Effect of density modulation on backward stimulated Raman Scattering in a laser-irradiated plasma

The influence of an arbitrary static sinusoidal density modulation on Raman backscattering is discussed theoretically and numerically. One-dimensional Vlasov simulations show a suppression of convective gain of seed with large modulation amplitude ϵ or small modulation vector k, which is consistent with the modulation theory, namely, the generation of harmonics. However, this initial suppression is soon replaced by a periodic enhancement of reflectivity in the nonlinear stage due to the different performance of Langmuir wave propagation in density peaks and valleys. Modulation effects are also instantiated in electron acceleration. With large k, finite orders of harmonics may lead to obvious layers of the phase island structure, and local phase island mixing occurs when the wave amplitude increases. With small k, electrons could be accelerated by a series of adjacent harmonics from low phase velocity to high phase velocity step by step, which may produce a large number of superthermal electrons even with relativistic kinetic energy.