FEL-like interaction regime in the up-shifted scattering of radiation by the laser-plasma wake

We study the process of resonant scattering of radiation by the laser excited plasma wake in the wavebreaking regime. The resonant interaction arises between the cusp electrons and the slow wave of ponderomotive potential which is formed by the incident and scattered electromagnetic waves. Scattered wave is amplified when the density cusp is moving slightly faster than ponderomotive wave, and is absorbed in the opposite case. This FEL-like interaction leads to nonlinearity in reflection, the intensity-dependent reflection coefficient is calculated.