Kevin Yates

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.

Electrothermal instability evolution on Z-pinch rods and imploding liners pulsed with intense current

Summary form only given. Magnetically imploded liners assemble high-energy-density plasmas for radiation effects and inertial confinement fusion experiments. The stagnation pressures and temperatures achieved are limited by the Magneto-Rayleigh-Taylor (MRT) instability, which can grow to large amplitude from a small seed perturbation. While the metallic liners used for experiments on the Sandia National Laboratories Z Facility are typically diamond turned to 10-30 nm rms surface roughness, the observed MRT amplitude is unexpectedly large. Early in the current pulse an electrothermal instability (ETI), driven by non-uniform runaway Ohmic heating, may provide a mass perturbation on the liners surface which exceeds the machining roughness; ETI may then provide the dominant seed from which MRT grows. First, data from Z experiments (20 MA in 100 ns) are presented which demonstrate enhanced implosion stability for magnetically accelerated liners that are coated with 70 μm of dielectric. While ETI is not directly observed in these experiments, data and simulation support that the dielectric tamps liner-mass redistribution from ETI, thus limiting the seed amplitude for MRT growth. Second, data from experiments on the U. of Nevada, Reno Zebra Facility (1 MA in 100 ns) are presented. Experiments directly observed the non-uniform temperature and phase-state evolution of the current-carrying surface of 1.0-mm-diameter solid Al rods. The self-emission from micron-scale surface temperature variations were observed directly through high-resolution (3 μm spatial, 2ns temporal) gated optical imaging. Data from aluminum alloys (6061 and 5N) and a variety of fabrication techniques (conventionally machined, single-point diamond turned, electropolished) enable evaluation of which imperfections most effectively seed non-uniform heating and phase changes.

PPPS-2013: Addressing short trapped-flux lifetime in high-density field-reversed configuration plasmas in FRCHX

Summary form only given. 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 plasmas (HEDPs) in strong, closed-field-line magnetic fields. These issues have relevance to such topics as magneto-inertial fusion (MIF), laboratory astrophysical research, and intense radiation sources, among others. To create the HEDP, 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 shell (solid liner), where it is trapped between two magnetic mirrors and then compressed by the magnetically-driven implosion of the shell. A requirement is that once the FRC is stopped within the shell, the trapped flux inside the FRC must persist while the compression process is completed. With the present shell dimensions and drive bank parameters, the total time required for implosion is ~25 microseconds. Lifetime measurements of recent FRCHX FRCs indicate trapped lifetimes now approaching ~14 microseconds, and with recent experimental modifications the liner compression can be initiated considerably earlier before formation is completed in order to close that gap further. A discussion of FRC lifetime-limiting mechanisms will be presented along with a description of FRCHX and recent changes that have been made to it. Results from recent experiments aimed at lengthening FRC lifetime will also be presented.

Plasma formation and evolution on a copper surface driven by megaampere current pulse

Summary form only given. Copper and aluminum mm-diameter rods have been driven by a mega-ampere current pulse at UNRs Nevada Terawatt Facility. The facilitys z-pinch delivers 1 M A in ~100 ns producing megagauss surface magnetic fields that diffuse into the skin layer, ohmically heating the load and causing plasma formation. The load radius is designed such that it is in the “thick wire” regime; the radius is much thicker than the skin depth. With the novel “barbell” design of our loads, plasma formation due to arcing or electron avalanche is avoided, allowing for the study of ohmically heated loads. Work presented here will show first evidence of a magnetic field threshold for plasma formation in copper and compare with previous work done with aluminum1. Similarities and differences between these metals will be presented, giving motivation for continued work with different material loads. During the current rise, the metal is heated to temperatures that cause multiple phase changes. When the surface magnetic field reaches a threshold, the metal ionizes and the plasma becomes pinched against the underlying cold liquid metal. Diagnostics fielded include visible light radiometry, two-frame shadowgraphy in both 266 and 532 nm wavelengths, 266 nm interferometry, time gated EUV spectroscopy, 12-frame/5 ns gated imaging, and single frame/2 ns gated imaging with an ICCD camera. Surface temperature, expansion speeds, instability growth, time of plasma formation and plasma uniformity are determined from the data. The interplay between an ohmically heated conductor and a magnetic field is important for the field of Magnetized Target Fusion (MTF). MTF compresses a magnetized fuel by imploding a flux conserving metal liner. During compression, fields reach several megagauss, with a fraction of the flux diffusing into the metal liner. The magnetic field induces eddy currents in the metal, leading to ionization and potential mixing of metal contaminant into the fusion fuel.