Surface plasma evolution from mockup Z current-return structures on Gamble II

Summary form only given. The Sandia Z generator produces high current density in the current-return conductors that can result in dense surface plasmas that move into the power-flow gaps and the slots that allow optical diagnostic access to the z-pinch. This blow-off plasma was diagnosed on the NRL Gamble II generator using two conductors to mimic the Z current density distribution at the edges of a slot corresponding to a 40 mm diameter return current "Z" experiment but without the associated z-pinch radiation flux on the conductor surfaces. The 2D profile of the blow-off material was measured using a pulsed nitrogen laser (500 ps) via schlieren imaging. Simultaneous 2-color time resolved single beam interferometry determined the blow-off density at points between the ribs, integrated along the length. Three different conductor cross-sections were studied under nominally the same conditions. Gold-coated and uncoated stainless steel conductors were compared. High-density (>10/sup 15/ cm/sup 3/) plasmas appear 1 mm from the ribs beginning about 100 ns after the start of the current. The plasma propagates at about 2/spl times/10/sup 6/ cm/s into the slot and into the power flow gap. These data can be used to benchmark simulation codes to address ultimate limitations for power flow and slot closure on the Z generator.