A Two Step Plasma Conditioning Scheme for High Current Plasma Implosions

Abstract

We consider the effect of using a limited energy current pre‐pulse to first heat and evaporate a solid material in wire or cylindrical foil geometry, and second to allow the material to expand until a suitable characteristic scale dimension, like the skin depth, before the application of the main high current pulse for compression and heating. In this way, due to the large‐scale density distribution formed during plasma expansion in the preparation phase, the short‐wavelength instabilities may be suppressed. This idea is confirmed by 2‐D RMHD simulations of the implosion of plasma from exploding a 13.6μm diameter tungsten wire evaporated with a low‐energy pre‐pulse, free expansion of the wire plasma until a necessary radius and subsequent implosion by a main pulse. The higher resistance against short‐wavelength instabilities results in a better quality compression and a higher energy density plasma structure. For implosion of a thin foil liner, the idea of two step implosion is considered by comparison of 2‐D RMHD simulations of the implosion of a 1.5μm copper foil at 18MA current amplitude in two cases: with and without pre‐pulse. The fastest short‐wavelength RT modes associated with high aspect ratio are suppressed due to a finite density gradient at the main pulse. The criteria for the optimum pre‐pulse are examined to improve the radiation characteristics for given pulsed power conditions.