Enhancement of Proton Acceleration by Hot-Electron Recirculation in Thin Foils Irradiated by Ultraintense Laser Pulses

MeV-proton production from solid targets irradiated by 100-fs laser pulses at intensities above $1\times {10}^{20}\mathrm{W}{\mathrm{cm}}^{-2\mathrm{}}$ has been studied as a function of initial target thickness. For foils 100 μm thick the proton beam was characterized by an energy spectrum of temperature 1.4 MeV with a cutoff at 6.5 MeV. When the target thickness was reduced to 3 μm the temperature was $3.2\pm 0.3\mathrm{MeV}$ with a cutoff at 24 MeV. These observations are consistent with modeling showing an enhanced density of MeV electrons at the rear surface for the thinnest targets, which predicts an increased acceleration and higher proton energies.