Analysis of Plasma Formation in an Experiment with Pulsed Megagauss Field on 1.0-mm Diameter Aluminum Rods

Abstract

Summary form only given. The physics of the interaction between large magnetic field and conducting media is important to wire-array z-pinches, high current fuses, magnetically insulated transmission lines, ultrahigh magnetic field generators, magnetized target fusion, and astrophysics. In an experiment on the 1 MA UNR Zebra Marx generator, megagauss magnetic field was pulsed on the surface of 1.0-mm-diameter aluminum rods. This rod diameter is large enough to confine current to a skin layer, so that the effects of magnetic diffusion are important, yet small enough to enable magnetic field in the range of a few megagauss; a regime where the formation of plasma on conducting surfaces is expected. Furthermore, to obtain experimental results with a one dimensional character to benchmark Radiation-MHD codes, loads were designed so that the growth of instability leaves the wire approximately axially uniform throughout the current rise. Rods with 1.0-mm-diameter fit this condition for the Zebra bank. An effort was made to distinguish plasma formation due to ohmic heating from plasma formation due to high electric fields or electrical contacts. Standard 1.0-mm-diameter wire loads were compared to loads machined from a solid aluminum cylinder to form a 1 -mm-diameter central length which transitioned smoothly to large-diameter contacts. Diagnostics included V-dot and B-dot probes, streak and time-gated intensified CCD cameras, photodiodes and photomultipliers, and laser shadowgraphy, schlieren, and interferometry. Filtered photodiodes measured radiation from the heated surface of the load. Assuming blackbody emission yields a surface temperature of order 10-eV near the time of peak current. Images from a time-gated intensified CCD camera and a streak camera give snapshots of complex surface phenomena, a time history of the expansion of the rod, and potentially uncover wave propagation speeds in the compressed aluminum. Images obtained support our expectation of a slowly expanding, highly axially symmetric surface during current rise, followed by fast expansion as the field strength diminishes. Laser diagnostics give evidence of plasma formation, as m=0 perturbation growth is observed after peak current. V-dot and B-dot data are being analyzed to obtain insight into the total energy deposition and the dynamic impedance of the load.