EVIDENCE FOR PRECURSOR PLASMA FORMATION RESULTING FROM HETEROGENEOUS CURRENT CHANNELS IN WIRE ARRAY LOADS

The initial current flow in single and double wire loads exploded by c urrent generators with dI/dt in the range of 10(10)-10(12) As has been investigated using laser absorption and interferometric techniques. T he results clearly show heterogeneous current channel formation with a plasma corona surrounding a predominantly neutral core. In the two wi re load case, the corona is observed to separate from the core and pre maturely implode on axis. Parasitic current channels were observed whe n material, outgassed by a low level prepulse current, is ionized to f orm a current carrying plasma channel that temporarily shields the wir e from the main current pulse. In a complementary study on a terawatt generator, the radiation spectra from Mg coated Al wire array loads co ntained Mg K-shell lines 10 ns before similar Al lines were observed, confirming the load straggling effects implied by the laser imaging me asurements. The results are consistent with theoretical predictions of heterogeneous current channels and precursor plasma formation that le ad to a softer implosion, reduced ion density, and reduced x-ray yield . (C) 1996 American Institute of Physics.