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 P₁, 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 P₂ 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 P₁ with a peak intensity of approximately 1.71 × 10 ¹⁹ W/cm², full-width at half-maximum(FWHM) duration of 325 fs and a CP P₂ with a peak intensity of 1.54 × 10 ²² W/cm², FWHM duration of 26.5 fs, and focal spot radius of 4 µm successively acting on a target with an initial density of 8nc, protons with cut-off energy of 940 MeV can be obtained from the cascaded acceleration scheme. Compared with the case without P₁, the cutoff energy increased by 340 MeV. Owing to the intervention of P₁, this scheme overcomes the limitation of laser contrast and is more feasible to be implemented experimentally.