Impact of ion mobility on nonlinear Compton scattering of an ultra-intense laser pulse in a plasma

We demonstrate that the nontrivial dispersion of a plasma driven by a high-intensity laser pulse qualitatively affects fundamental nonperturbative QED processes triggered by the laser pulse even in the case that no electrons remain in the interaction volume, e.g., due to ponderomotive expulsion. In the electron-free case this plasma effect is mediated by the response current of the residual ions as a dispersive effect on the laser propagation. The residual ions hence act as an effective background to the propagating electromagnetic laser field. We demonstrate that this has an impact on the fundamental nonlinear QED process of photon emission by an electron upon absorption of a large number of laser photons, called nonlinear Compton scattering. In two exemplary cases we find the ion plasma to suppress and enhance the photon emission rate by approximately $10\%$ for an intermediate and high laser intensity, respectively. The latter enhancement has no classical analog.