Energetic proton generation in ultra-intense laser–solid interactions

TLDR

Solid target experiments at intensities above 10²⁰ W cm⁻² routinely produce high‑energy electrons, hard bremsstrahlung, and energetic protons on the target backside. This report proposes an explanation for the energetic ions seen in PetaWatt experiments, aiming to account for their number, energy, and angular spread by suggesting that hot electrons from the front traverse the target and ionize the back hydrogen layer. Hot electrons generated on the front of the target travel through the solid, ionize the rear hydrogen layer, and accelerate the resulting ions to tens of MeV over distances of a few tens of micrometers. The model predicts ions reaching tens of MeV over tens of micrometers, in agreement with the energies measured by radiographic and spectrographic detectors.

Abstract

An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceeding 1020 W/cm2, high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In this report, an attempt is made to explain the physical process present that will 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 the target are sent through to the back off the target, where they ionize the hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of μm, whereupon they end up being detected in the radiographic and spectrographic detectors.