current distribution is established fairly early in the discharge current cycle, and for pressures above 2 T this distribution persists until about peak current (800 ns), after which pinching takes place. Preliminary Gamble II experiments have shown efficient ion beam transport over 1 m

Properties of intense ion beams from pinch-beam diodes

Summary form only given. Properties of pulsed ion beams from a pinch-beam diode on the Gamble II generator are being evaluated with several different diagnostics. An intense 1to 2-MeV ion beam is extracted from the diode through a 2-/spl mu/m thick Kimfol and transported up to 2.2 m in 1-Torr air. Multiple-pinhole-camera, Thomson-parabola-ion-analyzer, Rutherford-scattering, and step-wedge-filter diagnostics are fielded in the gas region to determine ion-beam properties. Uniformity of ion emission from the polyethylene anode and ion-beam divergence are determined with a multiple pinhole camera located 0.5 m from the anode. Images recorded on radiachromic film indicate extremely non-uniform emission from the anode and a beam divergence of 0.12 to 0.16 rad. Beam composition is determined with the Thomson parabola ion analyzer located about 2 m from the polyethylene anode in order to sample ions from the 10-cm-diam anode. The beam is primarily protons with a carbon fraction of <1%. When the transport region is at 10/sup -4/ Torr, a carbon component is observed. Beam fluence and pulse duration are determined by measuring protons Rutherford-scattered from small-area, thin, aluminum foils located at distances of 0.4 to 2.2 m from the anode.

Plasma source variations in plasma opening switch experiments

Summary form only given. Three plasma sources have been investigated for use in plasma opening switch (POS) experiments: flashboards, gas guns, and cable guns. These sources have been fielded on Hawk, a testbed for inductive store pulsed power switching. The purpose of the experiments is to determine the effects of changing the plasma species, flow velocity, and spatial distribution on POS performance (conduction current and time, voltage limitations, etc.) The plasma sources are characterized in a test chamber and diagnosed during POS shots using laser interferometry.

Vacuum Insulator Flashover Measurements For Microsecond Pulses

The flashover characteristics of the water/vacuum insulator assembly of the Gamble II pulsed power generator at the Naval Research Laboratory (NRL) were investigated to determine the insulators applicability for use with the recently assembled Hawk generator, which features inductive store/plasma opening switch (POS) technology. Hawk produces a 1-/spl mu/s voltage pulse with a peak value of 265 kV at the vacuum insulator during the POS conduction time. When the POS opens, a voltage pulse as high as 1.5 MV can be produced at the switch. The Gamble insulator was originally designed for /spl sim/2 MV, 75 ns voltage pulses. Gamble II was re-configured for a long-pulse mode to produce a 1 MV, 1.5 /spl mu/s wide output pulse for these tests. Flashover data will be presented for voltage pulses of 556 to 843 kV with the time from the beginning of the pulse to flashover ranging from 0.8 to 2.25 /spl mu/s. It was found that the results did not follow the familiar t/sup -1/6/ relationship for flashover. Nevertheless, the performance was more than adequate for this application and an identical insulator has been used successfully on Hawk for nearly a year.