Enhanced electron acceleration with wide-spot picosecond pulse relativistic laser

High power lasers with relativistic intensities with wide focal spot and picosecond (ps) pulse lengths are available in recent years. In over-ps relativistic laser-plasma interaction, both energy and number of hot electrons become higher than those predicted by the scaling used in the sub-ps regime. The electron energy spectrum changes from the thermal distribution to the nonthermal one, and the accelerated ion energy becomes much higher than the energy predicted by the isothermal model [1]. One of the key mechanisms of such the super-thermal electron generation is the stochastic heating in laser-foil plasma interaction, where hot electrons recirculate around the plasma and suffer multiple kicks from the laser field during the interaction [2]. The blowout of plasma towards the laser [3] enhances the stochasticity in the laser-plasma interaction. Two-dimensional PIC (PICLS) simulations demonstrate the significant enhancement of the electron heating with a large focal spot laser light over ps interaction, while a laser with small focal spot produces thermal hot electrons with the ponderomotive temperature. [1] N. Iwata et al, Phys. Plasmas {\bf 24}, 073111 (2017). [2] Y. Sentoku et al., Appl. Phys. B {\bf 74}, 207 (2002). [3] N. Iwata et al., Nat. Commun. {\bf 9}, 623 (2018).