Electron transport & ion acoustic dynamics in laser-produced plasmas
Date
2002-10
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Degree Level
Doctoral
Abstract
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 QNPIC 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 QNPIC 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 ID 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. Since this instability is of kinetic nature it cannot be reproduced by hydrodynamic simulations. Our preliminary results of the return current instability in electron temperature gradients using QNPIC indicate the feasibility of particle codes to treat this problem.
For the analytic description of ion dynamics we have developed kinetic closures of the ion fluid equations that allow a single representation of the dynamics over all regimes of collisionalities. We employ a Chapman-Enskog like procedure for the closure and use a 21-moment representation for the collisional terms which ensure that the classical collisional limit of the transport coefficients is recovered. Ion sound like waves, transverse to a nonlinear perturbation, are investigated in a cold ion fluid model. We demonstrated the applicability of a stability analysis from the study of solitons to an ion sheath and a double layer potential. In case of the double layer we find transverse ion oscillations with phase velocities approaching zero.
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Degree
Doctor of Philosophy (Ph.D.)
Department
Physics and Engineering Physics
Program
Physics and Engineering Physics