F. M. Lehr

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.

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.

Development and Testing of a High-Gain Magnetic Flux Compression Generator

The performance of a high-gain FCG is often limited by internal electrical breakdown caused by the high voltage generated during operation. Modern diagnostic techniques provide the opportunity to diagnose internal breakdowns so that generator designs can be improved. This paper describes the internal breakdowns observed in the JAKE FCG developed at the AFRL during the late 1990s. A revision to the stator winding pattern of the JAKE generator has led to improved control of the internal voltage. Designated JILL, the revised generator has substantially better flux transport efficiency, particularly at higher seed current. The techniques employed to design the new stator winding and the results of development testing are presented.

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.

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

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.

The Explosive Pulsed Power Test Facility at AFRL

The Air Force Research Laboratory has developed and tested a variety of explosive driven pulsed power devices over the past twenty-two years. Testing is performed primarily at a dedicated facility located at Chestnut Site on Kirtland Air Force Base. The facility is described in this paper, including details of recent upgrades.

Fiber-Optic Systems at the Explosive Pulsed Power Test Facility at AFRL

The Air Force Research Laboratory (AFRL) located on Kirtland Air Force Base performs explosive pulsed power experiments [1] - [3]. The large separation distances between the related subsystems of these shots increase the likelihood of inadvertent multiple electrical ground connections. This paper describes some of the fiber-optic devices routinely used during our explosive power tests to mitigate the problems associated with ground loops.

Magnetic pressure driven implosion of solid liner suitable for compression of field reversed configurations

Summary form only given, as follows. The initial design and performance of a magnetic pressure driven imploding solid liner with dimensions suitable for compressing a field reversed configuration (FRC) is presented and discussed. The nominal liner parameters are 30 cm length, 5 cm outer radius, /spl sim/0.1 cm thickness, Al material. The liner is imploded by magnetic pressure from an axial discharge driven by a 1300 microfarad capacitor bank. Other nominal discharge parameters are /spl sim/80 kV initial bank voltage, /spl sim/44 nanohenry initial total inductance, and /spl sim/milliohm series resistance. The discharge current exceeds 10 megamps in /spl sim/9 /spl mu/sec. Several types of calculations indicate that such a liner will implode in /spl sim/22 to 25//spl mu/sec, and will achieve a >0.3 cm//spl mu/sec implosion velocity by the time the liner has imploded to /spl sim/2.5 cm radius. This performance and these dimensions are suitable for FRC formation and compression. The diagnostics for the initial experiments include current (via Rogowski coils and inductive magnetic probes), voltage (via capacitive divider probes), flash radiography, and diagnostic magnetic field compression. Several types of simulations, including two dimensional magnetohydrodynamic simulations, are also discussed.