Large-angle stimulated Raman scattering induced by transverse density modulation

Large-angle stimulated Raman scattering (LA-SRS) in a longitudinally inhomogeneous plasma with a transverse density modulation is studied using a three-wave coupled model and numerical simulations. The simulations show that the scattering angle of SRS in a longitudinally inhomogeneous plasma can be significantly affected by transverse density modulation. Under transverse density modulation conditions, the laser focuses into underdense regions, owing to the transversely modulated refractive index. The angle of LA-SRS, neither a purely 90° angle side scattering nor purely backscattering, is almost consistent with the specific angle at which the density inhomogeneity vanishes. In modulated plasmas, the nonuniform distribution of laser intensity shifts the regions of scattering and gain compared with those in uniform plasmas, ultimately affecting the laser transmission. SRS is suppressed in weakly modulated regimes, whereas it is enhanced under strong modulation conditions, and a theoretical criterion distinguishing between strong and weak modulation is established.