Structure of the dense cores and ablation plasmas in the initiation phase of tungsten wire-array Z pinches

The early stages of tungsten (W) wire-array Z-pinch implosions have been studied using two-frame point projection x-ray backlighting on the 1MA COBRA pulsed power generator [J. D. Douglass, J. B. Greenly, D. A. Hammer, and B. R. Kusse, in Proceedings of the 15th IEEE International Pulsed Power Conference, Monterey, 2005 (to be published)]. X-pinch backlighter images with subnanosecond time resolution and 4–10μm spatial resolution have been obtained of individual W exploding wires in 8-wire arrays that show evolution of wire-core and coronal plasma structures. The timing of the X-pinch x-ray bursts relative to the Z-pinch initiation time was adjusted over a 50ns time interval by varying the X-pinch mass per unit length. Wire-cores seen in two images separated in view by 120° show that the expansion is remarkably azimuthally symmetric. A strong correlation is observed between the structure on the dense exploding wire-cores and the structure of the ⩾1018∕cm3 ablation plasma being drawn from radial prominences. Plasma ablation velocity was estimated to have a lower bound of 24km∕s. The wire-core expansion rate was found to be approximately constant with time over the interval 50–100ns after the start of the current pulse. Finally, micron-scale axial gaps, seen as early as 70ns into the current pulse and persisting from that time, were observed along the wire-core.