W. Sommars

Implosion of solid liner for compression of field reversed configuration

The design and first successful demonstration of an imploding solid liner with height to diameter ratio, radial convergence, and uniformity suitable for compressing a field reversed configuration is discussed. Radiographs indicated a very symmetric implosion with no instability growth, with /spl sim/13x radial compression of the inner liner surface prior to impacting a central measurement unit. The implosion kinetic energy was 1.5 megajoules, 34% of the capacitor stored energy of 4.4 megajoules.

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.

Design, fabrication, and operation of a high-energy liner implosion experiment at 16 megamperes

We discuss the design, fabrication, and operation of a liner implosion system at peak currents of 16 MA. Liners of 1100 aluminum, with initial length, radius, and thickness of 4 cm, 5 cm, and 1 mm, respectively, implode under the action of an axial current, rising in 8 /spl mu/s. Fields on conductor surfaces exceed 0.6 MG. Design and fabrication issues that were successfully addressed include: Pulsed Power-especially current joints at high magnetic fields and the possibility of electrical breakdown at connection of liner cassette insulator to bank insulation; Liner Physics-including the angle needed to maintain current contact between liner and glide-plane/electrode without jetting or buckling; Diagnostics-X-radiography through cassette insulator and outer conductor without shrapnel damage to film.

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.

Helical explosive flux compression generator research at the Air Force research laboratory

The inherent high energy density of explosives make them an obvious choice for pulsed power systems requiring high peak power and energy in compact packages. Ongoing research at the Air Force Research Laboratorys Directed Energy Directorate into helical explosive flux compression generators is discussed. These generators provide the initial pulsed power drive for a high voltage, long pulse system, which is the subject of a companion paper. The helical generator research described here centers on experiments utilizing two distinct generator designs, based on 7.6 cm. and 15.2 cm diameter aluminum armatures, respectively. Experiments using several different stator coil winding schemes with these armatures are described.

Flux Compression Generator Development at the Air Force Research Laboratory

The Air Force Research Laboratory (AFRL) maintains an extensive capability for the design, analysis, construction and testing of explosive pulsed power (EPP) components. Three flux compression generators (FCGs) were designed as part of an EPP technology development effort sponsored by AFRL and the Defense Advanced Research Projects Agency (DARPA). A secondary-stage, high-current FCG was designed to deliver 10 MA into a nominal load inductance of 80 nH from an initial generator inductance of 1.6 muH that is seeded with 1 MA. We have also developed a coaxial FCG to deliver more than 20 MA into a 2 nH load. The initial flux in the coaxial chamber (60 nH at 1.5 MA) is compressed uniformly using a copper armature, which is simultaneously initiated using a slapper detonator. Either of these two FCGs can be seeded with a third generator design: a high-gain, helical FCG. This model serves as our workhorse generator capable of delivering 2 MA into a 0.5 muH inductive load. It has also been operated into load inductances ranging from 0.1 to 2.0 muH with comparable flux delivery. All experiments are conducted on an explosive test range located on Kirtland Air Force Base [1]. The design effort is supported by powerful computer modeling using CAGEN [2], CALE and MACH2. Design features for all three FCGs are presented in this paper with results from recent explosive tests.

Refurbishment of the ETDL Rep-Rate Pulse Generator at AFRL

The Air Force Research Laboratory is refurbishing a high-voltage rep-rate pulser that generates 550 kV output pulses with a nominal FWHM of 550 ns at pulse repetition rates of up to 5 Hz. This pulser was originally constructed for the Army Research Laboratorys Electronic Test Devices Laboratory (ETDL). The pulser can drive 20-, 10-, or 5-Omega loads when either one, two, or four 20-Omega PFNs, respectively, are installed. Among the major changes and upgrades that are being made to the pulser is the replacement of the output switch with a triggered cascade switch. Initially, a two-electrode self-breaking spark gap was used for the output switch, but this had jitter in pulse-to-pulse timing and waveshape due to variation in the self-break voltage. An earlier implementation of a triggered output switch was successful in reducing pulse-to-pulse jitter [1], but the spread still remained approximately 500 ns. A significant effort was directed during this present upgrade towards the development of an improved triggered output switch. The new switching system has a 10 ns total spread in breakdown time with respect to the initial trigger to the pulser system and provides reproducible waveshapes. The charging system has also been upgraded. The original system used a computer-controlled high-voltage DC power supply and a filter bank that pulse-charged the pulse generators primary bank. This is being replaced with a commercially available, high-power (100 kV) DC charging supply to directly charge the primary bank. Eliminating the filter bank has led to a substantial reduction in size. Other modifications to the pulser include changes to the PFN design to allow greater ease and flexibility when tuning the pulse shape and to reduce the self-inductance of the PFN capacitors. This paper describes these modifications to the ETDL pulse generator in detail and presents data from initial tests following the upgrades.

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.

Explosive pulsed power experiments at the Phillips Laboratory

The application of pulsed power technology to advanced aerospace mission scenarios increasingly involves achieving higher peak power and energy while shrinking the deployment package. The inherent high energy density of explosives make them an obvious candidate for applications requiring extremely compact, single shot pulsed power drivers. However, explosive flux compression generators tend to be rather slow, low impedance, high current devices, while the loads of interest typically present a relatively high impedance and require short, high voltage pulses. In this paper, the results of experiments involving helical explosive generators and pulse shaping/impedance matching systems are discussed.