New X-ray backlighting results on small-scale structure formation, including liquid-vapor foam-like appearance, in exploding wire Z-pinches and X-pinches

Summary form only given. Exploding wire and wire array experiments have been carried out in Z-pinch and X-pinch geometry using low current (4.5 kA amplitude, 1.4 ps period damped sinusoid) and high current (up to 225 kA, 100 ns) drivers. X-ray backlighter images have been obtained using Mg, Al, Ti, Fe, Ni, NiCr, Cu, brass, Mo, Pd, Ag, Ta, W, Pt, Au, and Ni-coated Ti wires with initial diameters ranging from 7.5 pm (W) to 46 /spl mu/m (Mg). There were typically two Mo X-pinch X-ray backlighters which generated pulses of <0.5 ns duration from sources small enough to allow resolution of /spl sim/1 pm scale structure in the resulting images. In the high current experiments, images of dense cores within coronal plasmas are obtained with higher Z materials including Ni, Cu, Mo, Pd, Ta, W, Pt and Au; with lower Z materials, only the dense cores are visible in the backlighter images. In all cases a fine structure in the interior of the dense cores, and/or surface instabilities are observed. Some of the dense cores show a shock-wave-like structure and others show gaps that open up more rapidly than the wire cores are expanding. The wire interiors develop a foam-like liquid-vapor mix, with some of the bubbles breaching the surface and spewing vapor out of the dense core. The low current experiments follow the same general behavior but require several us to complete the process instead of 50-100 ns. With W and Al wire explosions, step wedges have been used to permit calibrated density measurements to be made of the core (W and Al) and coronal (W) plasmas.