Proton energy enhancement by optimizing a laser pulse profile

Based on current laboratory laser parameters and the low density target that is induced by the inevitable prepulse, we propose what we believe to be a new scheme to enhance the proton energy by employing a laser pulse with two different peak intensities. Initially, the lower-intensity peak of the laser pulse P 1 , irradiates the low-density plasma target induced by the prepulse to form a significantly denser plasma target. Such a compressed high-density target is critical for supporting the subsequent main pulse P 2 with higher peak intensity to drive proton acceleration. As an example, particle-in-cell (PIC) simulations reveal that when using a circularly polarized (CP) flat-top P 1 with a peak intensity of approximately 1.71 × 10 19 W/cm 2 , full-width at half-maximum(FWHM) duration of 325 fs and a CP P 2 with a peak intensity of 1.54 × 10 22 W/cm 2 , FWHM duration of 26.5 fs, and focal spot radius of 4 µm successively acting on a target with an initial density of 8 n c , protons with cut-off energy of 940 MeV can be obtained from the cascaded acceleration scheme. Compared with the case without P 1 , the cutoff energy increased by 340 MeV. Owing to the intervention of P 1 , this scheme overcomes the limitation of laser contrast and is more feasible to be implemented experimentally.