Dependence of high-energy proton acceleration in TNSA regime by fs laser on the laser pulse shape

TNSA regime is employed to accelerate protons from gold foils up to 20 MeV. Measurements were performed using a fs-pulsed laser at about 4× 1018 W/cm2. The laser source is characterized by a low contrast, high pre-pulse duration, main pulse within 40÷120 fs and presence of a second main pulse. The proton energy appears high compared to the values reported in the literature under similar experimental conditions, but the results can be explained by taking into account different mechanisms. The self-focusing effect, for instance, can produce increases in intensity of the main pulse. PIC simulations are used to account for these effects. The maximum electric field of ion acceleration, electron density as a function of space and time, and plasma temperature, are evaluated. The theoretical calculation is compared with the experimental measurements, confirming the very high proton acceleration under particular conditions of the laser parameters, irradiation conditions and target properties.