Energetic proton generation in ultra-intense laser-solid interactions

An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceedin g 10(20) W/cm(2), high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In thi s report, an attempt is made to explain the physical process present that w ill explain the presence of these energetic protons, as well as explain the number, energy, and angular spread of the protons observed in experiment. In particular, we hypothesize that hot electrons produced on the front of t he target are sent through to the back off the target, where they ionize th e hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of mum, whereupo n they end up being detected in the radiographic and spectrographic detecto rs. (C) 2001 American Institute of Physics