Microsecond plasma opening switch performance at currents above 2 mA

: The Hawk pulsed power generator is used in plasma opening switch (POS) experiments in the 1-micro(s) conduction time regime to study long conduction time switch physics. Experiments reported here include modifying the POS electrode geometry, injecting plasma into thee-beam diode, using gas gun plasma sources (with H2, He, and Ar gases), and using a helical cathode center conductor in the switch region to increase the total insulating magnetic field. Tapering the cathode center conductor over the 8 em POS length from 10 em to, typically, a 2.5 em diam produced peak load powers of0.7 TW with 55 kJ delivered to the diode--20% energy efficiency--with carboncoated flashboards as the plasma source. Performance (voltage, power generated) with a straight 10 em diam cathode deteriorated when the POS anode outer conductor just downstream of the switch was extended toward the load at the same radius as the switch. Load power was up to 70% higher with a plasma-filled diode (PFD) used in conjunction with the POS for short POS conduction times {400 ns and less). Use of a helical center conductor resulted in dramatically degraded switch performance for 350 ns conduction times. Switch performance with gas guns was generally comparable to that with flashboards in a given switch/load configuration and was independent of the gas (H2, He, and Ar) used.

Computer simulations for magnetic field penetration into plasma of plasma opening switch (POS), and current loop formation in it are presented for various initial plasma densities, currents, and POS geometries. It is shown that the current loop dynamics in the POS is determined by the fast magnetic field penetration in plasma due to the Hall effect. The strong dependence of the current loop longitudinal velocity on the transverse coordinate, together with the influence of the boundary conditions at the POS cathode and anode, lead to the formation of the narrow S-shaped current loop even in a homogeneous plasma. It is shown that the control parameters influencing the dynam-ics of the magnetic field and the motion of the current loop in the POS are the initial plasma density, driving current and cathode radius. The POS resistance is calculated for wide range of initial plasma densities, currents, and cath-ode radiuses. It is shown, that POS resistance is proportional to the total current, inversely proportional to plasma density, and is in the range 10-3…1 Ω for plasma densities 1012…1015 cm-3.

Summary form only given, as follows. Plasma opening switches (POS) are the key components of inductive energy store (IES) pulsed power systems. In the closed phase an IES system stores magnetic energy in the inductance of a vacuum transmission line. As the POS opens the stored magnetic energy flows past the POS and into a load (for example an electron or ion diode). To achieve power multiplication the opening time should be much shorter than the conduction time. There are three proposed mechanisms that control the conduction phase of the POS: 1) sheath effects at the electrodes, 2) rapid penetration of magnetic field into the body of the plasma due to EMH effects, and 3) displacement and distortion of the plasma. due to J/spl times/B (MHD) forces. In this paper, the PIC code, MAGIC, is used to study EMH and MHD effects in a POS. These simulations were performed in negative polarity with a strong radial density gradient. In regimes where EMH is expected to be dominant, the simulations show rapid axial penetration of magnetic field into the plasma in a narrow radial channel along the positive density gradient. No field penetration is observed along the negative portion of the density gradient. In positive polarity, field penetration occurs along the negative gradient and no field penetration is observed along the positive density gradient. This agrees with EMH theory of shock penetration of magnetic field. However, the speed of penetration is somewhat slower than predicted by fluid treatments. The PIC simulations also show the formation of vortices in the electron flow behind the EMH shock front.

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).

Plasma Opening Switches (POS) can be used to perform vacuum power conditioning for compact accelerators based on inductive energy storage. The characteristics of a microsecond conduction time radial disk geometry POS have been determined. POS performance data was obtained from both the ACE 2 accelerator, using a 240 kV, 1 MJ Marx with a 1.3/spl mu/s quarter period and the ACE 3 accelerator, using a 720 kV, 2 MJ Marx with a 1.5 /spl mu/s quarter period. The active POS area was an annular region where power flow converged radially inward. In the ACE 2 experiments a single POS provided switching while in the ACE 3 experiments two back-to-back POS provided parallel switching. Plasma for the POS was injected through a transparent anode by flashboards. POS performance into both short circuit and low impedance e-bearn loads are described.

Summary form only given, as follows. During the initial stages of opening in a long-conduction-time (/spl sim/ms) plasma opening switch (POS), a current propagation phenomenon is observed between the POS and the load. Experiments carried out on Double at the Institute of Electrophysics (Tomsk) showed that a current channel in the POS plasma is translated downstream for the original plasma region at high speed. Similar results were obtained on DM1 at Maxwell/Physics International and Hawk at NRL. Experiments on Hawk also showed difference in the dynamics of this plasma channel for different conduction times and loads, and between single- and multiple-species POS plasmas. Experiments at the Kurchatov Institute showed similar results in both positive and negative polarity and that the current channel width broadens during propagation. There have been various attempts to explain this phenomenon. Numerical simulations suggest that low-density plasma downstream of the POS location allows for this current channel propagation. Ion emission from the anode also increases the coupling of power to the load. It has also been suggested that lighter ions such as protons in the POS plasma separate from heavy species and are accelerated downstream in a current channel. Finally, it has been suggested that the current channel is a manifestation of nonlinear KMC waves which are predicted within the framework of electron magnetohydrodynamics. In this paper, we review the experimental characterization of this current propagation phenomenon and summarize the models proposed to describe it.

Summary form only given. Conduction and opening processes of plasma opening switches (POS) are examined with the aid of numerical simulations. In a POS, a plasma initially bridges a small axial region of the transmission line of an inductive energy storage generator. A section of vacuum transmission line then connects the POS region to a load. The plasma initially carries the generator current, allowing no current to flow to the load. Modifications to the plasma or to the current distribution in the plasma eventually open the POS, allowing power to flow to the load. These modifications can occur by a variety of different mechanisms: by plasma deformation and displacement, by magnetic field transport, or by a combination of such effects. Deformation and displacement are carried out by J/spl times/B forces and by electrostatic forces (ion erosion) associated with electron vortices. Magnetic field penetration can occur by electron-magneto-hydrodynamic (EMH) effects or resistive diffusion. Both particle-in-cell (PIC) and MHD codes are used to demonstrate many of these effects. In these simulations, a wide range of plasma densities (n/sub e/=5/spl times/10/sup 12/-5/spl times/10/sup 15/ cm/sup -3/) are examined.

Summary form only given, as follows. We are continuing the development of a low density plasma opening switch (POS) on the Double-EAGLE pulsed power generator, running in a 250 ns mode. The goal is to shorten the pulse and efficiently drive a=100 ns imploding PRS source (an aluminum wire array). This will provide a proof-of-principle demonstration of an alternative approach to long implosion PRS sources that are being developed for Decade Quad and the next generation of pulsed power drivers. In our previous experiment, the upper and lower current paths of Double-EAGLE strip-plate geometry were connected upstream of the POS to test the operation of a single-sided low density switch at linear current densities required for Decade Quad. Our present efforts involve using the lower inductance geometry of a double-sided POS and a post-hole convolute connecting the current paths downstream of the POS, driving either an inductive shun or an aluminum wire array. These tests will attempt to demonstrate high opened POS impedance and minimum electron sheath flow ("launched electron flow") downstream of the POS. We will discuss the interactions of the two sides of the POS and of the POS with the loads. The implications for Decade Quad will also be addressed.

Summary form only given, as follows. Experiments are being conducted on the HAWK inductive storage generator at the Naval Research Laboratory using a plasma opening switch (POS) to drive an electron-beam (e-beam) diode load. Utilizing a suite of current, voltage, and radiation diagnostics to measure power flow out of the POS as it opens into an e-beam diode, these experiments aim to elucidate the switch-load coupling, a phenomenon which is not currently well understood. Localized, shielded B-dot pickup coils located in the cathode and anode are used as current monitors to measure generator and load currents, to determine magnitude and location of current losses, and to determine the partitioning between electron-flow and cathode currents. The radiation diagnostics include a time resolved bremsstrahlung spectrometer, a Compton-Hall voltmeter, a scintillator photodiode, two pinhole cameras, and thermoluminescent dosimeters. The spectrometer is used to measure the high-energy tail of the bremsstrahlung spectrum, from which the electron energy may be inferred. This is compared with a direct load-voltage measurement using a vacuum voltmeter. By comparing the measured currents, load voltage, and spectrometer data, the effect of electron-flow current on the bremsstrahlung spectrum may be investigated. The effects of varying the POS conduction time, the anode-cathode gap, and the POS-to-load distance on POS-diode coupling will be reported.

Plasma opening switches (POS's) have shown excellent characteristics in pulsed power applications. Proposed POS scaling predicts that the fastest opening time for a given conducted current should occur using a high-velocity low-density plasma as the switch medium. The ion beam opening switch (IBOS) uses a charge-neutral ion beam of 100-300 kV, ? 120 A/cm2 as the switch plasma. Its velocity of up to 600 cm/?s and density of ~1012/cm3 make this a very fast low-density plasma compared with typical 10 cm/?s and 1013/cm3 POS plasmas. The IBOS has conducted ? 70 kA flowing in a parallel-plate transmission line driven by a 4-? pulser. IBOS opening time is load dependent, being ? 4 ns into a 15-nH load and about twice as long into a 4-? electron diode load. However, switch impedance is not zero during the entire conduction time, rising to ? 3 ? by the time of peak current. Peak current conducted before opening does not vary linearly with either injected ion current or switch axial length. Instead, the conduction current scales with plasma density in the switch, and is nearly independent of switch area until the area is restricted to a narrow (~1 cm) strip.

Experimental evidence suggests that long conduction time (≥ (R18)1 μs) plasma opening switches open more slowly than expected on the basis of magnetic forces, and perhaps even on the basis of plasma erosion. We propose that the slow opening is associated with charge exchange with and/or electron ionization of neutral particles produced in the switch gap during and/or prior to the power pulse. The possibility of such mechanisms bears on the choice of switch plasma and on surface conditioning procedures in the development of fast opening switch technology.

Plasma opening switches are believed to develop voltage through opening and magnetically insulating a gap formed in plasma injected between two conducting electrodes. Data will be presented to show that a relatively simple assumption about the behavior of this gap suffices to describe the overall operation of the switch. Specifically, the observed increase, then decrease, of operational voltage as a function of conduction time can be accounted for by the switch gap linearly decreasing with conduction time, while the conducted current increases. In addition to presentation of data to demonstrate this collapse, analysis is performed to couple the gap history to the voltage predicted for the switch, and to optimize this latter quantity.

Experiments have been conducted on a plasma opening switch (POS) test-bed to investigate the influence of cathode materials made of aluminum, stainless steel, molybdenum and tungsten on opening performance for conduction time up to 3 microseconds, conduction current up to 100 kA. Remarkably different opening characteristics have been shown for these materials, with tungsten being of the best opening performance.

The effects of contaminants on the electrodes of plasma opening switches have been studied. The contaminants introduce backscattered and sputtered neutral atoms into the carbon plasma that is impinging on the cathode. The process has been investigated with the TRIM.SP code. The distribution of density of these neutral atoms from the cathode surface has been obtained and found to be substantial compared to the density of the incident carbon plasma. The dependence of the sputtering yield on the composition of the contaminants has also been studied. >

Plasma Opening Switches have been examined numerically with the aid of the ANTHEM plasma simulation model. A generic bi-cylindrical switch is studied. The switching of generator pulses ranging from 50 ns to 1 usec is reviewed, for a variety of plasma fill lengths and densities, and for a range of resistive loads.