High-Energy Protons from closely stacked, ultra-thin foils

Ions accelerated from solid foils using ultra-intense laser pulses may have major impact on applications such as cancer therapy. While such ion beams typically have a low emittance and high charge density their maximum energy still falls short of therapy requirements. Analytic scaling laws for micrometer targets suggest an increase in maximum energy when reducing the pulse duration down to an optimum value. Further energy increase has recently been proposed when using ultra-thin foils or micro-structured targets.
We propose a new target design based on novel stacked foils which may lead to an acceleration of ions to even higher energies by a single high-intensity (~1020 -1021 W/cm² ) ultra-short laser pulse. In contrast to complex schemes relying on the use of synchronized laser pulses predicting a comparable ion energy gain, here the time interval between the irradiation of the individual foils is determined by their spacing.
We present an analysis of the fundamental acceleration mechanism and will focus on the electron dynamic during the laser interaction with the target. Based on thorough simulations and an analytic description of the laser interaction we will show how the enhanced electron dynamics in the early stages of the interaction leads to the gain in maximum ion energy observed in our simulations. Based on this analysis, for relevant target parameters we deduce optimum values and their scalings.