On the hot-electron generation produced by two-plasmon decay and stimulated Raman scattering in inhomogeneous plasmas On the hot-electron generation produced by two-plasmon decay and stimulated Raman scattering in inhomogeneous plasmas

Scalings of hot-electron fraction and temperature are obtained based on two-dimensional particle-in-cell simulations under parameters from direct-drive inertial confinement fusion to shock ignition. The scalings demonstrate a transition from two-plasmon decay (TPD) dominant regime to stimulated Raman scattering (SRS) dominant regime. It is observed that the transition occurs when convective SRS threshold is surpassed, which leads to a decrease in hot-electron fraction and temperature and increase in Raman reflectivity. We propose a new theoretical model for hot-electron generation by coherent Langmuir waves with varying phase velocity, and it is successful in explaining the simulation results in the regime of competing SRS and TPD. These results are coincident with recent experiments on direct-drive scheme and shock ignition, where an increasing importance of SRS is confirmed.