Effect of plasma peak density on energetic proton emission in ultrashort high-intensity laser-foil interactions

The influence of plasma peak density on energetic proton emission from the rear side of a foil irradiated by a 70-fs intense laser pulse at normal incidence has been studied. When the plasma peak density of the foil is overdense, forward energetic proton emission is dominant and attributed to rear surface acceleration. Along with the plasma peak density changed from overdense to underdense, a transition of proton acceleration from the forward to the radial direction is observed, and in the latter case energetic protons are emitted radially and slightly forward.