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

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. [ABSTRACT FROM AUTHOR]