Proton Acceleration Associated with Sheet Crossing in Petawatt-Laser-Irradiated Nanometer Foils

We explored the physics of efficient proton acceleration in a nanometer-thick polymer foil irradiated by an ultrahigh-contrast petawatt laser pulse. The longitudinal proton dynamics were experimentally investigated, indicating the acceleration of protons to over 90 MeV by a novel scheme associated with sheet crossing. Experimental evidence, such as a ringlike distribution of the most energetic protons and a reduced proton cutoff energy from diamondlike carbon foils, revealed that the highest-energy protons originated from the target forepart. Then, after a sheet crossing process inside a drifting longitudinal electric field induced by the laser, they overtook the protons from the rear surface. Such novel longitudinal dynamics of protons, supporting a favorable beam stability and a promising energy scaling, could prompt extensive applications of laser-accelerated protons.