A theoretical analysis is presented for the stimulated Raman scattering (SRS) driven by multiple beams with certain frequency differences or bandwidths. The concomitant electrostatic modes are excited by the scattering lights when the pump beams share a common electron plasma wave to drive SRS. This coupling can be suppressed when the frequency difference of the two beams is three times larger than the SRS growth rate of a single beam, where the beams develop their scattering lights independently. To mitigate SRS further under the multibeam irradiation, polychromatic pumps are introduced to reduce the SRS saturation level, where the weakly coupled common modes are heavily damped in hot plasmas. The sidescattering driven by polychromatic light in inhomogeneous plasmas is significantly reduced due to the wavevector mismatch of daughter waves. Particle-in-cell simulations confirm our theoretical analysis and demonstrate that the strength of multibeam SRS and the hot electron production in inhomogeneous plasmas can be controlled at a low level by polychromatic light with a few percentage bandwidth ( ≳ 3 % ), even in the non-linear regime.

A physical model is presented for the study of parametric instabilities in inertial confinement fusion (ICF), which considers the coupling of the incident and scattered electromagnetic waves with plasma electrons and ions. Specially, this model is solved numerically with the particle-mesh method, where the plasma is represented by macro-particles both for electrons and ions, and the velocity and position of each macro-particle are numerically updated by using the particle-mesh method. The developed particle-mesh code in one-dimensional geometry (PM1D) is utilized to study the development of parametric instabilities at the nonlinear stages, where electron plasma wave breaking, particle trapping, hot electron generation and density cavity formation can occur. Particle-in-cell (PIC) simulations are carried out to verify this PM1D code. By comparison, it is found that this PM1D code is able to capture the kinetic effects and precisely describe the developments of parametric instabilities at nonlinear stages as the PIC simulations while saving the computation time obviously. Furthermore, a test simulation of the stimulated Raman scattering evolution up to 200 ps verifies the robustness of this PM1D code.

Electromagnetic emission via linear mode conversion from electron plasma waves (EPWs) excited by stimulated Raman scattering (SRS) of an incident laser pulse in inhomogeneous plasma is investigated theoretically and numerically. It is found that the mode conversion can occur naturally in underdense plasma region below the quarter critical density provided that EPWs are generated due to the development of backward SRS when the laser pulse is incident at certain angle with the plasma density gradient. The produced radiation may cover a broad frequency range up to half of the incident laser frequency. The dependence of the radiation conversion efficiency on the laser intensity, incident angle, laser pulse duration, plasma density scale length, and initial electron temperature is analyzed based on one-dimensional particle-in-cell simulation. In two-dimensional geometry, due to the development of sideward SRS, it is found that the mode conversion to occur even at normal incidence of the laser pulse. The radiation frequency, bandwidth, duration, and amplitude can be well controlled by the laser and plasma parameters, suggesting that it may provide a new source of tunable broadband radiation as well as a diagnosis of the development of SRS.