Competition between stimulated Raman scattering and two-plasmon decay in inhomogeneous plasma

Abstract

We demonstrate competitions between stimulated Raman scattering (SRS) and two-plasmon decay (TPD) in the laser polarization plane in inhomogeneous near quarter-critical density plasma by using linear convective gain analysis and two-dimensional (2D) particle-in-cell (PIC) simulations. Linear theoretical analysis implies that convective SRS occurs in a wider and lower density region than absolute SRS and has a shared occurrence region with convective TPD. This convective SRS prefers a parameter space with the laser intensity larger than the order of 1015 W/cm2 and the density scale length about several hundreds microns, which may be common in large scale direct-drive scheme, shock ignition scheme, and hybrid-drive scheme. A convective nature and saturation mechanism under these parameter regions are identified to be Langmuir decay instability and strong pump depletion. The significance of this convective SRS is shown in our 2D PIC simulations that hot electrons are reduced through suppressing the electron staged acceleration by TPD in the lower density region due to its high phase velocity. Temperature induced competitions are also studied using a relativistic modification to the Langmuir wave dispersion relation when Te>5 keV. Both absolute and convective SRS are observed to be dominant in the simulations when the temperature is as high as 10 keV.