J. M. Grossmann

Plasma Erosion Opening Switch Research at NRL

This paper is a review of plasma erosion opening switch (PEOS) research performed at the Naval Research Laboratory (NRL). Several experimental and theoretical results are described to illustrate the present level of understanding and the best switching results obtained to date. Significant power multiplication has been achieved on the Gamble II generator, producing 3.5 TW with less than 10-ns rise time. Switching after nearly 1-¿s conduction time has been demonstrated on Pawn, producing a 0.2-TW 100-ns pulse. Scaling the switch to higher current, power, and conduction time should be possible based on theoretical analysis and the favorable results of scaling experiments performed thus far.

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...

Investigation of plasma opening switch conduction and opening mechanisms

Plasma opening switch techniques have been developed for pulsed power applications to exploit the advantages of electrical energy storage in a vacuum inductor compared to conventional, capacitive-based energy storage. Experiments are described that demonstrate the successful application of these techniques in conduction time ranges from 50 ns to over 1 mu s. Physics understanding of the conduction and opening mechanisms is far from complete; however, many insights have been gained from experiments and theory. Measurements of current distribution, plasma density, and ion emission indicate that conduction and opening mechanisms differ for the 50 ns and 1 mu s conduction times. For the 50 ns conduction time case, switching begins at a current level close to the bipolar emission limit, and opening could occur primarily by erosion. In the 1 mu s conduction time case, limited hydrodynamic plasma displacement implies far higher plasma density than is required by the bipolar emission limit. Magnetic pressure is required to augment erosion to generate the switch gap inferred from experiments. >

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.

Numerical simulation of a low‐density plasma erosion opening switch

Current conduction through a low density (∼1012 cm−3) collisionless plasma injected between two coaxial conducting cylinders is simulated using a 2 (1)/(2) ‐D, electromagnetic particle‐in‐cell code. Plasma is injected through the anode towards the cathode with flow velocity, VF, and is assumed to be azimuthally symmetric. Current is driven through the plasma so that the 100 kA level is reached in ≂5 nsec. The opening process, when current is diverted to a load, is also treated. Electrons are found to carry current in a narrow current channel across the plasma by E×B drift. A large electric field is established by charge separation in the plasma in order to provide the drift. The motion of the anode end of the current channel controls the time of opening and is found to be independent of VF and to depend strongly on density and length.

Gap formation processes in a high‐density plasma opening switch

A gap opening process in plasma opening switches (POS) is examined with the aid of numerical simulations. In these simulations, a high density (ne=1014–5×1015 cm−3) uniform plasma initially bridges a small section of the coaxial transmission line of an inductive energy storage generator. A short section of vacuum transmission line connects the POS to a short circuit load. The results presented here extend previous simulations in the ne=1012–1013 cm−3 density regime. The simulations show that a two‐dimensional (2‐D) sheath forms in the plasma near a cathode. This sheath is positively charged, and electrostatic sheath potentials that are large compared to the anode–cathode voltage develop. Initially, the 2‐D sheath is located at the generator edge of the plasma. As ions are accelerated out of the sheath, it retains its original 2‐D structure, but migrates axially toward the load creating a magnetically insulated gap in its wake. When the sheath reaches the load edge of the POS, the POS stops conducting curr...

Comparison of low-cost hyperspectral sensors

Recent advances in large format detector arrays and holographic diffraction gratings have made possible the development of imaging spectrographs with high sensitivity and resolution, at relatively low component cost. Several airborne instruments have been built for the visible and near IR spectral band with 10-nm resolution, and SNR of 200:1. Three instruments are compared, an all-reflective spectrography using a convex grating in an Offner configuration, and two off-the-shelf transmission grating spectrographs using volume holograms. The camera is a 1024 X 1024 frame transfer, back-thinned CCD, with four taps for obtaining high frame rates. Performance and scan data is presented and compared to the design for image quality, distortion, and throughput.

Hyperspectral analysis and target detection system for the Adaptive Spectral Reconnaissance Program (ASRP)

A multiprocessor version of the ORASIS hyperspectral analysis program has been implemented in support of the ASRP. In brief, the long-term technical objectives of the ASRP are to demonstrate the feasibility and military utility of real-time target detection from uncrewed air vehicles using hyperspectral data. This paper presents a preliminary assessment of ORASIS performance and describes the ORASIS development effort designed to meet the ASRP goals. Real-time performance of the analysis program and its potential effectiveness as a target detection method are demonstrated.

Implicit Collisional Three-Fluid Simulation of the Plasma Erosion Opening Switch

The plasma erosion opening switch (PEOS) has been studied with the aid of the ANTHEM implicit simulation code. This switch consists of fill plasma injected into a transmission line. The plasma is ultimately removed by self-electrical forces, permitting energy delivery to a load. Here, ANTHEM treats the ions and electrons of the fill plasma and the electrons emitted from the transmission-line cathode as three distinct Eulerian fluids-with electron inertia retained. This permits analysis of charge separation effects, and avoids the singularities that plague conventional MHD codes at low density. E and B fields are computed by the implicit moment method, allowing for time steps well in excess of the electron plasma period ?t >> ?p-1, and cells much wider than a Debye length, ?x >> ?D. Switch dynamics are modeled as a function of the driving electrical pulse characteristics, the fill plasma parameters, and the emission properties of the transmission line walls-for both collisionless and anomalously collisional electrons. Our low-fill-density (ne ? 4 × 1012 electrons/cm3) collisionless calculations are in accord with earlier particle code results. Our high-density computations (ne ? 2 × 1013 electrons/cm3) show the opening of the switch proceeding through both ion erosion and magnetic pressure effects. The addition of anomalous electron collisions is found to diffuse the driving B field into the fill plasma, producing broad current channels and reduced magnetic pressure effects, in some agreement with NRL experimental measurements.

Analysis of anomalous resistivity during the conduction phase of the plasma erosion opening switch

During the conduction phase of the plasma erosion opening switch, the current channel width in the body of the plasma away from the electrodes has been observed to be many times wider than the collisionless skin depth. Anomalous collisions have been invoked to explain this discrepancy. Here the problem is analyzed using an electrostatic Vlasov approach and an unstable ion acoustic mode is identified. The derived growth rate is fast enough and the nonlinear saturation level is high enough to explain the observed magnetic field penetration. 21 refs., 3 figs.