The Researches On Laser Plasma Instability And Its Saturation Mechanism In Inertial Confinement Fusion

With the development of inertial confinement fusion(ICF),the human energy crisis is more and more promising to be solved.However,the instability of laser and plasma is a big obstacle in the fusion process.The development of instability will consume the energy of the laser and generate superheat electrons that affect the compression of the target pellet.In inertial confinement fusion,the two most common laser plasma instabilities are stimulated Raman scattering(SRS)and stimulated Brillouin scattering(SBS).When the incident laser light decays into a Langmuir wave and a beam of scattered light,this instability is called stimulated Raman scattering.Langmuir waves accelerate electrons near their phase velocity,producing epithermal electrons that will preheat the pellet,making compression more difficult.When another type of instability,SBS,occurs,the incident laser light decays into ionic acoustic waves and scattered light.Because the frequency of ion acoustic waves is low,scattered light contains most of the energy of incident photons,and SBS is one of the main sources of reflected light in ICF.The study of instability in inertial confinement fusion and the search for the saturation mechanism of the instability are important research topics of ICF.In addition,laser plasma instability has great practical value in laser pulse amplification.Through three-wave coupling,the pump laser transmits energy to the seed laser through the plasma wave,so that the light intensity of the seed laser increases rapidly and the pulse width decreases rapidly.The ultra-intense laser generated by this method has extremely high application value for the study of relativistic optics and fundamental physics.In this paper,a series of procedures are developed for the specific problem of laser plasma instability in ICF,such as three-wave coupling program,one-dimensional and two-dimensional Vlasov program,etc.The relationship of plasma wave dispersion and the growth of SRS and SBS under any mass ratio and any charged state are studied;the influence of density perturbation in the linear and nonlinear regions on the inhomogeneous plasma SRS in the direct drive mechanism is studied;the developed 2D Vlasov simulation program systematically investigates the sideband instability of Langmuir waves and identifies sideband instability as a saturation mechanism for SRS.Specifically,it includes the following research contents:(1)Plasma waves in comparable-mass plasmas have always been a hot research topic,and their research ideas are very similar to those in inertial confinement fusion.Through the study of plasma waves with similar quality plasma,we can clarify the coupling relationship of each physical quantity in the plasma wave dispersion relationship.We show,through a simplified theoretical model and particle simulations,that SBS and SRS are enhanced in plasmas with similar electron-ion masses for different charge states.Compared with previous studies of electron-positron plasmas,we propose a more general plasma wave dispersion relation with arbitrary mass ratios(β=m_e/m_i)and ion charge states(Z=Z_i,Z_e=1),where Z represents the charge of the ion.Furthermore,we found that the ionic bulk mode(IBk)is responsible for the kinetic Brillouin amplification process.Higher intensity seed pulse amplification is observed inβZ=1 type plasma andβ=1 type plasma by stimulated Brillouin scattering;the former is due to kinetic effects,while the latter is due to unstable growth higher rate.Our results have potential applications for pulse amplification of plasmas with charge state Z>1 and electron-ion mass close.(2)Based on the envelope coupling equations of SRS,we compiled the three-wave coupling program of SRS.and verified with the theoretical results.Using this three-wave coupling procedure,full-motion physics Vlasov simulation and particle simulation(PIC)we investigate the effect of sinusoidal density modulation on SRS reflectivity in inhomogeneous plasmas.Through the three-wave procedure,we find that sinusoidal density modulation is capable of inducing absolute SRS even if the Rosenbluth gain is less thanπ,and we numerically obtain a modulation wavelength and amplitude region that satisfies the induction of absolute SRS,the boundary of this region is consistent with previous theoretical findings Consistent.The mean reflectance obtained from the Vlasov simulations has the same trend as the absolute SRS growth rate obtained from the three-wave equation.A modulation wavelength shorter than the fundamental gain length can suppress the inflation of the SRS by harmonics,rather than causing absolute instability.Furthermore,the PIC simulations agree qualitatively with our Vlasov simulations.Our results provide another possible explanation for the high reflectivity of the low-density plasma in the ICF experiments,which may be caused by the presence of long-wave modulation in the laboratory plasma.The mechanism of suppressing SRS using short-wave modulation has potential applications in inertial confinement fusion.(3)Electrostatic Vlasov simulation is a common method to study the instability of plasma waves.Because of the huge amount of computation,people are often limited to one-dimensional Vlasov simulations,and the high-dimensional effects of plasma waves cannot be described by one-dimensional simulations.We therefore developed a two-dimensional electrostatic Vlasov simulation program.This program solves the two-dimensional Vlasov equation and the Possion equation,and we use a parallel algorithm to improve the calculation speed.It is basically verified and applied to the study of high-dimensional plasma wave instability.Using the two-dimensional electrostatic Vlasov program,simulations perfrom the evolution of Langmuir waves over a long period of time.In multi-wavelength systems,trapped electrons can generate sidebands including longitudinal,transverse and oblique sideband.We found that the oblique sideband is an important attenuation channel of the Langmuir wave,and the growth rate of the oblique sideband is smaller than that of the longitudinal sideband,but higher than that of the transverse sideband.When the amplitude of the sidebands is comparable to that of the Langmuir wave,as the longitudinal and transverse wave numbers expand,vortices merge and occur,and the system eventually develops into a turbulent state.This work uses the nonlinear Schr?dinger equation model[Dewar–Rose–Yin(DRY)model]theory with Landau damping of a non-Maxwell distribution to give the growth rate of the high-dimensional sidebands.The theoretical results show that the nonlinear frequency shift is of great significance in the evolution of Langmuir waves,and the theoretically obtained sideband growth rates are qualitatively consistent with the Vlasov simulation results.