RAMAN UP-SCATTERING IN LONG-SCALE-LENGTH, LASER-PRODUCED PLASMAS

Frequency up-shifted light, produced by the interaction of a 351 nm laser with a plasma large enough to produce efficient down-shifted Raman backscattering, was measured. The plasma was created using low-intensity, 351 nm laser beams striking a 15-mu-m thick CH foil. During the period of interest, the electron temperature was estimated to be 1 keV, the peak density to be 0.15 of the critical density, and the plasma scale length to be 1500-mu-m at one-tenth critical density. The most straightforward explanation of the up-shifted light was found to be scattering from externally enhanced plasma waves. Scattering from a plasma mode created by coupling a large ion wave to a stimulated-Raman-scattering-produced plasma wave is also possible. Collisional damping precludes either scattering from a reflected electron-plasma wave produced by SRS or scattering from a plasma wave resulting from the onset of the Langmuir decay instability.