Three-dimensional modeling of a plasma in a strong azimuthal magnetic field.

Abstract

Three-dimensional magnetohydrodynamic simulations are able to model the generation of disk-shaped plasma, driven by laser ablation from a current-carrying rod in a pulsed-power machine producing azimuthal magnetic fields of 2-3 MG. The plasma at such extreme conditions is unique in that the parameter space for the plasma β and Hall parameter χ transition from below unity to greater than unity at different stages of the plasma generation. In simulations, the formation of the plasma disk in the azimuthal direction is driven by heat flux from the laser spot and depends on the set of transport coefficients used in simulations. The most recent set of transport coefficients leads to the formation of plasma ejecta at the back end of the rod, which qualitatively matches experiments. Specifically, the cross-gradient Nernst effect, which twists the magnetic field, is shown to have a large effect on the shape of the back-end ejecta. In the direction along the axis of the rod, there is propagation of perturbations from the disk as observed in experiments. In simulations, the period of temperature perturbations is in good agreement with experimental results. An instability due to coupling of heat flux and the magnetic field advection provides a possible explanation for perturbation growth along the axis of the rod, and the instability growth rate is consistent with experimental results.