Manipulation of the spatial distribution of laser-accelerated proton beams by varying the laser intensity distribution

We report on a study of the spatial profile of proton beams produced through target normal sheath acceleration (TNSA) using at target foils and changing the laser intensity distribution on the target front surface. This is done by either defocusing a single laser pulse or by using a split-pulse setup and irradiating the target with two identical laser pulses with variable spatial separation. The resulting proton beam profile as well as the energy spectrum are recorded as functions of the focal spot size of the single laser pulse and of the separation between the two pulses. A shaping of the resulting proton beam profile, related to both an increase in ux of low-energy protons in the target normal direction and a decrease in their divergence, in one or two dimensions, is observed. The results are explained by simple modelling of rear surface sheath field expansion, ionization and projection of the resulting proton beam