Spectral control in proton acceleration with multiple laser pulses

We address the question of whether multiple high intensity (> 10(18) W cm(-2)) laser pulses ( each O( 100 fs)) can produce proton beams with a modified energy spectrum on irradiating a foil target. This has been studied in one dimension with both Vlasov and particle-in-cell codes. A reduction in the maximum proton energy, and the generation of spectral peaks, is observed. This is the first theoretical demonstration of spectral peak generation by purely optical means. The mechanism, which has been termed multiple pulse sheath acceleration, that produces these spectral peaks is described, and the same mechanism occurs in both sets of simulations.