Microstructured snow targets for high energy quasi-monoenergetic proton acceleration

Compact size sources of high energy protons (50-200MeV) are expected to be key technology in a wide range of scientific applications (1-8). One promising approach is the Target Normal Sheath Acceleration (TNSA) scheme (9,10), holding record level of 67MeV protons generated by a peta-Watt laser (11). In general, laser intensity exceeding 10(18) W/cm(2) is required to produce MeV level protons. Another approach is the Break-Out Afterburner (BOA) scheme which is a more efficient acceleration scheme but requires an extremely clean pulse with contrast ratio of above 10(-10). Increasing the energy of the accelerated protons using modest energy laser sources is a very attractive task nowadays. Recently, nano-scale targets were used to accelerate ions (12,13) but no significant enhancement of the accelerated proton energy was measured. Here we report on the generation of up to 20MeV by a modest (5TW) laser system interacting with a microstructured snow target deposited on a Sapphire substrate. This scheme relax also the requirement of high contrast ratio between the pulse and the pre-pulse, where the latter produces the highly structured plasma essential for the interaction process. The plasma near the tip of the snow target is subject to locally enhanced laser intensity with high spatial gradients, and enhanced charge separation is obtained. Electrostatic fields of extremely high intensities are produced, and protons are accelerated to MeV-level energies. PIC simulations of this targets reproduce the experimentally measured energy scaling and predict the generation of 150 MeV protons from laser power of 100TW laser system(18).