Dramatic Improvement of X-Ray Power Maximum and Shape on Aluminum Z-pinches using Slow Current Prepulse on SPHINX Machine

Summary form only given. The SPHINX machine based on microsecond LTD technology is used in direct drive mode to implode nested aluminum wire arrays with outer diameter up to 140 mm and maximum current from 3.5 MA to 4.5 MA. Wire array implosions were usually done with no prepulse, only current from the LTD lines was switched to the load. Best results obtained in this configuration were 1-3 TW radial total power, 100-300 kJ total energy and 20-30 kJ energy above 1 keV; axial total power reaches around 0.1 TW. The physical processes of z-pinch wire array implosion in this 800 ns regime were shown to be very similar to those observed on shorter implosion time (60 to 300 ns) machines. SPHINX machine configuration has been modified to allow a multi-microsecond current to be injected through the wire array load before the start of the mam current. Amplitude and duration of this current prepulse is adjustable, with maxima ~10 kA and 50 mus. This prepulse dramatically changes the ablation phase and the implosion leading to a delayed implosion and an improvement of the axial homogeneity of both the implosion and the final pinch. First shots were done without any optimization of this prepulse. However, radiation pulse was strongly improved: the full width at half maximum (FWHW) went from 40-50 ns to less than 20 ns with a reproducible behaviour and only one single peak: the total radiated energy was multiplied by a factor of 1.6 and the total power by a factor of 4. Optimization of the prepulse current amplitude and duration was then performed showing that total power can be multiplied by 6 to 7 and power above 1 keV by a factor of 2. Experimental results as well as 2D and 3D simulations are shown to analyze the phenomenology of this improvement and first explanations are proposed.