Threshold target thickness in high-contrast laser-driven ion acceleration

Abstract

The experiments have revealed a "threshold"target thickness for proton acceleration by a femtosecond petawatt class laser above which the forward accelerated proton energy weakly depends on the target thickness, while for thinner targets, it gradually decreases. For targets thicker than the threshold, the pre-pulse-induced preplasma at the target front can boost ion acceleration by increasing laser absorption, while this is ineffective for thinner targets due to prepulse-induced plasma formation at the target rear. This dual nature of the preplasma is described analytically, and particle-in-cell simulations confirm this concept. A linear dependence of threshold target thickness on the prepulse intensity is predicted.