DYNAMICS OF A NONUNIFORM HOLLOW GAS PUFF Z-PINCH

We present results from a Hollow Gas Z-Pinch (HGP) with nonuniform lin ear density distribution together with a numerical simulation of the d ischarge. The calculation is performed from immediately after the form ation of the discharge until the minimum radius (''pinch'') is reached . We use a two-dimensional (2D), time dependent, two fluid (ions and e lectrons), two-temperature model that includes, besides the magneto-hy drodynamics, diffusive processes such as thermal diffusion, magnetic d iffusion and ion viscosity. We try to discriminate upon the influence of different processes upon the formation of ''mushroom''-type structu res, 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 c athode, the initial distribution of the gas is not uniform in space. B esides the non-uniform ra.dial distribution, there is a density gradie nt along the axis of the device. Using the 2D code we have studied the influence of the initial distribution on the dynamics of the collapse . The main observation is that in most cases the initial linear densit y distribution determines the dynamics. Another important parameter is the variation of the initial thickness as a function of the axial coo rdinate. Other effects (for example, the cooling of the plasma in cont act with the metal electrodes or the diffusion of the magnetic field) have little or negligible influence on the converging dynamics.