Investigation of laser ion acceleration in low-density targets using exploded foils

Intense research is being conducted into sources of laser-accelerated ions and their application. Particular attention is now given to the low-density regime of laser ion acceleration. In this regime, volume effects are expected to dominate, while for solid foils, ion acceleration is directly related to the electron surface density and the number of accelerated ions is limited. Simulations therefore show that it is possible to reach high ion energies with a high number of accelerated ions and a high conversion efficiency. This scheme also leads to less debris than solid foils and is more adapted to high repetition lasers. Due to the difficulty in generating short and dense gas jets experimentally, we have decided to study this regime using very thin foils exploded by a longer, lower intensity pulse. As this regime scales well with laser energy, experiments were recently performed with a high laser energy (∼180 J) on the LLNL Titan laser. A secondary long pulse laser was used to control the density profile of the target. Preliminary analysis suggests that, in this high-energy regime, protons of high energies and with good beam quality were obtained when exploding the foil. We present new simulation results exploring the laser ion acceleration mechanism in laser and plasma conditions close to those of these experiments. These results demonstrate that low-density targets are a promising candidate for an efficient proton source. This source can be optimized by choosing appropriate plasma conditions.