Generation of ultra-energetic ions by interaction of petawatt lasers with micrometer-scale foils

Abstract

The interaction of an ultraintense (2 × 1021 W/cm2) and ultrashort (18 fs) laser pulse with micrometer-scale aluminum foils is studied via the use of two and three dimensional (2D and 3D) particle-in-cell simulations. One shows that for a plane target with a steep density step, total (collisional and collisionless) absorption is weak. On the other hand, the use of structured targets (gratings in our case) or the presence of a sharp density gradient at the front of the slab allows a better coupling between the laser pulse and the target, and this yields a high absorption rate and ultraenergetic aluminum ions (in the 1 GeV range) generated by target normal sheath acceleration. By changing the characteristics of the grating, it is possible to control, to some extent, the absorption rate and the cutoff of the ion spectrum. It is also shown that a relatively simple model based on the adiabatic self-similar expansion of the target in the ultrarelativistic limit provides reasonable agreement with the simulation results. Finally, the comparison of 2D and 3D results for the interaction with gratings shows that 2D simulations accurately describe the absorption by 2D gratings but, on the other hand, they significantly overestimate the ion-spectrum cutoff.