Laser Channeling and Hosing in Millimeter-Scale Underdense Plasmas in Fast Ignition.

摘要

This thesis studies laser channeling under parameters relevant to fast ignition with two-dimensional (2D) and three-dimensional (3D) particle-in-cell (PIC) simulations. Laser channeling is found to be a highly nonlinear and dynamic process. Channeling in 2D and 3D simulations displays qualitatively similar physical features, including laser self-focusing/filamentation, laser hosing, and channel bending/bifurcation/self-correction. Residual electrons in the channel are heated to relativistic temperatures, which reduce the electron quiver momentum and cause the decoupling of the plasma and the laser. The eventual formation of a straight, low density channel allows the trailing ignition pulse to transmit with a > 80% transmittance. There are quantitative differences between 2D and 3D channeling simulations. Channel advancing is faster in 3D due to easier channel formation and a larger ponderomotive force from laser self-focusing. The intensity scalings of the channeling time and energy show that channeling in fast ignition is essentially a ponderomotive process and low intensity channeling pulses are preferred to reduce the required laser energy.;This thesis also studies dependences of the hosing instability on plasma temperatures and dispersion. Coupled laser envelope and plasma density perturbation equations for short and long pulses are derived from the relativistic fluid theory. Hosing equations are then derived from these coupled equations using a variational method. A parameter α, which is the sum of the normalized relativistic plasma pressure and internal energy, is introduced to represent the plasma thermal effects. In a relativistically hot plasma, α is much larger than 1, and suppression of the laser ponderomotive force and laser hosing is found. Analysis of the hosing equations finds that the fastest growing mode shifts to longer wavelengths as α increases. This solves a long-standing puzzle that the hosing modes observed in both experiments and simulations have much longer wavelengths than predicted by the hosing theory for a cold plasma. Dispersion is found to be unimportant for long-pulse hosing. For short-pulse hosing, dispersion is found to stabilize hosing at wavelengths longer than a critical value. PIC simulations on both long-pulse and short-pulse hosing have largely verified these analyses.