Modeling of gas dynamics for a laser-generated plasma: Propagation into low-pressure gases

The physical phenomena involved during three-dimensional axisymmetric laser -induced plasma expansion into background gas are numerically studied. For this purpose, a multispecies hydrodynamic model is developed which consider s the effects of mass and ambipolar diffusions, thermal conduction, viscosi ty, and nonequilibrium conditions for ionization. This model is applied to describe quantitatively the Si plasma plume expansion into Ar or He gases. It is shown that the mechanism of plasma expansion depends critically on bo th the pressure and mass of the background gas. The shock front expansion i s found to be strongly correlated with ion dynamics. A pronounced differenc e between heavy-particle and electron temperatures indicates a persistent l ack of equilibrium between the heavy particle and the electron in the plasm a plume expansion. The Si atoms of the rarefied plume are essentially drive n by the backward-moving background gas as a result of a mass diffusion pro cess. It is also noted that the diffusion processes are only important in t he last expansion stage, and are less significant in the first stage. There fore, it is shown that a computation which does not include diffusion effec ts (Euler equations) can adequately describe only the earliest stage of pla sma expansion into background gas. The ability of the Navier-Stokes hydrody namic multispecies model to predict the key role of the background gas type (Ar, He) and pressure is demonstrated.