Compact sources of high energy protons (50-500MeV) are expected to be key technology in a wide range of scientific
applications 1-8. Particularly promising is the target normal sheah acceleration (TNSA) scheme 9,10, holding record level
of 67MeV protons generated by a peta-Watt laser 11. In general, laser intensity exceeding 1018 W/cm2 is required to
produce MeV level protons. Enhancing the energy of generated protons using compact laser sources is very attractive
task nowadays. Recently, nano-scale targets were used to accelerate ions 12,13. Here we report on the first generation of
5.5-7.5MeV protons by modest laser intensities (4.5 × 1017 W/cm2) interacting with H2O nano-wires (snow) deposited
on a Sapphire substrate. In this setup, the plasma near the tip of the nano-wire is subject to locally enhanced laser
intensity with high spatial gradients, and confined charge separation is obtained. Electrostatic fields of extremely high
intensities are produced, and protons are accelerated to MeV-level energies. Nano-wire engineered targets will relax the
demand of peak energy from laser based sources.
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