| Laser plasma physics is a new branch subject established with the development of laser pulse technology.The continuous improvement of laser intensity makes the laser plasma interaction from the initial linear response to nonlinear optics,and finally to relativistic nonlinear interaction.These rich and interesting nonlinear phenomena make laser plasma interaction has important application in fusion scheme,particle acceleration,soft X-ray and so on.Among them,excited plasma wave,relativistic self-focusing and laser plasma wakefield and soliton generated by laser plasma interaction are the important nonlinear phenomena that people pay grate attention.These abundant physical phenomena and broad application prospect have aroused great interest of scientists.However,the nonlinear interaction between relativistic laser pulses and hot plasma is rarely studied.Based on the relativistic laser and hot plasma interaction,this paper mainly studies the nonlinear frequency shift of electron sound wave and the related characteristics of the physical mechanism of the transition from wakefield to soliton.The research results provide some theoretical guidance for the experiment.In the first part,we give the nonlinear frequency shift equation of electron acoustic wave based on the magnetic fluid model of relativistic hot plasma,and analyze the influence of plasma density and electron temperature on the nonlinear frequency shift of electronic sound waves.The results show that the plasma density,first harmonic amplitude and electron temperature are the main factors that determine the nonlinear frequency shift in relativistic hot plasma.For long wave excitation,the nonlinear frequency shift increases with the increase of electron temperature,and the plasma density inhibits the nonlinear frequency shift.For short wave excitation,the nonlinear frequency shift decreases with increasing electron temperature,and the plasma density promotes the nonlinear frequency shift.The results provide a theoretical basis for understanding the laser plasma interaction and the generation and application of harmonics.In the second part of our work,we investigate the physical mechanism of the wakefield to soliton transition in the laser-plasma interaction.Propagation of an electromagnetic(EM)pulse in an underdense plasma can either generate a wakefield or excite soliton wave,which depends on the competition between the linear dispersion and nonlinear self-modulation of the wave.Here,we study the interaction of the EM pulse and relativistic hot plasma analytically and numerically and reveal the physical mechanism of the transition from wakefield generation to soliton excitation in terms of soliton stability and modulation instability(MI)of a plane wave.Starting from the relativistic hot fluid-Maxwell model,a nonlinear Schr¨odinger equation(NLSE)governing the amplitude of scalar potential is obtained by using a multiscale perturbation technique.The bright and dark soliton solutions of the NLSE are obtained analytically.The stability phase diagram of solitons is given numerically.Furthermore,the MI of the plane wave is studied,and the stability phase diagram of MI is obtained.The results indicate that,when the plasma density increases,the propagation of the EM pulse in the plasma experiences wakefield–soliton transition,which depends on the thermal effect.Our results provide theoretical evidence for deep understanding of high-power laser plasma interaction. |
