Realistic modeling of short-pulse high-intensity lasers in underdense plasmas using particle-in-cell simulations.

摘要

In this dissertation, a parallelized particle-in-cell computer model was developed to model short-pulse high-intensity laser-plasma experiments. Using this code, we modeled several recent experiments in full scale. The simulations were in many instances in quantitative agreement with the experiments and they elucidated the complex nonlinear phenomena associated with such laser-plasma interactions.; First, the anomalous absorption of the laser as it propagated through an underdense plasma is examined. We found that the transmission loss can be more than 50% within just 1mm of propagation in a {dollar}1{lcub}-{rcub}4times 10sp{lcub}19{rcub}cmsp{lcub}-3{rcub}{dollar} plasma for a 600fs l{dollar}mu m{dollar} laser. The simulations showed that the sources of this anomalous absorption is a complex interplay between Raman scattering (including backward (RBS), near-forward side (RFSS), and direct-forward (RFS)), relativistic self-focusing, filamentation, and hosing. For the parameters of an experiment at Lawrence Livermore National Laboratory, the simulation found the transmission loss to be 50% within 0.64mm in agreement with the experimental results.; Next, the generation of high-current ({dollar}{lcub}>{rcub}kA){dollar}, relativistic electron beams from the wavebreaking of plasma waves by a highly nonlinear interaction between Raman scattering, self-heating, and self-focusing of high-power ({dollar}{lcub}>{rcub}5TW{dollar}), short-pulse ({dollar}{lcub}<{rcub}1ps){dollar} lasers is examined. We found that the resulting beams have a continuous energy spread with a maximum energy exceeding simple dephasing estimates. These results were in agreement with experiments at Rutherford Appleton Laboratory. For a 5J laser, a total of {dollar}2times 10sp{lcub}11{rcub}{dollar} electrons are accelerated within a 1mm of interacting distance, but {dollar}2times 10sp8{dollar} electrons are at {dollar}50 pm 1MeV{dollar} with a normalized emittance of {dollar}13pi mm{lcub}cdot{rcub}mrad.{dollar} We find that 10% of the laser energy is converted into multi-MeV electrons.; Last, we examined in detail the physics of self-focusing and cavitation. We gave analytic solutions of cavitation in 2D for various conditions. By rerunning a PIC simulation without allowing particles to move in the laser's propagation direction, we unequivocally demonstrated that RFS suppresses self-focusing and cavitation. We also showed analytically and in simulation that laser heating is another factor which can suppress cavitation.