Influence of Pre-Plasma on Accelerated Protons in Ultraintense Laser Interaction with Not-So-Thin Foils

Abstract We report on recent experimental results on proton acceleration from laser interaction with foil targets at ultra-relativistic intensities. We show a three-fold increase in the proton cut-off energy when a micrometer scale-length pre-plasma is introduced by irradiation with a low energy femtosecond pre-pulse. The foil target is sufficiently thick to prevent disruption of the sheath field at the rear surface by the shock launched by the pre-pulse. Measurements are compared with accurate, numerical hydrodynamic and Particle-In-Cell simulations where the role of the finite plasma scale-length at the laser-target interface is taken into account and the role of stochastic heating in enhancing fast electron production is discussed.