D.J. Amdahl

Experimental and Computational Progress on Liner Implosions for Compression of FRCs

Magnetized target fusion (MTF) is a means to compress plasmas to fusion conditions that uses magnetic fields to greatly reduce electron thermal conduction, thereby greatly reducing compression power density requirements. The compression is achieved by imploding the boundary, a metal shell. This effort pursues formation of the field-reversed configuration (FRC) type of magnetized plasma, and implosion of the metal shell by means of magnetic pressure from a high current flowing through the shell. We reported previously on experiments demonstrating that we can use magnetic pressure from high current capacitor discharges to implode long cylindrical metal shells (liners) with size, symmetry, implosion velocity, and overall performance suitable for compression of FRCs. We also presented considerations of using deformable liner-electrode contacts of Z-pinch geometry liners or theta pinch-driven liners, in order to have axial access to inject FRCs and to have axial diagnostic access. Since then, we have experimentally implemented the Z-pinch discharge driven deformable liner-electrode contact, obtained full axial coverage radiography of such a liner implosion, and obtained 2frac12 dimensional MHD simulations for a variety of profiled thickness long cylindrical liners. The radiographic results indicate that at least 16 times radial compression of the inner surface of a 0.11-cm-thick Al liner was achieved, with a symmetric implosion, free of instability growth in the plane of the symmetry axis. We have also made progress in combining 2frac12-D MHD simulations of FRC formation with imploding liner compression of FRCs. These indicate that capture of the injected FRC by the imploding liner can be achieved with suitable relative timing of the FRC formation and liner implosion discharges.

FRC lifetime studies for the Field Reversed Configuration Heating Experiment (FRCHX)

The goal of the Field-Reversed Configuration Heating Experiment (FRCHX) is to demonstrate magnetized plasma compression and thereby provide a low cost approach to high energy density laboratory plasma (HEDLP) studies, which include such topics as magneto-inertial fusion (MIF). A requirement for the field-reversed configuration (FRC) plasma is that the trapped flux in the FRC must maintain confinement of the plasma within the capture region long enough for the compression process to be completed, which is approximately 20 microseconds for FRCHX. Current lifetime measurements of the FRCs formed with FRCHX show lifetimes of only 7 ∼ 9 microseconds once the FRC has entered the capture region.

Addressing Short Trapped-Flux Lifetime in High-Density Field-Reversed Configuration Plasmas in FRCHX

The objective of the field-reversed configuration heating experiment (FRCHX) is to obtain a better understanding of the fundamental scientific issues associated with high-energy density laboratory plasmas (HEDLPs) in strong, closed-field-line magnetic fields. These issues have relevance to such topics as magneto-inertial fusion, laboratory astrophysical research, and intense radiation sources, among others. To create HEDLP conditions, a field-reversed configuration (FRC) plasma of moderate density is first formed via reversed-field theta pinch. It is then translated into a cylindrical aluminum flux conserver (solid liner), where it is trapped between two magnetic mirrors and then compressed by the magnetically driven implosion of the solid liner. A requirement is that, once the FRC is stopped within the solid liner, the trapped flux inside the FRC must persist while the compression process is completed. With the present liner dimensions and implosion drive bank parameters, the total time required for implosion is ~25 μs. Lifetime measurements of recent FRCHX FRCs indicate that trapped lifetimes following capture are now approaching ~14 μs (and therefore, total lifetimes after formation are now approaching ~19 μs). By separating the mirror and translation coil banks into two so that the mirror fields can be set lower initially, the liner compression can now be initiated 7-9 μs before the FRC is formed. A discussion of FRC lifetime-limiting mechanisms and various experimental approaches to extending the FRC lifetime will be presented.

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.

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.

Full Axial Coverage Radiography of Deformable Contact Liner Implosion Performed with 8 cm Diameter Electrode Apertures

diameter ratio, radial convergence, uniformity, and implosion velocity suitable forcompressing anFRC[3]. We obtained full axial coverage radiography ofa Ourrecent progress hasbeentoreplace themorestandard deformable contact imploding liner. Thisradiographic data sliding liner-electrode contacts withdeformable linerindicates thefeasibility ofusing avarying thickness inalong electrode contacts, whichenables theuseoflarge cylindrical solid liner, driven asa 12megampZ-pinch, to electrode apertures, suitable forFRCinjection. SeeFig. 1 achieve factor - 16cylindrical convergence, while using 8cm foraillustration ofthis concept. diameter aperture electrodes. TheAlliner was30cmlong, with9.78cminner diameter forits full length, 10.0cmouter Research ontheuseofimploding liners to diameter forthecentral 18cm ofitslength, andouter compress plasmas hasbeenreported byanumberof diameter increased linearly to10.2cmat1cmfromeitherresearchers. Thisincludes suggesting thegeneral concept electrode, andto11cmatelectrode contacts. Theelectrode ofusing liners tocompress plasma, andresearch on apertures allow injection ofField Reversed Configurations in shorter orlowervelocity liner implosions [4-17], and proposed future experiments onmagnetized target fusion. implosion ofaCu-Wliner withexplosives tocompress Indexterms: capacitor bank, Field Reversed Configuration, flux to200T[18]. FRC,Magnetized Target Fusion, MTF,imploding liner, radiography, megamp Uniform-thickness liner Variable-thickness

Virtual prototyping of directed energy weapons

This paper gives an overview of how RF systems, from pulsed power to antennas, may be virtually prototyped with the improved concurrent electromagnetic particle-in-cell (ICEPIC) code. ICEPIC simulates from first principles (Maxwells equations and Lorenzs force law) the electrodynamics and charged particle dynamics of the RF-producing part of the system. Our simulations focus on gigawatt-class sources; the relativistic magnetron is shown as an example. Such simulations require enormous computational resources. These simulations successfully expose undesirable features of these sources and help us to suggest improvements

Deformable contact liner implosion performed with 8 cm diameter electrode apertures

We obtained full axial coverage radiography of a deformable contact imploding liner. This radiographic data indicates the feasibility of using a varying thickness in a long cylindrical solid liner, driven as a 12 megamp Z-pinch, to achieve factor- 16 cylindrical convergence, while using 8 cm diameter aperture electrodes. The Al liner was 30 cm long, with 9.78 cm inner diameter for its full length, 10.0 cm outer diameter for the central 18 cm of its length and outer diameter increased linearly to 10.2 cm at 1 cm from either electrode, and to 11 cm at electrode contacts. The electrode apertures allow injection of Field Reversed Configurations in proposed future experiments on magnetized target fusion.

Field Reversed Configuration (FRC) formation, translation and compression

Experiments on FRC formation and translation into the interior of a metal shell or liner have been conducted at AFRL. Flux exclusion, collimated light, and interferometer data on magnetized plasma injection will be presented. These are a pre-requisite for FRC compression by liner implosion, experiment progress on which will also be presented. FRC translation, capture, and compression experiments all use primarily axial ∼ 2 Tesla guide and mirror fields established inside the liner, using ∼ 5 millisecond rise time discharges into an array of pulsed magnet coils surrounding the liner implosion portion of the device. A 12 MA, 4.5 MJ axial discharge drives the liner implosion for compression experiments. The FRC capture experiments use 3 capacitor discharges into a segmented theta coil surrounding the FRC formation region to establish a bias field, accomplish pre-ionization of deuterium gas, and provide the reverse field main theta discharge (∼ 1 Megamp) which forms the FRC. This is aided by two cusp field discharges. The guide and mirror fields enable translation of the FRC and its capture in the liner interior region. Diagnostics include pulsed power (current and voltage), magnetic field, field exclusion, He Ne laser interferometry, imaging and spectroscopy, radiography, and both activation and time-of-flight neutron detection. Design features and operating parameters are guided by 2D-MHD simulations.

Results from compression of field reversed configuration using imploding solid liner

Summary form only given. The AFRL Shiva Star capacitor bank (1300 μF, up to 120 kV) used typically at 4 to 5 MJ stored energy, 10 to 15 MA current, 10 μs current rise time, has been used to drive metal shell (solid liner) implosions for compression of axial magnetic fields to multi-megagauss levels, suitable for compressing magnetized plasmas to Magneto-Inertial Fusion (MIF) conditions. MIF approaches use embedded magnetic field to reduce thermal conduction relative to inertial confinement fusion (ICF). MIF substantially reduces required implosion speed and convergence. Using a profiled thickness liner enables large electrode apertures and the injection of a field-reversed configuration (FRC) version of a magnetized plasma ring. Using a longer capture region than originally used, the FRC trapped flux lifetime was made comparable to implosion time and an integrated compression test was conducted. The FRC was compressed cylindrically by more than a factor of ten, with density up more than 100x, to >1018 cm-3 (a world FRC record), but temperatures were only in the range of 300-400 eV, compared to the intended several keV. Although compression to megabar pressures was inferred by the observed time and rate of liner rebound, we learned that heating rate during the first half of the compression was not high enough compared to the normal FRC decay rate. Principal diagnostics for this experiment were soft x-ray imaging, soft x-ray diodes, and neutron measurements. Measures that could double the trapped flux lifetime and pre-compression temperature of the FRC will be discussed.