Stimulated Raman Scattering Space-time Evolution Characteristics Of One Dimensional Simulation Studies

Stimulated Raman scattering (SRS) is a three-wave parametric process in which an incident light decays into a scattered light and a plasma wave. It happens under the quarter critical density region of plasmas. In indirect drive inertial confinement fusion (ICF), the plasma density in the hohlraum for ignition is about one tenth critical density, the temperature is about several kev, and the laser intensity is about1015W/cm2(triple frequency). Then SRS is in the convective region and the scattering level satisfies a linear relationship with laser intensity, plasma scale and plasma wave damping rate. For plasmas with low temperature or intense laser (for example, caused by filamentation or crossed lasers), SRS enters the absolute region. More generally, the instability is in the transition region between absolute and convective regimes. However the spatiotemporal transition characters are still unclear, which makes it difficult to assess the impact of SRS in the hohlraum. Besides, when the plasma wave amplitude is large enough, nonlinear saturation effects come out. The changing of Landau damping and frequency mismatch would affect the occurrence and development of the SRS.In view of above questions, we developed a three-wave interaction (3WI) program and rewrote a one-dimensional particle-in-cell (PIC) code to study the spatiotemporal evolution characteristics and regularity of one dimensional back SRS in a broad range of laser plasma parameters as well as the particle trapping effects on SRS growth rate and scattering level.The main results obtained are as follows:1) The3WI equations are derived in detail; we get the spatial factor in different instability phases and the temporal growth rate in the first phase theoretically. The results are verified by3WI code and PIC code. We find the spatial factor must be taken into consideration in order to get the correct temporal growth rate.2) We use a simple set of fluid equations to describe the particle trapping effects under the quasi-linear approximation. We get the equations of the amplification of the plasma wave as well as the saturation of Landau damping and the nonlinear frequency shift, and succeed in coupling them to the3WI model. We find the Landau damping rate vanishes more slowly if the plasma wave amplitude grows.3) In the absolute regions, the PIC code is used to study the instability growth rate and scattering level under different plasma scales. We find the noise level in PIC might affect the spatial factor and thereby affect the temporal growth rate. In the convective regions, the relationship between scattering level and plasma scale is derived theoretically and the results are verified by3WI code.4) The differences between absolute and convective instabilities are studied and transition regimes between them are also checked.5) We use the3WI code to determine the frequency mismatch effects on the temporal growth rate. We find the introduction of frequency shift raises the threshold of SRS and reduces the growth rate. And for a same frequency shift, we find the instability threshold increases significantly when the spatial factor is considered.6) The Landau damping changing and frequency mismatch effects caused by particle trapping on SRS are also studied. We find the two nonlinear processes are competing with each other. In some conditions, the effects of vanishing of Landau damping on SRS are more important than the effects of frequency downshift of plasma wave. The maximum frequency shift predicted by3WI model is also checked by PIC.