Optically tunable proton acceleration with a controlled prepulse in ultrashort intense laser double-foil target interaction

Competition and transition of the dominated mechanisms for proton acceleration were investigated in experiments by optically tuning the preplasma density profile using an additional femtosecond pre-ablation laser beam. Two groups of proton beams with angular separation were measured along the laser propagation axis and target normal direction from a vacuum-gapped double-foil target. A transition of proton acceleration from a target normal sheath acceleration regime to relativistically induced transparency (RIT) domination was observed when increasing the prepulse intensity. Two-dimensional particle-in-cell simulations qualitatively verify the experimental observations that a proton component along the laser axis is mainly generated by the RIT induced breakout afterburner from the tailored pre-expanded ultrathin front-layer foil with spatial-intensity distribution improvement by the second-layer foil. Our method can be popularized in manipulating the laser-driven proton acceleration and beam spatial quality for wide applications.