Coherent radiation and radiation friction at moderate particle energies and laser intensities

We investigate the coherent radiation of a charged particle bunch colliding with an intense laser pulse. Depending on the bunch density and size we determine which frequencies are amplified due to coherency and analytically reconstruct the coherent part of the radiation spectrum. We demonstrate that coherency strongly (by many orders of magnitude) increases the total energy radiated by the dense bunch as compared to the incoherent radiation of the same number of particles. Even in the mildly relativistic regime coherent radiation significantly affects particles’ motion, making radiation friction observable at notably lower particles' energies and laser intensities than commonly accepted. Our analytical findings are in very good agreement with the numerical three-dimensional particle-in-cell simulations.