Plasma Flow Switch Research Articles

Characterisation of the current switch mechanism in two-stage wire array Z-pinches

In this paper, we describe the operation of a two-stage wire array z-pinch driven by the 1.4 MA, 240 ns rise-time Magpie pulsed-power device at Imperial College London. In this setup, an inverse wire array acts as a fast current switch, delivering a current pre-pulse into a cylindrical load wire array, before rapidly switching the majority of the generator current into the load after a 100–150 ns dwell time. A detailed analysis of the evolution of the load array during the pre-pulse is presented. Measurements of the load resistivity and energy deposition suggest significant bulk heating of the array mass occurs. The ∼5 kA pre-pulse delivers ∼0.8 J of energy to the load, leaving it in a mixed, predominantly liquid-vapour state. The main current switch occurs as the inverse array begins to explode and plasma expands into the load region. Electrical and imaging diagnostics indicate that the main current switch may evolve in part as a plasma flow switch, driven by the expansion of a magnetic cavity and plasma bubble along the length of the load array. Analysis of implosion trajectories suggests that approximately 1 MA switches into the load in 100 ns, corresponding to a doubling of the generator dI/dt. Potential scaling of the device to higher current machines is discussed.

Experimental Studies of an Ultrahigh-Speed Plasma Flow

In 1991, Turchi et al. reported evidence for a 2000 km/s aluminum plasma that originated from the upstream boundary of a wire array armature in a plasma flow switch (PFS). The 2008 article by Turchi et al. posits that if such high $Z$ plasma could instead be composed of deuterium or a deuterium–tritium mixture, then the resultant multi-keV plasma would make an effective target for magnetized plasma compression to fusion conditions. This report documents several experiments executed in an effort to achieve an ultrahigh-speed flow in a deuterium plasma. The first phase of this research concentrated on extension of the earlier work to a lower current system that would emulate the PFS used in series with an imploding liner load. The apparatus was also modified to permit pulsed injection of deuterium gas along the insulated coaxial electrodes between the PFS armature and the vacuum power feed. The experiments met with limited success, exhibiting evidence of a 550 km/s plasma flow which convected a small fraction of the total magnetic field. Two subsequent tests were conducted using foam armatures. In both cases, current prematurely shunted upstream in the vacuum feed. Several possible causes were explored for the shunting of the current. Among the modifications implemented, the gas injection system was altered to increase both the quantity of gas adjacent to the armature while facilitating an increased pressure gradient between the armature and the current feed. A series of low-energy shots were conducted to examine the impact of several proposed design modifications on current delivery to the armature. These experiments demonstrated that the hardware assembled for this investigation was unlikely to forestall breakdown in the injected gas as required by Turchi et al. Nevertheless, two experiments were conducted to evaluate performance with foam armatures. Both experiments exhibited good current delivery to the armature, behaving initially like the low-energy experiments. The magnetic flux convected downstream was greater than in any of the prior experiments, though significant work remains to demonstrate the ultrahigh-speed plasma flow concept.

Review of Some Plasma Gun Techniques for Fusion at Megagauss Energy Densities

Plasma guns offer opportunities to generate and direct plasma flows at high energy density. Typically, such guns comprise coaxial electrodes that are connected to high-current sources (e.g., capacitor banks, pulse lines, inductive stores, or magnetic-flux-compression generators). The basic interactions include ionization of materials such as injected gas or preinstalled wires/foils, acceleration of these materials by the Lorentz force, and expulsion of the resulting plasma flows. We review the use of a particular arrangement in the form of a plasma flow switch that acts as a multimegampere commutator, but it can also provide a magnetized-plasma target for compression by an imploding liner. In a quite separate concept, a plurality of quasi-steady plasma guns in a spherical array provides converging, collimated jets to compress plasma with stand-off from the plasma generators and chamber walls. Such stand-off in a repetitively pulsed system can be crucial for the development of fusion power reactors at megagauss energy densities.

Combined Flux Compression and Plasma Opening Switch on the Saturn Pulsed Power Generator

A wire-array flux-compression cartridge installed on Sandia's Saturn pulsed power generator doubled the current into a 3-nH load to 6 MA and halved its rise time to 100 ns. The current into the load, however, was unexpectedly delayed by almost 1 micros. Estimates of a plasma flow switch acting as a long-conduction-time opening switch are consistent with key features of the power compression. The results suggest that microsecond-conduction-time plasma flow switches can be combined with flux compression both to amplify currents and to sharpen pulse rise times in pulsed power drivers.

Preparation and Liner Compression of Plasma From an Ultrahigh Speed Flow

Preparation of the target plasma represents a critical issue in liner compression techniques to achieve fusion conditions. We consider the use of an ultrahigh speed plasma flow from a special coaxial-gun arrangement known as the plasma flow switch. Experiments have demonstrated that this arrangement can provide plasma flows with speeds in excess of 2000 km/s. Stagnation of such a plasma flow results in fully stripped aluminum plasma with electron temperatures of 30 keV. Substitution of deuterium or a deuterium-tritium mixture could provide target plasma at kilovolt temperatures within an imploding liner. Such temperatures suggest that, even if substantial heat loss occurred during liner compression, fusion-level temperatures would be possible. The concatenation of events to generate the ultrahigh speed flow, to direct it into the implosion chamber, and to arrange liner dynamics for effective compression demands numerical simulation, which is based on initial analytical estimates. Both types of calculation for exploring this concept are discussed.

Peaking the current of a microsecond generator operating into a plasma load

A configuration of a plasma flow switch intended for peaking the current of a capacitor bank operating into a liner load has been proposed and tested where the current bridge and the liner are formed with the help of a single pulsed gas valve. The design of the load unit has been optimized for the efficiency of the energy transfer from the energy store to the liner. The peaker was tested in experiments on generation of soft x-radiation and K-radiation of neon. High efficiency of conversion of the magnetic energy of the primary store to x-radiation has been demonstrated.

Effects of plasma surface layers on the efficiency of plasma flow switching

The plasma flow switch is a possible fast opening-switch for Z-pinches on inductive-store pulsed-power machines. This paper presents results of two-dimensional, radiation-magnetohydrodynamic models for a well-characterized plasma flow switch operating into a load region at current levels of 5-6 MA. The calculations predict that motion of the switch plasma along coaxial electrodes leads to the formation along the inner electrode of a boundary layer that can interfere with switching. The simulations describe the effect of the boundary layer on switching and show that it may be removed by means of a boundary-layer trap. Predictive calculations were used to eliminate the effects of the boundary layer and led to an improvement in the switch's performance. Comparison with experiments lends credence to the model.

Two-dimensional modeling of x-ray output from switched foil implosions on Procyon

A series of two-dimensional radiation magnetohydrodynamic calculations are presented of a Z-pinch implosion using a plasma flow switch. Results from a recent experiment using the high explosive driven generator Procyon, which delivered 16.5 MA to a plasma flow switch and switched about 15 MA into a static load, are used to study the implosion of a 29 mg load foil [J. H. Goforth et al., ‘‘Review of the Procyon Explosive Pulsed Power System,’’ in Ninth IEEE Pulsed Power Conference, June 1993, Albuquerque, edited by K. R. Prestwich and W. L. Baker (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1993), p. 36]. The interaction of the switch with the load plasma and the effects of background plasma on the total radiation output is examined. Models which assume ideal switching are also included. Also included are the effects of perturbations in the load plasma which may be associated with initial vaporization of the load foil. If the background plasma density in the switch region and in the load region does not affect the dynamics, the pinch is predicted to produce a total radiation output of about 4 MJ. Including perturbations of the load plasma associated with switching and assuming a background plasma density after switching in excess of 10−7 g/cm3 results in a total output from the pinch of about 0.6 MJ.

Initiation and assembly of the plasma in a plasma flow switch

A series of fast opening switch experiments was carried out on the Pegasus pulsed power facility. The first series of experiments characterized the assembly and run down of a 50 mg plasma flow switch operating in the 6 MA regime. The switch plasma arises from the vaporization of a wire array and a barrier foil. Arrays with different numbers of wires were examined. Two-dimensional (2-D) radiation magnetohydrodynamic simulations of the initial vaporization stage of the array plasma and its assembly on the barrier foil are presented, which include the effects of initial perturbations on the evolution of the switch plasma as it moves down a coaxial gun barrel. Sparse wire arrays were observed to perform poorly. A change in the experimental design, motivated by 2-D simulations, led to improved performance. Computational results which lead to these changes are discussed, and the results are compared with experimental data.

MJ capacitive energy store matching with dynamic load at puma installation

Abstract The description of a high power pulse gun driven by capacity energy store called PUMA is presented. Experimental results concerning some possible ways of coordination between the energy store and the inductive plasma load are described. A power amplification scheme based upon the plasma flow switch is discussed.

Enhancement of the radiation yield, in plasma flow switch experiments

The Shiva Star fast capacitor bank, an inductive store, and a plasma flow switch were used to deliver multimegaampere currents with submicrosecond rise times to cylindrical foil loads. A series of numerical simulations of the plasma flow switch/imploding load system were performed with the goal of discovering a way to boost the total power radiated by the imploding plasma as it stagnates on the axis of symmetry. The changes to the experimental design that were investigated include variations of the shape of the electrodes, size and mass of the load foil, structure of the axial view vanes, shape and mass of the switching plasma, material from which the load is constructed, the degree to which the load is bowed, and the energy of the capacitor bank. Radiation yields in the range 6-9 TW are predicted for future experiments on Shiva Star.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Generation of high-energy x-radiation using a plasma flow switch

Experiments with coaxial plasma guns at currents in excess of ten megamperes have resulted in the production of high-voltage pulses (0.5 MV) and hard x radiation (10–200 keV). The x-radiation pulse occurs substantially after the high-voltage pulse suggesting that high-energy electrons are generated by dynamic processes in a very high speed (≳106 m/s), magnetized plasma flow. Such flows, which result from acceleration of relatively low-density plasma (10−4 vs 1.0 kg/m3) by magnetic fields of 20–30 T, support high voltages by the back electromotive force-u×B during the opening switch phase of the plasma flow switch. A simple model of classical ion slowing down and subsequent heating of background electrons can explain spectral evidence of 30-keV electron temperatures in fully stripped aluminum plasma formed from plasma flows of 1–2 × 106 m/s. Similar modeling and spectral evidence indicates tungsten ion kinetic energies of 4.5 MeV and 46 keV electron temperatures of a highly stripped tungsten plasma.

Enhanced load current delivery from the SHIVA star vacuum inductive store/plasma flow switch

The SHIVA Star device is a 1313- mu F, 120-kV capacitative storage device capable of storing 9.4-MJ electrical energy. The experimental operating voltage is 90 kV. An increased current delivery from the SHIVA Star capacitor band to a plasma-flow-switch-driven cylindrical foil load is reported. Modification of a plasma-flow-switch electrode produces a current delivery that is a factor of two better than the best previous comparable efforts. Peak driving current is 12.3 MA; the peak load current is over 9.4 MA. At peak load current, driving current is 11.2 MA for a current delivery of 85% or more. Current delivery in the experimental system is reported. The modification specified entails partially closing the outflow electrode of a plasma-flow switch. >

Review of Plasma Flow Switch Development

The plasma flow switch utilizes the nonlinear and nonuniform dynamics of a plasma discharge in vacuum to accumulate magnetic energy in times of several microseconds and then release this energy to a load in times of a few hundred nanoseconds. Experiments have been performed with capacitor banks up to 6 MJ, providing currents in excess of 107 A and peak voltages over 0.5 MV. Theoretical models include simple slug dynamics coupled to lumped-circuit analyses, magnetoacoustic considerations of one- and two-dimensional aspects of the plasma flow, and two-dimensional magnetohydrodynamic code calculations. The present article reviews both experimental and theoretical efforts, discusses the use of the plasma flow switch to drive plasma liner implosions and high-energy ion flows, and indicates directions for plasma flow switch applications to very high current, high-energy inductive pulsed power systems.

Simulations of a Plasma Flow Switch

In a portion of the experimental program using the SHIVA Star capacitor bank at the Air Force Weapons Laboratory (AFWL), a cylindrical foil load is imploded using an inductive store and a plasma flow switch. We have performed a number of two-dimensional simulations of the switch and load using the MHD code MACH2. In addition to explaining the data from the first series of experiments, the simulations led to design modifications of the basic plasma flow switch that resulted in improved current delivery and in enhanced radiation yield. The experimental results are reported in a companion paper by Degnan et al. The key modification was closing portions of the vane structure. The switch must be sealed shut or else substantial current will flow in the diffuse gas that is ablated from the walls of the switch barrel.

Pulsed Power Opening Switch Research at the University of New Mexico

Opening switch research at the University of New Mexico (UNM) is directed toward moderate-current (~10 kA) devices with potential applications to high-power charged particle accelerators. Two devices with the capacity for controlling gigawatt high-voltage circuits, the grid-controlled plasma flow switch and the scanned-beam switch, are under investigation. Both switches are conceptually simple; they involve little collective physics and are within the capabilities of current technology. In the plasma flow switch, the flux of electrons into a high-voltage power gap is controlled by a low-voltage control grid. Plasma generation is external to, and independent of, the power circuit. In the closed phase, plasma fills the gap so that the switch has a low on-state impedance. Pulse repetition rates in the megahertz range should be feasible. In single-shot proof-of-principle experiments, a small area switch modulated a 3-MW circuit; a 20-ns opening time was observed. The scanned-beam switch will utilize electric field deflection to direct the power of a sheet electron beam. The beam is to be alternately scanned to two inverse diodes connected to output transmission lines. The switch is expected to generate continuous-wave pulse trains for applications such as high-frequency induction linacs. Theoretical studies indicate that 10-GW devices in the 100-MHz range with 70-percent efficiency should be technologically feasible.

High Power Switches for Ion Induction Linacs

The success of linear induction ion accelerators for accelerator inertial fusion (AIF) applications depends largely on innovations in pulsed power technology. There are tight constraints on the accuracy of accelerating voltage waveforms to maintain a low momentum spread. Furthermore, the non-relativistic ion beams may be subject to a klystronlike interaction with the accelerating cavities, leading to enhanced momentum spread. In this paper, we describe a novel high power switch with a demonstrated ability to interrupt 300 A at 20 kV in less than 60 ns. The switch may allow the replacement of pulse modulators in linear induction accelerators with hard tube pulsers. A power system based on a hard tube pulser could solve the longitudinal instability problem while maintaining high energy transfer efficiency. The problem of longitudinal beam control in ion induction linacs is reviewed in Section 2. Section 3 describes the principles of the plasma flow switch. Experimental results are summarized in Section 4.