Rescatters of stimulated Raman side scattering in nonuniform plasmas

Rescatters of stimulated Raman side scattering (SRSS) in nonuniform plasmas have been studied via two-dimensional (2D) particle-in-cell (PIC) simulations. Theoretical analysis predicts that rescatter of SRSS, denoted as the 2nd order SRSS, occurs below one-ninth critical density, once the amplitude of the 1st order side-scattered light exceeds the threshold. If the pump amplitude is strong enough, it can trigger higher order SRSS, located at lower density regions. We have confirmed the phenomenon via 2D PIC simulations. For moderate laser intensity, I=10¹⁵W/cm², a robust 2nd order SRSS grows exponentially as SRSS excited, and then saturates at a level one-tenth of the 1st order SRSS. The rescatters contribute to the transverse superhot electrons with respect to the density gradient due to the transversely-propagating Langmuir wave of the 2nd order SRSS. At higher laser intensity, I=10¹⁶W/cm² , higher order SRSS has been observed; while I=10¹⁴W/cm², there is only SRSS but no rescatters. It shows that rescatters of SRSS are a common nonlinear phenomenon of SRSS and it may contribute to the saturation of SRSS and superhot electrons.