Two- and three-dimensional modeling of the different phases of wire-array z-pinch evolution

Abstract

A series of specialized multidimensional resistive magnetohydrodynamic (MHD) models have been developed to tackle the different phases of evolution of wire array z-pinch implosions. Two-dimensional (r–z) “cold-start” or “wire initiation” simulations of single wires indicate the persistence of a two-component structure with a cold, dense core embedded within a much hotter, low density, m = 0 unstable corona. Cold-start simulations with similar conditions to wires in an array show a general trend in the plasma structure from discrete wires with large m = 0 perturbation amplitudes to partially merged wires with smaller perturbation amplitudes as the number of wires is increased. Two-dimensional (r–θ) simulations then show how the persistence of dense wire cores results in the injection of material between the wires into the interior of the array, generating radial plasma streams which form a precursor plasma upon reaching the axis. Higher-resolution 2-D (r–θ) simulations show similar behavior for large number wire arrays in use at Sandia National Laboratories. This model is also used to predict which modes of implosion are in operation in nested wire array experiments. Separate r–θ plane simulations of the flux of plasma imploding towards the axis from the outer array and the bombardment of the inner array by this flux are presented. Finally, 2-D (r–z) simulations of the Rayleigh–Taylor instability during the final implosion phase are used to illustrate the effect upon the power and duration of the radiation output pulse. The results of low-resolution 3-D resistive MHD simulations are also presented. The need for much higher resolution 3-D simulations of certain aspects of wire array evolution is highlighted.