Longitudinal laser ion acceleration in low density targets: Experimental optimization on the Titan laser facility and numerical investigation of the ultra-high intensity limit

Recent theoretical and experimental studies suggest the possibility of enhancing the efficiency and ease of laser acceleration of protons and ions using underdense or near critical plasmas through electrostatic shocks. Very promising results were recently obtained in this regime. In these experiments, a first ns pulse was focused on a thin target to explode it and a second laser with a high intensity was focused on the exploded foil. The delay between two lasers allowed to control the density gradient seen by the second laser pulse. The transition between various laser ion acceleration regimes depending on the density gradient length was studied. With a laser energy of a few Joules, protons with energies close to the energies of TNSA accelerated protons were obtained for various exploded foils configurations. In the high energy regime (∼180 J), protons with energies significantly higher than the ones of TNSA accelerated protons were obtained when exploding the foil while keeping a good beam quality. These results demonstrate that low-density targets are promising candidates for an efficient proton source that can be optimized by choosing appropriate plasma conditions. New experiments were also performed in this regime with gas jets. Scaling shock acceleration in the low density regime to ultra high intensities is a challenge as radiation losses and electron positron pair production change the optimization of the shock process. Using large-scale Particle-In-Cell simulations, the transition to this regime in which intense beams of relativistic ions can be produced is investigated.