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 ${\mathrm{CO}}_2$ laser is focused into plasmas with densities ${\mathit{n}}_{\mathit{e}}$ spanning (0.4–6)×${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$. The fluctuations corresponding to backscatter are probed using Thomson scattering. At low ${\mathit{n}}_{\mathit{e}}$, the scattered spectrum peaks at a frequency shift Δω=${\mathit{kv}}_{\mathit{e}}$ and appears to be in a linear convectively saturated regime. At the highest ${\mathit{n}}_{\mathit{e}}$, a nonlinear saturation of the SCS instability is observed possibly due to a self-induced perturbation of the electron distribution function.