Control of energy spread and dark current in proton and ion beams generated in high-contrast laser solid interactions

By using temporal pulse shaping of high-contrast, short pulse laser interactions with solid density targets at intensities of 2 x 10(21) W cm(-2) at a 45 degrees incident angle, we show that it is possible to reproducibly generate quasimonoenergetic proton and ion energy spectra. The presence of a short pulse prepulse 33 ps prior to the main pulse produced proton spectra with an energy spread between 25% and 60% (Delta E/E) with energy of several MeV, with light ions becoming quasimonoenergetic for 50 nm targets. When the prepulse was removed, the energy spectra was broad. Numerical simulations suggest that expansion of the rear-side contaminant layer allowed for density conditions that prevented the protons from being screened from the sheath field, thus providing a low energy cutoff in the observed spectra normal to the target surface.