Stimulated Raman backscattering instability in short pulse laser interaction with helium gas

Experimental and theoretical results on the stimulated Raman backscattering (SRS) reflectivity of a short laser pulse (120 fs) interaction with an optically ionized helium gas are presented. The reflectivity is measured as a function of the gas pressure from 1 to 100 Torr. A monodimensional (1-D) theoretical model, including the refraction induced during the ionization process, describes the dependence of the SRS reflectivity with the gas pressure and explains its maximum at around 35 Torr. In the very low pressure case (<15 Torr), the radial ponderomotive force expels the electrons out of the propagation region before the laser pulse reaches its peak intensity and significantly reduces the observed reflectivity. A 1-D hydrodynamic calculation, included in the model, describes this density depletion and a good agreement is obtained between theory and experiments in the whole range of pressures. (C) 1996 American Institute of Physics.