Electron transport and ion acoustic dynamics in laser-produced plasmas.

摘要

In this thesis laser-plasma processes are studied at transport and ion time scales. In order to study these processes the particle-in-cell code QN-PIC with one spatial dimension and three dimensions in velocity space was developed. Collisional effects are included by a Monte Carlo procedure, and an electric field solver based on the quasineutrality condition has been implemented. This allows long time scale simulations without having to resolve the electron plasma frequency. Collisional heating of the electrons in the laser electric field is one of the major restrictions on the time step in particle-in-cell codes. We have developed a collisional heating procedure that is based on a Langevin equation. It utilizes a Fokker-Planck equation that describes heating time averaged over the laser frequency. This procedure, in conjunction with the fast field solver and procedures to represent collisions, allows simulation of long time scale in laser-plasma interactions without the need to resolve the short time scales to ensure numerical stability and suppress numerical artifacts. We have studied in detail homogeneously heated plasmas and the effects of electron-electron collisions and collisional heating on the electron distribution function. We have suggested a nonlocal, nonlinear heat transport model based on a earlier self-consistent nonlocal transport theory that is formally restricted to small (linearized) temperature perturbations. Our model extends this model to the case of finite temperature perturbations. The model is tested successfully in simulations of hot spot relaxation of an initial temperature distribution that corresponds to the instantaneous release of heat into a spatially Gaussian temperature profile and Maxwellian velocity distributions of the electrons. In simulations of collisionally heated hot spots we qualitatively describe the effects of non-Maxwellian velocity distributions on the heat flux and the change of the distribution function due to transport in nonheated regions. For the representation of ion dynamics in QN-PIC we have conducted studies of the two stream instability with counterstreaming electrons and ions. Predicted growth rates of ion sound waves are recovered and we find it possible to study anomalous heating and resistivity in 1D ion sound turbulence. In laser plasmas ion sound turbulence due to the return current instability is thought to be one of the factors in the reduction of heat flux from hot spots. (Abstract shortened by UMI.)