  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.
