OBSERVATION OF FORWARD RAMAN-SCATTERING ENHANCED BY BACKWARD RAMAN-SCATTERING IN A LASER PLASMA

The coupling between backward and forward Raman scattering was studied experimentally in a preformed underdense parabolic shaped plasma irradiated with a CO2 laser. The forward and backward Raman spectra were simultaneously recorded, and together with interferometry indicated that stimulated Raman scattering was occurring at the top of the profile. The forward Raman was observed well below its parabolic threshold, and it is proposed that plasma waves from backward Raman scattering act as an enhanced noise source for the forward Raman process. A model, whereby the forward and backward Raman processes are coupled together to produce an absolute instability, based on the Koch and Williams coupled Raman model [Phys. Fluids 27, 2346 (1984)], is shown to explain many of the observed features. This is shown to be very similar to a model of two backward Raman amplifiers displaced from the top of the plasma profile.