Theoretical And Simulative Study On Stimulated Raman Scattering And Production Of Super Thermal Electrons In A Density Modulated Plasma

As an laser-plasma.instability,Stimulated Raman Scattering(SRS)plays an extremely important role in Inertial Confinement Fusion(ICF).On the one hand,the electron plasma wave(EPW)and scattered light,which are the two child wave of SRS,will take energy from the pump and decrease the coupling efficien-cy of energy transferred to the pellet.On the other hand,a.high phase velocity EPW excited by SRS will produce super thermal electrons through wave-particle interaction,leading to a.result that the pellet will be preheated and has a lower compressibility.SRS usually takes place under 1/4 critical density of plasma and has a low threshold,existing in several ways of ICF ignition.The evolution is influenced by plasma.parameters(density,temperature and scale length,etc.)and laser parameters(intensity,frequency,etc.)with different levels,and plasma density is absolutely one of the most important factors.In the actual inertial pro-cess,there exists different forms of space density distribution,which will result in separate SRS performances and characteristics.This thesis aims at the evo-lution of SRS and production of super thermal electrons in a density-modulated plasma.,including the following three parts:In Part I(chapter ?),we introduce a basic theory of SRS linear gain expo-nent,including a linear modification of density modulation.Through adding a.space modulation term in Vlasov equation,we get harmonic solution in Fouri-er space.These harmonics will take energy from the fundamental mode,which equally increases the EPW damping rate.The values of harmonic intensity,dis-tribution and max order are all related to modulation parameters.We mainly study two parameters,i.e.,the modulation amplitude ? and wave number ks.The rough trend of damping rate is a positive correlation to ? and a negative correlation to ks.At the same time,the influence of modulation is obvious in small k?D parameter regime.In Part II(chapter ?),we study the influence of density modulation to the growth and saturation of SRS linear and nonlinear evolution separately in convective and absolute regime through Vlasov simulations.In convective con-dition,we make some comparison of SRS backward gain exponents in various modulation conditions to those of theoretical results and get a consistent result.In absolute SRS,we pick up a long wavelength condition ks = 0.1kl with more obvious harmonic effects,discovering a periodical structure of EPW and a burst of SRS reflectivity.That means in nonlinear stage,a proper density modula-tion will enhance the SRS process periodically and affect nonlinear wave-particle interaction.Part ?(chapter ?)is a.continued work of part ?,which studies how density modulation affects electron acceleration.Here the modulation wave number ks is a crucial parameter.In the case of short wavelength modulation ks = 0.1kl,the number of harmonics is limited and the gap in spectra space is large,so the result of modulation is layered trapping structure in phase space.If the intensity of harmonics reaches a certain level and satisfies resonant overlapping condition,it it possible for the two adjacent harmonics to merge.On the contrary in the case of ks = 0.1kl,the harmonics are close and has a large number.Before the electrons are fully trapped by the original mode,they are also probable to be accelerated by another harmonic which has a higher phase velocity.The mechanism of cascade acceleration is able to produce a large number of super thermal electrons in a wide velocity range.