Effect of Foil Target Thickness in Proton Acceleration Driven by an Ultra-Short Laser

Proton acceleration experiments were carried out by a 1.2x10(18) W/cm(2) ultra-short laser interaction with solid foil targets. The peak proton energy observed from an optimum target thickness of 7 mu m in our experiments was 2.1 MeV. Peak proton energy and proton yield were investigated for different foil target thicknesses. It was shown that proton energy and conversion efficiency increased as the target became thinner, on one condition that the preplasma generated by the laser prepulse did not have enough shock energy and time to influence or destroy the target rear-surface. The existence of optimum foil thickness is due to the effect of the prepulse and hot electron transportation behavior on the foil target.