Nonlinear evolution of stimulated Raman scattering near 1/4 nc driven by a broadband laser

The nonlinear evolution and energy transfer process of broadband laser-driven stimulated Raman scattering (SRS) near 1/4 critical density (nc) have been investigated by particle-in-cell simulations. We find that in this regime, where wave-wave interactions dominate the evolution of SRS, laser reflectivity increases at moderate bandwidth (Δω = 0.6%) and subsequently decreases as the bandwidth becomes larger. And at moderate bandwidth, the bursts of SRS lead to increased trapping of super-hot electrons (>50 keV) by electron plasma wave (EPW). We also find that the EPW undergoes a mode-coupling process: under the density modulations introduced by the ion-acoustic wave (IAW, k ∼ 2k0) generated via Langmuir decay instability (LDI), the energy of the SRS-generated EPW(k ∼ k0) is transferred to a coupled-EPW at k ∼ 3k0, which has a lower phase velocity. The coupled-EPWs trap a significant number of electrons in the 10-50 keV range, resulting in heating of the background plasma. Both moderate and large bandwidths are effective in suppressing the LDI and mode-coupling process, thereby reducing the trapping of electrons in the 10-50 keV range and limiting the increase in plasma temperature.