Relativistic Laser Self-Channeling in Underdense Plasmas: A Simulation of Key Experimental Parameters.

摘要

Relativistic and ponderomotive self-channeling of intense ultrashort laser pulses in underdense plasmas has been studied under more realistic experimental conditions in this thesis in order to optimize the controlled power compression and stability of the channel -- elements that are critical to the applications associated with these channels, including the generation of coherent x-rays. In experiments for coherent x-ray generation, the electron plasma column is created by the front temporal region of the laser pulse through ionization. A number of inter-connected experimental parameters will determine the quality of the transition from the incident laser spatial profile to that of a channel eigenmode. Proper control of these parameters enables the incident transverse laser radiation profile be matched adiabatically with minimal coupling losses to the spatial character of the desired fundamental channel eigenmode which provides a stable and robust zone for power compression. In order to study the effect of these experimental parameters on the formation and stability of laser self-channeling in underdense plasmas, the relativistic model under simplified conditions is modified with the inclusion of gas jet density and ionization conditions, laser mode structure and focusing conditions, and laser wavelength. The model results are in good general agreement with the experimental observations for the self-channeling of TW-level 248 nm laser pulses in Xenon and Krypton gas jets employed for the generation of coherent amplified keV x-ray pulses and capture the salient features of the relativistic self-channeling dynamics. The results outline the laser-target conditions that must be met to initiate the efficient self-channeling of laser. As the generation of a straight channel is required for x-ray amplification, the results in this thesis underline the importance of a high laser beam quality. The simulations also show that the combination of laser wavelength and target gas species is important, and the combination of 248nm laser radiation and a Xe gas target is one of the best choices for relativistic self-channeling. The relativistic laser self-channeling simulations presented in this thesis have examined the root causes of experimental observations more accurately than before and have computed results over a wider range of conditions than have been performed in experiments.