Three-dimensional effects in the Radiation Pressure Acceleration.*

Radiation Pressure Acceleration (RPA) is considered one of the most efficient regimes of laser driven ion acceleration. Previous analytical and computer simulation results single out this regime as a way of generating GeV-class ion beams. Here a simple analytical model is presented, which describes multi-dimensional effects, especially the transverse expansion of the target, and advances our understanding  of the properties of the RPA-generated ion source. It is shown how laser pulse temporal and spatial profiles can be used to alter the divergence of the ion beam and optimize the energy transfer from the laser to the ions.