Plasma density measurements within Tungsten wire-array z-pinches on the cobra accelerator

The COBRA pulsed-power generator, with a nominal peak current of 1.1 MA and a minimum zero-to-peak rise-time of 95 ns, is being used to study the early phases of wire-array Z-pinch development with a variety of diagnostics. Here we present the results of applying point-projection X-ray radiography using up to five X pinches in the return current circuit to make accurate, high-resolution measurements of the plasma density distributions in tungsten (W) wire-array Z-pinch implosions. Plasma density measurements are calibrated by comparing X-ray transmission recorded on photographic films to transmission through W steps with known thicknesses. Plasma density distributions as a function of time with sub-ns time resolution and few-mum spatial resolution are presented for the coronal plasma close to each wire (10¹⁸-10²⁰/cm³), ablation streams moving towards the axis (<10²⁰/cm² areal density) and on-axis (<10¹⁹/cm³) plasmas. X-ray images ranging from 70 to 170 ns after the start of the current pulse have been analyzed. Dense cores appear to expand to a radius, r_f, that is linearly proportional to initial wire radius. The final radius is reached before 70 ns for 7.4-12.7 mum wires, with r_f = 7.0 r₀ - 9.6, where r₀ is the initial radius and all units are in mum, within ~10%. Larger wires appear to continue to expand during the entire 70-170 ns observation window. The final radius, r_f, is found from the Abel inversions of coronal plasmas, taking the radius at which the plasma density drops to 3times10²⁰/cm³. This trend was not affected by the number of wires in the array or the array radius, implying that the initial wire diameter is more significant than the inter-wire gap for a fixed array mass. The coronal plasma areal density outside the dense core radius, r_f, scales as 1/(r-r_f/2)²- - starting from the value at r_f. The areal density of the plasma in the ablation streams moving toward the axis falls off as 1/r, where r is the distance from the center of the dense wire core. Detailed results will be presented.