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

: 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.

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.

Summary form only given, as follows. DSWA is presently supporting plasma opening switch (POS) development focused on improvement of single DECADE module performance and the development of a monolithic POS for the DECADE Quad. The goal for the single module POS is to conduct 1.8 MA in 300 ns and to efficiently drive a nominal 1 Ohm bremsstrahlung diode load with minimal current losses in the POS. Presently, the single module POS conducts 1.4 MA in /spl sim/265 ns, with only 60% of the current into the POS producing radiation. A DECADE Quad POS will be required to conduct the combined current from four modules, approximately 10 MA in 300 ns, and to drive a nominal 0.2 Ohm bremsstrahlung diode load. The ACE 4 POS configuration which conducts 3.5 MA in /spl sim/900 ns, with the POS opening to about 0.3 Ohms into a high inductance load, is being developed for the DECADE Quad. This paper will report on both the overall technical approaches and the current work to improve the present single DECADE module POS and to extend the present ACE 4 POS technology to the DECADE Quad. An alternate POS concept applicable to the DECADE program, the inverse pinch source POS tested on HAWK, will be introduced. The DSWA R&D program emphasizes well diagnosed experiments, tightly coupled to theoretical modeling.

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.

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.

The successful development of the Plasma Opening Switch (POS) for inductive storage applications has been largely confined to negative polarity operation. Some models of POS behavior suggest that this is because in a positive polarity coaxial configuration, the weaker magnetic field at the cathode position retards the switch opening process. This article describes experiments in which both conductor radii in the POS region were significantly reduced. Anode- and cathode-side current monitors indicate that voltages greater than open-circuit are generated at the POS position, but there is a significant amount of electron flow out of the POS, depending upon load impedance. Flow impedance analysis indicates that a relatively small gap appears in the POS plasma after switch opening. Switch performance is also compared between flashboard and carbon gun plasma sources, with the latter operated both in positive and negative polarity.

Plasma Opening Switch (POS) experiments to support Sandia National Laboratories PBFA II ion beam accelerator were conducted on Maxwell Laboratories Blackjack 5 accelerator. The first experiment tested an improved flashboard plasma source to generate a uniformly opening POS. The results showed that the J X B force caused plasma motion into the diode load region prior to the opening of the POS. Plasma motion was inferred by comparing experimental data to circuit models which used a variable resistance POS and a variable inductance (L-dot) term. The L-dot term was necessary in the model to reproduce the experimental results.

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.

A coaxial geometry plasma opening switch (POS) has been tested on the 4 MJ ACE 4 pulsed power driver. Simple models of POS operation were used to scale successful results from drivers such as HAWK at NRL and ACE 1 which operate at the desired conduction time but at currents below 1 MA. For the conduction phase, a snowplow model indicates that the square root of the plasma density is proportional to the product of conduction time and peak conduction current for fixed radius and length switches. This relationship was verified for the ACE 4 POS. For the opening phase, a model of gap formation enabled plasma density thinning during the conduction phase was applied. The gap distance can be estimated from the voltage and current measurements using a magnetic insulation model.

Problems related to creating a superpower pulsed current generator (Baikal project, ≈10 MV, ≈50 MA, ≈150 ns) for inertial confinement fusion are considered. In order to test the circuit design of the generator, the MOL unit (3.7 MA, 5 MV, 150 ns), in which a plasma opening switch (POS) is proposed for use as an output power sharpener, is now under construction. Results are presented from experimental testing of a number of proposals enabling the POS operation under the MOL conditions: (i) plasma guns were employed to connect the POS to the inductive storage circuit, (ii) a separating discharger was used to avoid a POS reclosure and to switch the current to a load whose impedance was initially low and increased with time, and (iii) a programmable filling of the POS gap with plasma was implemented in order for the POS to pass a long current prepulse with parameters close to those in actual systems.

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.

Plasma opening switches (POS) are a key technology in inductive energy storage based pulsed power applications. It has been observed on HAWK, DPM1, and ACE 4 that the geometry downstream of the POS plasma injection region can have a significant effect on POS-load coupling. However, the correlation between performance and geometry is not well understood. POS-load coupling has been investigated for the 9 cm radius, ACE 4 coaxial POS. This study provides insight into the coupling of the POS driving an inductive load and guidance in the design for coupling to an e-beam diode. It also complements the work reported at this meeting by Coleman et al. and Don Parks et al.

The performance of a plasma opening switch (POS) was studied, experimentally using a nanosecond pulsed power generator (600-800 kV, 40-60 ns) as well as a small capacitor bank (20 kV, 15-20 kA, 0.25-1 mu s). It was shown that the initial stage of POS action is related to the formation of the cathode plasma. The reproducibility of results is related to that of the cathode plasma light emission pattern. Switch opening occurs at any load, and the increase of POS resistance weakly depends on that of the load. The physics of opening is related mainly to some internal mechanisms. It was shown that a partially neutralized high-current electron beam directed to the load is formed at the final stage of switch opening. The appearance of this beam in the downstream coaxial structure or in the load diode limits the open POS impedance and explains the reported effect of decreased diode impedance that occurs even for large anode-cathode gaps. >

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.

A method is proposed to increase the linear charge density transferred through a plasma opening switch (POS) and, accordingly, reduce the POS diameter by enhancing the external magnetic field in the POS gap. Results are presented from experimental studies of the dynamics of the plasma injected into the POS gap across a strong magnetic field. The possibility of closing the POS gap by the plasma injected across an external magnetic field of up to 60 kG is demonstrated.