STIMULATED RAMAN BACKSCATTERING INSTABILITY IN SHORT-PULSE LASER INTERACTION WITH HELIUM GAS

Experimental and theoretical results on the stimulated Raman backscatt ering (SRS) reflectivity of a short laser pulse (120 fs) interaction w ith an optically ionized helium gas are presented. The reflectivity is measured as a function of the gas pressure from 1 to 100 Torr. A mono dimensional (1-D) theoretical model, including the refraction induced during the ionization process, describes the dependence of the SRS ref lectivity with the gas pressure and explains its maximum at around 35 Torr. In the very low pressure case (<15 Torr), the radial ponderomoti ve 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 obtai ned between theory and experiments in the whole range of pressures. (C ) 1996 American Institute of Physics.