Dynamics of hot refluxing electrons in ultra-short relativistic laser foil interactions

We investigate the dynamics of hot refluxing electrons in the interaction of an ultra-short relativistic laser pulse with a thin foil target via particle-in-cell (PIC) simulations, that is governed by the multidimensional spatio-temporal evolution of self-generated sheath field. The comparison of time-integrated energy spectra of refluxing and escaping electrons indicates the refluxing efficiency is higher than 95\% in average for each bounce. The characteristics of wide transverse spatial distribution and energy-resolved angular distribution caused by the refluxing electrons show a direct correlation with the angular-dependent photon yield of Bremsstrahlung emission, as verified by the hybrid simulations of coupling the PIC results with Monte-Carlo particle transport code. We further clarify the energy dissipation mechanisms of refluxing electrons through the recirculation in the thin target under the electron-refluxing dominated regime, and conclude that the self-generated sheath field plays a dominant role over the competing processes such as the radiation loss, collisional stopping and anomalous inhibition via the resistive field. The lifetime of recirculation is calculated to be few hundred femtoseconds, that is one order of magnitude shorter than the time scale of collisional dissipation, while one order of magnitude longer than the laser pulse duration. The results could provide useful insight to understand the hot electron transport and stopping, secondary radiation generation and ion acceleration in the high energy density plasmas.