Increase of the Backward Raman Reflectivity Caused by the Langmuir Decay Instability in an Inhomogeneous Plasma: The Loss of Gradient Stabilization

We investigate the nonlinear evolution of the backward stimulated Raman scattering (BSRS) in the regime where the nonlinear saturation mechanism is the Langmuir decay instability resulting from the coupling of the BSRS-generated Langmuir wave with the ion acoustic waves. We present numerical results obtained with a fluid-type code in one- and two-dimensional spatial dimensions, in the case of an inhomogeneous plasma. The plasma density is under quarter-critical and depends linearly on the longitudinal spatial coordinate, in the regime where the Rosenbluth gain factor for the amplitude, denoted as G(Ros), is in the range pi/2< or =G_(Ros)< or =6. We observe that the Langmuir decay instability is able to suppress the gradient stabilization and restore the absolute nature of BSRS, thus leading to a significantly increased BSRS reflectivity.