Stimulated Raman scattering in large plasmas

In long pulse, high-energy experiments (4000 J, 2 nsec, 5×1014 W/cm2, 1.064 μm) at the Shiva laser facility, several percent of the laser light has been observed to be Raman scattered. The spectrum of the Raman-scattered light was measured from 1.48 to 2.17 μm. The Raman scattering occurred principally at electron densities much lower than the quarter-critical electron density. The high-energy electrons expected in Raman scattering were observed indirectly in these experiments via their bremsstrahlung radiation. Additional experiments show that the Raman instability has a much lower intensity threshold for longer laser pusle length and larger laser spot size. Raman light measurements for 5320 Å irradiated disk target experiments are also reported. The light near 2λ0 or 1.064 μm had both a red- and blue-shifted component. At high intensities, Raman scattering also occurred in the very underdense plasma.