Hybrid proton acceleration with an ultra-intense laser of low contrast

Proton acceleration from the interaction of an ultra-intense femtosecond-class laser pulse at varying picosecond contrast levels with an ultrathin-foil target is investigated numerically. It is found that lower contrast increases the optimal target thickness without reducing the proton cutoff energy at an ultrahigh laser intensity of I similar to 8 x 10(21 ) W/cm(2), contrasting with previous experimental results at low laser intensity I < 1x10(21) W/cm(2). By employing particle tracking techniques, we show that due to the intense radiation pressure of the main pulse, the acceleration of protons in the pre-expanded target via the Hole-Boring mechanism under low contrast surpasses that in the steep-edge target under high contrast, which is driven by a hybrid regime of light-sail and sheath acceleration before the target is penetrated. After that, a thinner optimal target thickness under high contrast results in stronger electron heating, enabling the proton energy to catch up gradually in relativistic-induced transparency enhanced acceleration. Ultimately, a similar cutoff energy is obtained for both scenarios. Our work demonstrates that high laser intensity can offer some advantages to proton acceleration at the radiation-pressure-dominated stage when laser contrast control on the picosecond level is challenging, and a thicker target is necessary. This implies that the demanding requirement for laser contrast could potentially be relaxed for multi-petawatt laser facilities, simplifying experimental setups and enhancing proton energy.