MeV proton generation and efficiency from an intense laser irradiated foil

The proton energy distribution generated from the interaction of an intense (Iλ² ≈ 10²⁰ W/cm² μm²) short-pulse (100 fs) laser with a thin foil is investigated using energy resolved measurements and 2D collisional PIC-hybrid simulations. The measured absolute proton spectrum is well matched by a 1.7 MeV exponential function for energies <11 MeV. The proton conversion efficiency from hot electrons ≈6%. Simulations predict a strong radial dependence on the maximum proton energy and on the radial extent of 12 Å hydrocarbon depletion region. C and O ions in the hydrocarbon layer gain significant energies, limiting the efficiency to the protons. The efficiency scaling for ion mixtures is derived using a simple model, and is shown to strongly depend on the cooling rate of the hot electrons. Simulations using hydrogen-rich, layered targets predict much higher efficiencies.