Effects Of Landau Damping And Collision On Stimulated Raman Scattering With Different Distribution Functions

The interaction between laser and plasma is an important process in inertial confinement fusion(ICF)and the instabilities of this process seriously affects the ignition.Stimulated Raman scattering(SRS)is one of the common instabilities.It is a process in which the laser decays into two sub waves,namely scattered light wave and Langmuir wave,when the laser interacts with the plasma.This process will lead to a series of problems that are not conducive to ignition,such as loss of energy;destroying radiation symmetry;preheating fusion fuel,etc.Therefore,the mitigation of SRS instability has become a research hot spot of inertial confinement fusion.In this thesis,we explore the effects of Landau damping and collision on stimulated Raman scattering(SRS)with various distribution functions with the analytic formulas and Vlasov simulations.Second,the interaction between Laguerre-Gaussian(LG)light and plasma is discussed.This thesis includes the following three parts:The first part(Chapter two)introduces the basic theory and simulation methods.Langmuir wave and scattering wave are both products of the SRS process.It’s well known that the nature of Langmuir wave is plasma oscillation,so we start our theoretical derivation from plasma oscillation,and use the method of kinetics to find the dispersion relation,Landau damping of Langmuir wave and SRS growth rate with the Maxwellian distribution.The formula shows that Landau damping is mainly related to the product of wave number k and Debye length_D and increases with the increase of the product.The SRS growth rate negatively correlated with Landau damping.Since we cannot separate the Landau damping of the Langmuir wave from the SRS simulation,we simulate SRS process by electromagnetic Vlasov code to obtain SRS growth rate,and simulate corresponding Langmuir wave generated in SRS by electrostatic Vlasov code to get Landau damping.The second part(Chapter 3)there are three electron distribution functions in the evolution of stimulated Raman scattering:initial Maxwellian distribution,flattened distribution due to trapped-particle and Bi-Maxwellian distribution formed by late heating.The Landau damping formulas with different velocity distributions is deduced by kinetic theory and verified with Vlasov simulation.The amplitude of Landau damping with Maxwellian distribution is mainly related to6)_D and increases with the increase of the6)_D.As the derivative of the set distribution at the initial phase velocity is 0,the theoretical value of Landau damping with the flattened distribution is 0.However,it is difficult to have an analytical expression for the real platform distribution function.We preset the platform distribution so that the first derivative at the phase velocity is 0,but the simulated Landau damping value is not zero,and increases with the increase of the6)_D.The Landau damping increases with the6)_D increasing,then decreases and finally increases again in Bi-Maxwellian distribution.Second,the electromagnetic Vlasov procedure is used to simulate the stimulated Raman scattering process with different distribution functions,and the electrostatic Vlasov procedure is used to simulate the Langmuir wave with the corresponding parameters,to explore the relationship between Landau damping and SRS growth rate.The simulation results show that Landau damping is inversely correlated with the SRS growth rate.The SRS growth rate of Maxwell distribution decreases with the increase of kλ_D which is consistent with previous studies.The smaller the width of platform or the larger kλ_D with flattened distribution,the smaller the SRS growth rate.The SRS growth rate decreases with the higher proportion of hot electrons in Bi-Maxwell distribution.The SRS growth rate can be well mitigated when the temperature ratio of hot to cold electron is about 7.Finally,we also study the impact of collision damping on SRS growth rate,and find that under our parameters,collision damping has a small modified effect on our study.The above research gives us a deeper understanding of SRS and may mitigate SRS by controlling the distribution function in a high Landau damping state.The third part(Chapter 4)explores the interaction between Laguerre-Gaussian light and plasma through PIC simulation,paving the way for the subsequent exploration of SRS using LG light as the pump light.The results show that the interaction between LG light and plasma will affect the mode of LG light,the mode of electric field of reflected light remains unchanged,and the mode of electric field of transmitted light changes,which shows that high-order Laguerre-Gaussian light can be generated by plasma reflection,but the disadvantage of this method is that the energy conversion efficiency is a little low.When the oblique incidence is carried out,the last reflection point of LG light is inconsistent with the initial incident point,the reflection angle is equal to the incident angle.These researches have made a deeper understanding of the interaction between Laguerre-Gaussian laser and plasma and laid a certain foundation for further research.The main innovations of this thesis are as follows:1.Through the combination of kinetics theory and simulation,the relationship between Landau damping and SRS growth rate under different electron velocity distributions during the evolution of SRS is studied,and the relationship between Landau damping and SRS growth rate is inversely correlated.2.The influences of width of platform,hot-electron fraction and hot-to-cold electron temperature ratio on SRS in the electron distribution function provide us with ideas to suppress SRS.3.Laguerre-Gaussian light is used as the pump light of laser-plasma interaction,the characteristics are discussed.