Effects Of Pre-channel And Plasma Temperature On Propagation Characteristics Of A Dark Hollow Gaussian Laser Beam

The guidance and propagation of a strong laser in plasmas have important scientific significance and application prospect in the fields of desktop accelerator,high harmonic generation,X-ray source,THz radiation and laser fusion.At present,the research mainly focuses on the ordinary gaussian laser beam.This dissertation takes hollow Gaussian beams?HGB?,a typical non-gaussian laser beam,as the research object to study the influence of pre-plasma channel and plasmas temperature on the propagation of hollow gaussian laser beam.The main work includes:1.By considering the nonlinear effects of relativity,ponderomotive force and pre-plasma channel,the influence of the channel on the propagation characteristics of the hollow Gaussian beams is studied.Firstly,the evolution equation of the focal spot size is obtained by using WKB method and paraxial approximation.Based on the evolution equation of the focal spot size,the critical curve and the initial laser plasma parameters of the HGB for periodic focusing oscillation,periodic defocusing oscillation and con-stant focal spot size propagation are given by the extreme value theory.Furthermore,according to the specific parameter space,the evolution equation of the focal spot size is solved numerically by the fourth-order Runge-Kutta method,which gives the detailed propagation images of the HGB in the pre-plasma channel.The results show that when n is constant,the focusing ability of the HGB increases with the increasing channel pa-rameter N_c.The parabolic pre-plasma channel can not only enhance the focusing ability of ordinary Gaussian lasers,but also effectively enhance the focusing ability of the HGB.Moreover,when N_cis constant,as the increase of the order n of the HGB,self-focusing is more pronounced for low irradiation intensity HGBs.Conversely,defocusing is more pronounced for high irradiation intensity HGBs.2.The influence of plasmas temperature on the propagation characteristics of HGB in plasmas was studied by considering the nonlinear effects such as collision and Ohmic heating.Using the same theory and method as in the Chapter Two,the evolution equation of HGB focal spot size and the initial laser plasmas parameters of HGB propagating with constant focal spot size are obtained.The research results show that HGB focal spot size will go through periodic defocusing oscillation amplitude decreases,propagation with constant focal spot radius,periodic focusing oscillation amplitude increases first and then,propagation with constant focal spot radius,periodic defocusing oscillation amplitude increased gradually,direct divergent,with the increase of plasmas temperature.On the other hand,with the increase of HGB order n,the conclusion is similar to Chapter Two,when the plasmas temperature T₀is constant,focusing is easier to small initial irradiation intensity and defocusing is easier to large initial irradiation intensity.