EFFECTS OF LASER-BEAM SMOOTHING ON STIMULATED RAMAN-SCATTERING IN EXPLODING FOIL PLASMAS

Time-resolved spectra of backward stimulated Raman scattering (SRS) we re measured from the interaction of a 527 nm laser with a preformed pl asma. The effect of laser smoothing by spectral dispersion (SSD) was s tudied using laser bandwidth (Delta lambda/lambda=0.1%) and varying th e laser intensity (2-20x10(14) W/cm(2)). A broad SRS spectrum extendin g to short wavelengths was observed for the high-intensity, Delta lamb da/lambda=0 case. Narrow spectra corresponding to the peak plasma dens ity were observed for cases with either high intensity and Delta lambd a/lambda similar to 0.1%, or with low intensity and Delta lambda/lambd a=0. Simulations of the filamentation process were performed for the c onditions of these experiments. The simulations show that laser smooth ing stabilizes filamentation for high-intensity interactions, and that filaments are stable without smoothing for low intensity. The predict ed onset of filamentation without smoothing correlates with the growth of short wavelength SRS. These experimental results are presented and models are discussed that may help explain the data. (C) 1996 America n Institute of Physics.