  We theoretically investigate high energy, collimated proton beam with three
dimensional particle in cell simulations of ultrashort petawatt laser
interaction with cryogenic hydrogen target of various shapes. Here we show that
under appropriate conditions between the laser and target parameters, the
protons are accelerated to high energies mainly due to collisionless shock
acceleration mechanism combined with TNSA. The dependence of the protonic
energy on the laser field, target shape and thickness is reported. It is
demonstrated that the irradiation of intense laser (20fs, 2PW) with cryogenic
hydrogen target at optimal thickness allows the efficient generation of high
energy proton beam (100 MeV) of small divergence. Our results also indicate
that diffracted laser field strongly affects the collimation of ions as it
passes beside the mass limited target. This approach predicts a possible
pathway to control laser driven ion sources.
