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

Abstract

Summary form only given. Copper and aluminum mm-diameter rods have been driven by a mega-ampere current pulse at UNR's Nevada Terawatt Facility. The facility's 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 aluminum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> . 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.