Nonlinear evolution of stimulated Raman scattering near 1/4 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 () 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 () 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 () generated via Langmuir decay instability (LDI), the energy of the SRS-generated EPW () is transferred to a coupled-EPW at , 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.