Gareth Adam Bird

Research on forming, compressing, and accelerating milligram-range compact toroids using a meter diameter, two-stage, puffed gas, magnetic field embedded coaxial plasma gun is described. The compact toroids that are studied are similar to spheromaks, but they are threaded by an inner conductor. This research effort, named marauder (Magnetically Accelerated Ring to Achieve Ultra-high Directed Energy and Radiation), is not a magnetic confinement fusion program like most spheromak efforts. Rather, the ultimate goal of the present program is to compress toroids to high mass density and magnetic field intensity, and to accelerate the toroids to high speed. There are a variety of applications for compressed, accelerated toroids including fast opening switches, x-radiation production, radio frequency (rf) compression, as well as charge-neutral ion beam and inertial confinement fusion studies. Experiments performed to date to form and accelerate toroids have been diagnosed with magnetic probe arrays, laser interferometry, time and space resolved optical spectroscopy, and fast photography. Parts of the experiment have been designed by, and experimental results are interpreted with, the help of two-dimensional (2-D), time-dependent magnetohydrodynamic (MHD) numerical simulations. When not driven by a second discharge, the toroids relax to a Woltjer–Taylor equilibrium state that compares favorably to the results of 2-D equilibrium calculations and to 2-D time-dependent MHD simulations. Current, voltage, and magnetic probe data from toroids that are driven by an acceleration discharge are compared to 2-D MHD and to circuit solver/slug model predictions. Results suggest that compact toroids are formed in 7–15 μsec, and can be accelerated intact with material species the same as injected gas species and entrained mass ≥1/2 the injected mass.

Using a 1313-μF, 3-nH, 120-kV, 9.4-MJ SHIVA Star capacitor bank, we have performed vacuum inductive store/plasma flow switch (PFS) driven implosions of low mass (200-400 μg/cm2) cylindrical foil liners of 2-cm height and 5-cm radius. This technique employs a coaxial discharge through a plasma armature, which stores magnetic energy over 3-4 μs and rapidly switches it to an imploding load as the plasma armature exits the coaxial gun muzzle. The current transferred to the load by the PFS has a rise time of less than 0.2 μs. With 5-MJ stored energy, we have driven fast liner implosions with a current of over 9 MA, obtaining an isotropic equivalent 2.7-TW 0.5-MJ X-ray yield.

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.

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.

Vacuum inductive-store, plasma flow switch-driven implosion experiments have been performed using the Shiva Star capacitor bank (1300 µf, 3 nH, 120 kV, 9.4 MJ). A coaxial plasma gun arrangement is employed to store magnetic energy in the vacuum volume upstream of a dynamic discharge during the 3- to 4-µs rise of current from the capacitor bank. Motion of the discharge off the end of the inner conductor of the gun releases this energy to implode a coaxial cylindrical foil. The implosion loads are 5-cm-radius, 2-cm-long, 200 to 400 µg/cm2 cylinders of aluminum or aluminized Formvar. With 5 MJ stored initially in the capacitor bank, more than 9 MA are delivered to the implosion load with a rise time of ∼200 ns. The subsequent implosion results in a radiation output of 0.95 MJ at a power exceeding 5 TW (assuming isotropic emission). Experimental results and related two-dimensional magnetohydrodynamic simulations are discussed.

The acceleration of solid material to velocities > 100 km/s using reasonable length accelerators can best be accomplished if electrostatic techniques are utilized. An accelerator for nanogram microprojectiles is being developed to demonstrate the basic principles for a multistage system based on the sequential application of voltage pulses /spl ges/ 100 kV. Experiments have been performed stressing carbon material to electric fields > I.6x1O/sup 9/ V/m, a field adequate to attain charge-to-mass ratios (q/m) of 5.0 C/kg for micron-diameter projectiles. An injector for charging and launching the microprojectiles into an accelerator has been constructed and operated with q/m = 1 C/kg. Specialized diagnostics for recording the microprojectile's charge and trajectory include a Faraday cup, and a schlieren optical system coupled to an electronic streak camera and to a microchannel-plate framing camera. A five-stage system is presently being and tested to attain a microprojectile velocity of 1 km/s as a proof-of-concept demonstration.

Experiments to form, compress, and accelerate compact toroids are described. A 1-m-diam, two-stage, puffed gas, magnetic field embedded coaxial plasma gun is used. Emphasis is on conical compression. Discharges were in the several mega-ampere, few microsecond rise time range. Magnetic probe data suggest that l/(r{center_dot}{delta}r) compression of the toroid field is achieved, consistent with theoretical prediction. The magnetic field pulse and electron density pulse due to the compact toroid correlate in space and time. The compact toroid species is the injected gas species and precedes electrode plasma by several microseconds. The poloidal magnetic field precedes the azimuthal magnetic field. The time of arrival of the axial magnetic field compared with the axial position is consistent with the mean current axial position trajectory obtained from inductance growth. 8 refs., 6 figs.

Summary form only given, as follows. A compact toroid (CT) formation experiment is discussed. The device has coaxial electrode diameters of 0.9 m (inner) and 1.25 m (outer) and an electrode length of ~1.2 m, including an expansion drift section. The CT is formed by a 0.1-0.2-T initial radial magnetic field embedded coaxial puff gas discharge. The gas puff is injected with an array of 60 pulsed solenoid driven fast valves. The formation discharge is driven by a 108-μF, 40-100-kV, 86-540-kJ, 2-5-MA capacitor discharge with ~20-nH initial total discharge inductance. The hardware includes transmission line connections for a Shiva Star (1300-μF, up to 120-kV, 0.4-MJ) capacitor bank driven acceleration discharge. Experimental measurements include current and voltage; azimuthal, radial, and axial magnetic field at numerous locations; fast photography and optical spectroscopy; and microwave, CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser, and He-Ne laser interferometry. Auxiliary experiments include Penning ionization gauge, pressure probe, and breakdown gas trigger diagnostics of gas injection, and Hall probe measurements of magnetic field injection

Abstract : The Plasma Flow Switch utilizes the dynamics of a plasma discharge in vacuum to accumulate magnetic energy in several microseconds and then release this energy to a load region in a few hundred nanoseconds. Experiments have previously been performed using the plasma flow switch on the Shiva Star capacitor bank to drive imploding plasma loads at multi-megajoule, multi-megampere levels. Recently, experiments have been conducted in which a portion of the switch plasma is used as the current carrying load. These experiments employed the Shiva Star capacitor bank, charged initially to 84 kV (4.6 MJ) to achieve currents in excess of 10 megamperes at the switch region. Comparisons of voltage measurements in the bank transmission line with numerical simulations (MACH2 computer code) indicate that the low density, very high speed flow in the switch supports voltages in excess of 0.5 megavolts at the coaxial gun muzzle. Measurements of hard X-radiation ( 10-100 keV) imply the existence of high energy electrons and are consistent with the generation of high voltages in the plasma flow switch.

A solution is obtained for the temperature, density, total number, and energy of the particles trapped between magnetic mirrors which, at zero time, acquire a uniform velocity of convergence. It is assumed that collisions between particles may be neglected and that the collisions with the mirrors are adequately described by the guiding center theory. It is found that, for a given mirror strength and rate of convergence, there are mirror spacings at which the energy reaches a maximum and the temperature and density reach limiting values. Furthermore, there are optimum rates of convergence at which these maximum and limiting values are a maximum. During the early stages of compression at high rates of convergence, the temperature rises at a far greater rate than the square of the density.

The Phillips Laboratory's MARAUDER Compact Toroid acceleration experiments are described. The acceleration discharge is the second stage of a two-stage co-axial plasma gun discharge. The first stage discharge forms a compact torus. First stage discharge parameters are 110 microfarad, 60 to 80 kv initial voltage, 40 nanohenry initial inductance, 6 milliohm series resistance, 2 to 2.6 Megamps peak current, 1 to 2 milligrams initial gas mass (puff injected), 0.1 Tesla initial radial field (double solenoid injected). Second stage discharge parameters are 108 microfarad, 40 to 80 kv initial voltage, 25 nanohenry initial inductance. The two-stage gun geometry parameters are 44.8 cm inner conductor radius, 52.4 cm first stage outer conductor radius, 62.8 cm second stage outer conductor radius, transition from first stage to second stage at 20 cm axial distance downstream from gas injection plane, and variable (up to 80 cm) axial length of second stage. Data from fast photography, arrays of radial, azimuthal, and axial magnetic probes, He-Ne and microwave interferometry, time resolved optical spectroscopy, time gated and spectral line filtered photography, and X-ray detectors will be presented and discussed. The acceleration discharge appears to sharpen and strengthen radial magnetic field reversals, suggesting driven reconnection. The effects of varying the second stage geometry on acceleration discharge current delivery will be discussed.

An electrostatic accelerator technique for microprojectiles is being developed based on a multistage system using the sequential application of moderate-voltage pulses (>or=100 kV). Preliminary experiments have shown that carbon fibers have adequate tensile strength and conductivity to achieve charge-to-mass ratios >or=1 C/kg, a value consistent with hypervelocity goals. The carbon microprojectiles have been used in a five-stage proof-of-principle prototype accelerator at stage voltages of 35 kV to attain velocities of 0.5 km/s. Through the use of schlieren imaging techniques, data have been obtained showing that good control of the projectile trajectory can be achieved with electrostatic aperture focusing methods. Information from these experiments is being used to design and construct a 10-20 km/s prototype accelerator. To obtain a relatively short accelerator, encapsulation techniques are being developed so that acceleration gradients approaching the high dielectric strengths of the encapsulants can be achieved. A reflex transmission line arrangement has been devised that permits the longitudinal accelerating field to follow the projectile motion along the multiple stages with minimal switch action and without reversing electric field vectors, which would degrade dielectric strength. Details on the accelerator concept, the experimental results, and hardware designs are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>