Neutral accumulation from plasma-wall interactions in plasma opening switches

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.

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.

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.

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.

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.

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.

: 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.

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. >

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.

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.

Summary form only given, as follows. Plasma opening switch (POS) with an applied extrinsic magnetic field allows to Teach maximum voltage multiplication. However, this regime limits the charge density through the POS that reduces current value and pulse duration. The increase of the POS conduction phase duration allows to obtain a substantial growth in charge and energy densities transferred through the POS. This problem is important for the "Baikal" program of superpower generator creation. The machine as assumed will provide a long high current pulse which should be sharpened employing POS technology. To increase the conduction phase time it was proposed to use a special programmable plasma filling of the POS gap. This allows to maintain plasma concentration at low level providing an erosion regime for the conduction phase. The experiments aimed to increase the conduction phase time up to 40 ms used a complex form pulse obtained from a two-stage generator with a POS as a power intensifying element shortened with an inductive load. The first "long" stage utilized a 6 mF and 50 kV battery with a 40 ms current rise time. The second "short" part of the pulse was formed by a Marx generator (0.4 mF, 200 kV) with a quarter of period at 1 ms. The time interval between two parts of the pulse was 20-40 ms.

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).

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.

The influence of electric fields and neutral particles on the plasma sheath at ITER divertor conditions

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.