Effects of uneven mass distribution on plasma dynamics in cylindrical wire array Z-pinches

The formation and dynamics of cylindrical wire array Z-pinch plasmas using uneven mass distribution is studied through experiments and computer simulations. In this study, the initial mass distribution is modified using different wire sizes within the same array, in contrast to the most standard wire arrays. Since the ablation rate of a particular wire material is related to current driver characteristics but not to the wire size, the use of different wires within an array produces time differences for complete ablation of each size. This changes the plasma dynamics and precursor plasma formation as compared to standard arrays. The experiments have been carried out on the Llampudken pulsed power generator (~350kA in ~300ns) using a 1:6 mass ratio among different wires of a single array. Plasma dynamics are studied using time-resolved laser interferometry (532nm, 5ns FWHM) and XUV imaging (5ns exposure time) in both side-on and end-on directions respect to the array. Experimental results show the formation of a dense, precursor plasma column on the array axis at early times, which shifts its position toward the thicker wires at later times at velocities of the order of 104 m/s. Numerical simulations using the 3-D MHD code GORGON are able to reproduce the experimental observations. They show that the larger mass of thicker wires induces modifications in the global magnetic field topology, producing the shifting in the precursor position as observed in the experiments. Further details on the changes of ablation dynamics and precursor formation are presented and discussed.