Going from nested to single wire array for 800ns implosion time Z-pinches, using microsecond prepulse technique on sphinx machine

Summary form only given. The Sphinx machine is a 6 MA, 1 mus driver based on the LTD technology, used for Z-pinch experiments. Despite the long implosion time (600 ns to 800 ns), Sphinx wire arrays with large diameter (~14 cm) have already shown to behave and to be controlled by the same physical processes as faster Z-pinches. This allowed to find wire arrays configurations able to mitigate instabilities and reach good radiation outputs. Nested aluminum wire arrays loads, typically 150/75 wires, 5 cm height and diameter 140/70 mm leads to 0.25 TW/cm total radiation power and 10 to 20 kJ of Al K-shell yield with a full width at half maximum of about 50 ns. An improvement of these results has been possible using the new multi-microsecond prepulse technique described in [H. Calamy, Phys. Plasmas 15, 012701(2008)]. Early breakdown of the wires during the prepulse phase leads to axially homogeneous ablation rate of the wires. Therefore, during the implosion process, cathode bubble issue observed on shots without prepulse is removed and the implosion is axially homogeneous. During the final stage, the implosion is centered and zippering effect is dramatically reduced, leading to simultaneous X-ray emission of the whole pinch length. A great reproducibility of this improved load is obtained. Total power is increased up to 1.5 TW/cm and FWHM is reduced to 20 ns. Further study of this prepulse technique are described here. Results of single wire array loads of typical dimensions 5 cm in height with implosion time between 700 and 900 ns and diameter varying between 80 and 140 mm are given. Best result was obtained with a W single array which radiates 2.54 TW/cm of total power and 520 kJ total yield, which is more than 25% of the energy stored in the generator. Parameters of the loads were varied in terms of radius, number of wires. Comparisons with nested wire array loads are done and trends are proposed. These experiments highlight the great benefit of using the long prepuls- - e technique for stabilization of wire array Z-pinch loads with 700 ns implosion time.