Spectrally peaked proton beams shock accelerated from an optically shaped overdense gas jet by a near-infrared laser

We report on the generation of impurity-free proton beams from an overdense gas jet driven by a near-infrared laser ($lambda_L=1.053$ $mathrm{mu} m$). The gas profile was shaped prior to the interaction using a controlled prepulse. Without this optical shaping, a 30$pm$4 nCsr$^{-1}$ thermal spectrum was detected transversely to the laser propagation direction with a high energy 8.27$pm$7 MeV, narrow energy spread (6$pm$2 %) bunch containing 45$pm$7 pCsr$^{-1}$. In contrast, with optical shaping the radial component was not detected and instead forward going protons were detected with energy 1.32$pm$2 MeV, 12.9$pm$3 % energy spread, and charge 400$pm$30 pCsr$^{-1}$. Both the forward going and radial narrow energy spread features are indicative of collisionless shock acceleration of the protons.