Dense laser-driven electron sheets as relativistic mirrors for coherent production of brilliant X-ray and γ-ray beams

Several techniques exist to obtain brilliant X-ray beams by coherent reflection from relativistic electrons (Ee=γmc2) with Doppler frequency upshift of 4γ2. We describe a new approach starting with an ultra-thin solid target. Larger ‘driver’-laser intensities with high contrast are required to produce dense electron sheets. Their acceleration in vacuum results in a transverse momentum component besides the dominant longitudinal momentum component. The counter-propagating ‘production’ laser for optimum Doppler boost in X-ray production by reflection has to be injected opposite to the electron direction and not opposite to the driver laser. Different measures to increase the reflectivity of the electron sheet via laser trapping or free-electron-laser-like micro-bunching are discussed, extending the photon energy into the MeV range. Here, first-order estimates are given.