中空高斯激光束在抛物预等离子体通道中的聚焦

Focusing of the hollow Gaussian laser beams in a parabolic pre-plasma channel

在考虑相对论、有质动力和预等离子体通道等非线性效应的基础上,研究了抛物预等离子体通道对中空高斯激光束(Hollow Gaussian beams,HGB)传播特性的影响.首先运用WKB方法和傍轴近似,得到了HGB在抛物预等离子体通道中传播时激光焦斑尺寸参量的演化方程.针对激光焦斑尺寸参量的演化方程,一方面用极值理论得到了HGB以周期性聚焦振荡、周期性散焦振荡和恒定焦斑尺寸传播时的初始激光等离子体参数;另一方面,根据具体参数平面,通过四阶Runge-Kutta法数值求解激光焦斑尺寸参量的演化方程,得到了HGB在预等离子体通道中传播的具体演化图像,并研究了等离子体通道参数和HGB阶数对激光自聚焦和传播特性的影响.研究结果表明,当HGB阶数n一定时,HGB的聚焦能力随着通道参数N c的增加而不断增强,抛物预等离子体通道不仅可以增强普通高斯激光的聚焦能力,也可以有效增强HGB的聚焦能力;当通道参数N c一定时,随着HGB阶数n的增加,初始辐照强度小的HGB更容易聚焦,而初始辐照强度大的HGB更容易散焦.

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(HGB)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 constant 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 parameter.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 c is 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.