Laser driven ion acceleration (LDIA) has the potential to deliver compact and affordable accelerators for applications in many fields of science and medicine. Specifically, radiotherapy of cancerous tumors requires ion energies in the range of 200-300 MeV/a.m.u. and with energy spreads on the order of 5%, parameters thus far beyond the LDIA experimental results using the most powerful lasers in the world. Recently, it was shown experimentally that laser-driven collisionless shocks can

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... proton beams to 20 MeV with extremely narrow energy spreads of about 1% and low emittances [1]. This was achieved using a linearly polarized train of CO 2 laser pulses having a peak power of 4 TW interacting with a hydrogen gas-jet target. Motivated by these results, presented here is a systematic study of the basic physics of collisionless shock waves using 1D OSIRIS simulations. Shock formation, Mach number, and reflection of protons are key processes observed versus the initial density and drift velocity of two interpenetrating plasmas.