Dynamics of a non-uniform hollow gas puff Z-pinch

The paper presents results from a Hollow Gas Z-Pinch (HGP) with non-uniform linear density distribution together with a numerical simulation of the discharge. The calculation is performed from immediately after the formulation of the discharge until the minimum radius ('pinch') is reached. It's used a two-dimensional (2D), time dependent, two fluid (ions and electrons), two-temperature model that includes, besides the magneto-hydrodynamics, diffusive processes such as thermal diffusion, magnetic diffusion and ion viscosity. The paper tries to discriminate upon the influence of different processes upon the formation of 'mushroom'-type structures, experimentally observed using optical measurements in the Z-pinch at the University of Mar del Plata (NOVA). Due to the fact that this Z-pinch works as a 'gas puff', that is with gas injection from the cathode, the initial distribution of the gas is not uniform in space. Besides the non-uniform radial distribution, there is a density gradient along the axis of the device. Using the 2D code it has been studied the influence of the initial distribution on the dynamics of the collapse. The main observation is that in most cases the initial linear density distribution determines the dynamics. Another important parameter is the variation of the initial thickness as a function of the axial coordinate. Other effects (for example, the cooling of the plasma in contact with the metal electrodes or the diffusion of the magnetic field) have little or negligible influence on the converging dynamics