Transition of backward stimulated Raman scattering from absolute to convective instability via density modulation.

The effect of a static sinusoidal density modulation on the temporal growth rate of backward stimulated Raman scattering (BSRS) is discussed by kinetic theory and Vlasov simulation. One-dimensional Vlasov simulations show that the temporal growth rate will decrease with the increasing density modulation amplitude ε, which is consistent with the kinetic theory. Moreover, the transition from an absolute instability to a convective instability via the density modulation is also observed from the variation of the temporal growth rate of BSRS. The temporal growth rate in the case of density modulation wave-number k_s = 0.1k_l (kl is wave-number of Langmuir wave) decreases faster with increasing ε than that in the case of k_s = 0.5k_l because of the generation of more modes, which will make Landau damping of Langmuir waves stronger. In addition to the generation of other modes, the decrease of the resonant region where BSRS occurs with the increasing density modulation amplitude ε is also a reason for the reduction of the temporal growth rate. [ABSTRACT FROM AUTHOR]