Stimulated photon cascade and condensate in a relativistic laser-plasma interaction

The propagation of a linearly polarized relativistic laser pulse in an underdense plasma is studied by fluid-Maxwell and particle-in-cell simulations. A nonlinear interplay between backward and forward stimulated Raman scattering instabilities produces a strong spatial modulation of the light pulse and the down cascade in its frequency spectrum. The Raman cascade saturates by a unique photon condensation at the bottom of the light spectra near the electron plasma frequency, related to strong depletion and possible break-up of the laser beam. In the final stage of the cascade-into-condensate mechanism, the depleted downshifted laser pulse is gradually transformed into a train of ultra-short relativistic light solitons