Nonisothermal asymmetric expansion of laser induced plasmas into vacuum

Abstract

A theoretical model is developed that describes an asymmetric expansion of radiative, thermally anisotropic, laser induced plasmas into vacuum. The model is based on the conventional fluid dynamic equations supplemented by the equations of state and radiative transfer. The plasma is assumed to be nonviscous and nonconducting. It is proven that the self-similarity expansion law, when there is a rectangular coordinate system in which the velocity vector field components of plasma species are proportional to the corresponding coordinates, cannot hold true for generic thermally anisotropic plasmas with radiative losses. When the effects of radiative losses on the plasma expansion are neglected, a simple analytical model is obtained for fast numerical simulations of plasma dynamics. The model is shown to have analytical solutions for axially symmetric plasma shapes which explain the flip-over effect observed experimentally. Numerical calculations are performed for the silicon plasma adiabatically expanding into vacuum. The radiative properties of such plasma are investigated. It is shown that spectral line intensities and Stark shifts depend strongly on the direction in which the emission spectrum is observed. This result is relevant for improving spectroscopic methods of plasma diagnostics.