Enhanced proton acceleration from ultrashort intense laser irradiated thin foil targets with controlled pre-plasma

The presence of a pre-plasma before the solid affects the dynamics of ultrafast, ultra-intense laser-solid interaction, thereby increasing laser absorption and the hot electron flux and temperature. This change in the hot electron parameters can be pivotal for the laser-accelerated protons. In this work, we experimentally investigate proton energy enhancement through controlled pre-plasma. A low-intensity ps pulse (250 mJ, 400 ps, 1013 W cm−2) derived from the uncompressed pulse of 150 TW laser has been used for this purpose. The relative time delay between the external pre-pulse and the main fs pulse (1.7 J, 180 fs, 9 × 1018 W cm−2) arriving at the target can be independently controlled, and its effect on the accelerated proton spectrum has been studied using Cu (2, 7, 12.5 μm) and Ti (6 μm) targets. An enhancement in proton cutoff energies was observed at an optimal delay for all the targets. Proton cutoff energies are found to be increased by more than ∼30% for Cu targets (4.6–6.1 MeV) and ∼50% for Ti targets (4.9–7.8 MeV) at optimal pre-plasma conditions. Correlated hot electron measurements have also been performed, where enhanced hot electron flux and temperatures are observed. These experimental findings are corroborated by numerical simulations involving a two-stage approach using radiation hydrodynamics and particle-in-cell simulations with the experimental conditions. Simulation results indicate the role of enhanced laser energy absorption—owing to the relativistic self-focusing and long plasma channel formation in the extended underdense plasma, and hot electron divergence.