Studies of the initial phases of exploding W and Al wire and wire-array plasma formation using x-ray backlighting

Exploding W and Al wires and wire arrays have been investigated using direct (point-projection) x-ray backlighting as the principal diagnostic method. A 4.5 kA amplitude, 350 ns quarter-period rise time sinusoidal current source which damps in about 5 micros is delivered to one or more fine W (7.5, 10 or 13.5 microm) or Al (13.5 or 25 microm) wires approximately 6 cm away from one or two Mo X-pinch x-ray backlighter sources. The X-pinches are placed in parallel between the output electrodes of the 450 kA, 100 ns XP pulser at Cornell University, each thereby producing a sub-nanosecond x-ray pulse. The source size is small enough to permit micron-scale spatial resolution images of the exploding wires on x-ray film. By varying the relative timing between pulsing the current source for the W or Al wire or wires and the XP pulser, images of the initial explosion phase of W and Al wires have been obtained at times ranging upwards from about 100 ns after the start of the 4.5 kA current source. Al wires expand substantially with the linear current rise for 100 ns to 1 kA in a single wire. By the time (350 ns) of peak current in a single Al wire, the Al has expanded sufficiently to be undetectable using the 3--4.8 keV x-ray backlighter source. However, two Al wires with 2 kA per wire expand more slowly and are still visible up to at least 450 ns. By contrast, W wires do not expand significantly during the first ∼ 1 micros. A laser schlieren imaging system to image the coronal plasma around the wire cores has been synchronized with the x-ray backlighter sources in order to see the tenuous outer coronal plasma which is invisible in the x-rays images. Comparisons of schlieren and x-ray backlighting images will be presented. The implications of these results to cylindrical plasma formation in exploding wire arrays by the prepulse on the Z accelerator, and to 2 and 3 D MHD code validation will be discussed