Laser and plasma conditions at the onset of Raman scattering in an underdense plasma

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Abstract
The laser and plasma conditions at the onset of Raman scattering have been measured in plasmas formed from thin-foil targets. The targets were Al disks 50 to 250 nm thick and 200 to 500 μm in diameter mounted on 100 nm polymer films. The incident wavelength was 526 nm. Raman scattering was observed for nominal intensities from 1014 W/cm2 to 5 × 1014 W/cm2. The most important observation of the experiment is that the density-gradient scale-length at onset is independent of intensity. One would expect the scalelength at onset to be inversely related to the intensity, since the convective gain for stimulated Raman scattering (SRS) is proportional to the product of intensity and scalelength.