Measurements of surface plasma evolution from a Z current-return structure fielded on Gamble II

Plasmas generated near conductor surfaces with /spl sim/MA/mm/sup 2/ current density are diagnosed on Gamble II (1 MA/100 ns) for conditions relevant to a Z (20 MA/100 ns) wire-array z-pinch configuration. Plasma density greater than 10/sup 14/ cm/sup -3/ is detected 1 mm from the conductor beginning 100 ns after the start of the current. The plasma expands with 2 cm//spl mu/s velocity. High-density (10/sup 17/-10/sup 18/ cm/sup -3/) plasma migrates more slowly (/spl sim/0.5 cm//spl mu/s) away from the ribs. Three different rib cross sections appear to have little influence on the shape of the expanding plasma or its initial density time-history. Plasma expansion from gold-coated ribs appears to be about the same as from stainless ribs if the stainless ribs are cleaned in acetone prior to the shot. Stainless ribs that are not cleaned appear to generate plasma sooner. Implications for Z experiments and future experiments are discussed.