Wire Array Dynamics with Varied Current Risetime on COBRA

The 1 MA COBRA accelerator is used to drive wire array loads with current risetimes from 95 to 220 ns. The dynamics of 8 x 12 micron Al arrays show a clearly different character when driven by the fast and slow current pulses. The fast ($\sim $100 ns, 1 MA) drive produces a hot pinch, high total x-ray energy in a long-duration, slowly rising x-ray pulse, multiple hot spots in the pinch, a relatively small load voltage that stays up after x-ray peak, and a large inferred current radius at x-ray peak. In contrast, the slow ($\sim $200 ns, 900 kA drive) produces a cooler pinch with lower total x-ray energy in a short, fast-rising x-ray pulse, absence of hot spots but clear evidence of high-energy ``electron beam,'' large load voltage that drops fast after x-ray peak, and small inferred current radius at x-ray peak. The diagnostic evidence of these characteristics especially x-ray imaging and spectral information, will be presented and the dynamics underlying these phenomena will be discussed.