  We have investigated proton acceleration in the forward direction from a
near-critical density hydrogen gas jet target irradiated by a high intensity
(10^18 W/cm^2), short-pulse (5 ps) laser with wavelength of 1.054 micron. We
observe the signature of shock acceleration driven by the laser pulse, leading
to monoenergetic proton beams with small divergence in addition to the commonly
used electron-sheath driven proton acceleration. The proton energies we
obtained are modest (~MeV), but prospects for improvement are offered through
tailoring the gas jet density profile. Also, we observe that this mechanism is
very robust in producing those beams and thus can be considered as a future
candidate in laser-driven ion sources driven by the upcoming next generation of
multi-PW near-infrared lasers.
