Convective amplification of stimulated Raman rescattering in a picosecond laser plasma interaction regime

Yu Ji,Chen Wang,Rui Yan,Chang-Wang Lian,Zhen-Hua Wan,Bin Zhao,Dong Yang,Chuang Ren,Jian Zheng,Zhi-Heng Fang

doi:10.1063/5.0026379

Abstract

We present particle-in-cell (PIC) simulations of laser plasma instabilities (LPIs) with a laser pulse duration of a few picoseconds. The simulation parameters are appropriate to the planar-target LPI experimental conditions on SG-II. In this regime, the plasmas are characterized by a long electron density scale length and a large electron density range. It is found that when the incident laser intensity is well above its backward stimulated Raman scattering (backward SRS, BSRS) threshold, the backscattered light via the primary BSRS is intense enough to excite secondary SRS (Re-SRS) in the region below one-ninth of the critical density of the incident laser. The daughter light wave via the secondary BSRS (Re-BSRS) is amplified as it propagates toward the higher-density region in the bath of broadband light generated through the primary BSRS process. A higher intensity of the incident laser not only increases the amplitude of the BSRS light but also increases the convective amplification lengths of the Re-BSRS modes by broadening the spectrum of the BSRS light. Convective amplification of Re-BSRS causes pump depletion of the primary BSRS light and may lead to an underestimate of the primary BSRS level in SP-LPI experiments. A significant fraction of the generation of energetic electrons is strongly correlated with the Re-BSRS modes and should be considered as a significant energy loss.

Highlights

Laser plasma instabilities (LPIs) such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and twoplasmon decay (TPD), are among the most critical factors hindering inertial confinement fusion (ICF) ignition
It is found that when the incident laser intensity is well above its backward stimulated Raman scattering threshold, the backscattered light via the primary backward SRS (BSRS) is intense enough to excite secondary SRS (Re-SRS) in the region below one-ninth of the critical density of the incident laser
Short laser pulses last for only ∼4 ps, the simulations are performed for ∼35 ps to allow all of the light signals to be eventually collected by the diagnostic at the left boundary so that the integrated fraction of the scattered light can be obtained; see Fig. 2

Summary

INTRODUCTION

Laser plasma instabilities (LPIs) such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and twoplasmon decay (TPD), are among the most critical factors hindering inertial confinement fusion (ICF) ignition. Hot electrons generated via LPI may benefit the formation of the shock rather than representing a fuel-preheating risk if they can be controlled at an appropriate energy level Such high-intensity ignition schemes are expected to involve rather complicated LPI processes, whose details can be comprehensively studied and understood with the help of SP-LPI simulations and experiments. When the incident laser is intense enough, the backward SRS (BSRS) daughter light wave can be intense enough to further excite a secondary SRS in its own resonant density region These so-called SRS rescattering modes alter the interpretation of experimental diagnostics such as scattering losses and generation of energetic electrons.

RESONANT REGION OF SRS RESCATTERING

SIMULATION CONFIGURATIONS

SIMULATION RESULTS AND DISCUSSION

SUMMARY AND CONCLUSIONS