Kinetic Simulation Of Stimulated Raman Scattering In Inertial Confinement Fusion

Parametric instability in laser plasma interaction(LPI)is an important issue to the ignition of inertial confinement fusion(ICF).Stimulated Raman scattering(SRS)and Stimulated Brillouin scattering(SBS)can reduce the coupling efficiency of incident laser and destroy the uniformity of the radiation field.The former accelerates electrons to extremely high energy resulting in the fusion target gain reduction.In the past few decades,people have made remarkable achievements in the research of LPI.However,the failure on the American national ignition facility means that there are still insufficient understating of LPI and lack of effective control methods.So,it is necessary and urgent to further study the LPI,especially the mechanism and suppression means of SRS.Since the characteristic time of ICF ignition is on the order of nanoseconds,the previous simulation of LPI mostly used the radiation fluid code,which could show the saturation process of instability over a long time.Nevertheless,this method ignores the reverse effect of particles kinetic behavior on the evolution process so that it is not sufficient to accurately describe the physical process of LPI.With the aim to solving the above problems,we propose three SRS suppression methods and examine the physical evolution by fluid theory and Particle-in-cell(PIC)simulations.We also conduct theoretical and simulation studies on the experimental results on Shenguang ?.The main structure and results of this dissertation are as follows:Firstly,we investigate the evolution process of SRS driven by finite-bandwidth laser.The non-resonant coupling between the incident laser and plasma wave dominates in the linear stage.It suppresses the development of SRS and changes the evolution characteristics of plasma wave and scattered light.The relaxation time is defined by electron spectrum distribution function and the nonlinear frequency shift of plasma wave.It divides the evolution of SRS driven by broadband laser into two stages,i.e.suppression stage and enhancement stage.After the relaxation time,the broadband laser will enhance the SRS,leading to the higher energy plasma wave,wider nonlinear frequency shift and more superthermal electrons.The above effects are valid in the range of(0.0225₀,0.03₀).Secondly,we investigate the evolution process of SRS driven by rotated polarized laser.The rotating linearly polarized laser has a significant suppression on linear stage of SRS with a suppression factor of 2/?.In the nonlinear stage,with the faster of rotating,reflectivity behaves unlike linear stage for the different saturation threshold under each rotating frequency.It generally remains stable in the low frequency region and then decreases until it becomes stable again.The rotating polarization effect destroys the periodic growth structure of the plasma wave,changes the gain mode,reduces the instantaneous reflectivity and slows down the generation rate of the superthermal electrons.This laser still has suppression effect on the nonlinear SRS.Both theoretical and simulation results show that,with the increasing of rotating frequency,the stimulated Raman backscattering(SRBS)is effectively alleviated while the stimulated Raman forward scattering(SRFS)is enhanced.Thirdly,we investigate the evolution process of SRS driven by linearly polarized laser in weak longitudinal magnetized plasma.The induced Faraday rotating effect rotates the direction of laser polarization so as to suppress the linear stage of SRS.In the near cut-off density plasma,the magnetic field changes the refractive index of the left and right vortex light.It makes the four-wave coexisting in the plasma wave spectrum when the magnetic field is above 200 T.The beat patterns between each mode causes the envelope of plasma wave to oscillate at a high frequency and appear a periodically absolute gain characteristic in the space.The suppression effect of longitudinal magnetic field on nonlinear SRS disappears,while both reflectivity and plasma wave maintain stable below the saturation value.Fourthly,we carry out the numerical simulation study on the experimental results via Shenguang ? facility.In the simulated parameter space,the non-uniform plasma density causes the plasma wave appears an arched distribution in the spectrum.SRS evolves into deep nonlinear stage,stimulating wide nonlinear frequency shift.At the same time,SBS is also strongly stimulated.With the increasing of laser intensity,leads to the SBS reflectivity firstly decreases and then rises owing to the competitive relationship between SBS and SRS.This is similar to the experimental results.