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Rescattering of stimulated Brillouin scattered light is investigated. The net reflection coefficient for light scattered by a plasma slab is calculated and compared with the single scatter result.

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PHYSICAL REVIEW LETTERS 2 JANUARY 1978 etry yields a value l(1/n)/(dn/dz) I ' =L = 2+ 1 mm at r=0. It may be shown that a,t resonance 9 T~96k~2 T2 4', c' 2 k, m, c' ' ing this work. This work was supported by the U. S. Energy Research and Development Administration, Contract No. E Y/76-C-02-3073. and that at any position (&~)' = to~, '+36k'T, /m,. (The other sideband has~' =e, +«u.) From Eq. (4) it can be shown that if we depart a. distance « from resonance, the plasma wave detected at this &~is at resonance at distance «' = (2b.k z/k,)bz away. «' is then just the width of the convective instability, which for our experimental parameters may be numerically calculated to be 30+ 10 p m. Using these values, we may calculate I; matching in? to the exponent in Eq. (3) and letting the pump power be 10" W/ cm', we obtain I-=2.7 mm, in qualitative agreement with the experimentally determined value. In conclusion, we have observed the parametric decay of CO,-laser light into plasma waves at quarter-critical density; the amplitude and experimental parameters allow us to infer that this decay is convective. The technique of ruby-laser Thomson scattering in a CO,-laser-heated gastarget plasma seems to hold great promise as a tool for diagnosing parametric turbulence of interest to laser-fusion schemes.