Stimulated Compton scattering from preformed underdense plasmas

The electron-density fluctuation spectra induced by stimulated Compton scattering (SCS) are directly observed for the first time. A CO₂ laser is focused into plasmas with densities nₑ spanning (0.4--6)×{}10¹⁶ cm^-3. The fluctuations corresponding to backscatter are probed using Thomson scattering. At low nₑ, the scattered spectrum peaks at a frequency shift {Δ}{ω}=kvₑ and appears to be in a linear convectively saturated regime. At the highest nₑ, a nonlinear saturation of the SCS instability is observed possibly due to a self-induced perturbation of the electron distribution function.