Plasma Formation and Evolution from an Aluminum Surface Driven by a MG Field

Summary form only given. Applying a magnetic field of several megagauss to a surface drives an interesting interplay of magnetic diffusion, hydrodynamics, and radiative energy transfer. This physics is important in wire-array Z-pinches, high current fuses, magnetically insulated transmission lines, ultrahigh magnetic field generators, magnetized target fusion, and astrophysics. To investigate such plasmas experimentally, 1 MA was driven through a 1 -mm-diameter cylindrical aluminum rod, using the UNR Zebra generator. The 70-ns current rise was sufficiently short that the current skin depth was a small fraction of the conductor radius. Diagnostics included optical imaging to a time-gated intensified CCD camera and a streak camera, magnetic field probes, photodiodes, photomultipliers, and laser shadowgraphy, schlieren, interferometry, and Faraday rotation. These yielded information on the threshold for plasma formation, the expansion of the aluminum, the temperature at the transition between optically thick and optically thin matter, and the growth of the unstable m=0 mode driven by the curvature of the magnetic field. Plasma formation due to ohmic heating was distinguished from plasma formation due to high electric fields or electrical contacts by comparing shots with wire loads vs. loads machined from a solid aluminum cylinder to have a 1-mm-diameter central length but large-diameter contacts. Time-gated images show markedly more uniform light from the machined load than from the wire load. The relatively simple experimental setup was chosen in the hope of providing a benchmark with which to test and improve radiation-magnetohydrodynamics modeling. Measurements have been compared with the results of RAVEN and MHRDR computer simulations, using various assumptions for equation of state, electrical conductivity, and radiation. The simulations yield observed quantities such as luminosity, laser shadowgraphs, and m=0 mode growth. They also yield many additional interesti- ng details, such as the propagation of a compression wave from the surface to the axis and back, with a resultant rapid radial expansion of the surface after peak current.