Implosion and stagnation of wire array Z pinches

Abstract

Detailed measurements of the dynamics of aluminum wire array Z pinches from immediately prior to implosion until stagnation and dissipation on axis are presented. Before implosion, the ∼0.5mm axial modulation seen in earlier laser probing images is observed as ablation on the surface of the wire cores facing away from the array axis. This results in the complete ablation of sections of the wire cores and a redistribution of current at the start of implosion. The dynamics of implosion are then strongly influenced by the number of wires in the array. With only eight wires, discrete snowplough bubbles expand from each wire toward the precursor. There is little, if any, correlation between the bubbles from adjacent wires, and a large temporal spread over which the bubbles arrive at the precursor is observed, along with a long rise time, low power soft x-ray pulse. With 32 or more wires, bubbles from adjacent wires merge close to the array edge to form an imploding sheath. The front edge of the sheath is well defined with a small spatial spread, and upon reaching the precursor, the start of a fast rising high power soft-x-ray pulse is seen. As x-ray emission increases, the stagnating column on axis starts to decrease in diameter, reaching a minimum at peak x-ray emission, which also coincides with the time when the rear edge of the snowplough reaches the column. Thereafter, the stagnated column is seen to go unstable, and trailing mass left behind during the implosion is accelerated toward the axis. Intense x-ray emission ends as this mass becomes cleared out.