The radiation spectrum of a dense bunch of charged particles colliding with a laser pulse is coherently enhanced at low frequencies. We analytically derive the main features (spectral range and shape, angular spread) of the coherently enhanced radiation of particles randomly distributed in a cylindrical bunch colliding head on with a pulsed plane wave with a Gaussian envelope.
A dense bunch of scattering particles can lose a substantial amount of energy due to radiation. We identify that in such conditions coherent radiation friction mostly originates from forward and backward scattered radiation and develop a model, which predicts the dependence of the average momentum loss of the particles on all the relevant parameters of the particle bunch and laser pulse.
Three-dimensional PIC simulations demonstrate high accuracy of our models for both radiation and radiation friction. Our findings demonstrate that  observation of radiation friction with dense bunches is feasible at lower particle energies and laser intensities than commonly presumed.

high-intensity laser-plasma interaction, coherent radiation, simulation