Emission of energetic protons from relativistic intensity laser interaction with a cone-wire target

Emission of energetic protons (maximum energy \ensuremath{\sim}18 MeV) from the interaction of relativistic intensity laser with a cone-wire target is experimentally measured and numerically simulated with hybrid particle-in-cell code, lsp [D. R. Welch et al., Phys. Plasmas 13, 063105 (2006)]. The protons originate from the wire attached to the cone after the OMEGA EP laser (670 J, 10 ps, 5 \ifmmode\times\else\texttimes\fi{} 10${}^{18}$ W/cm${}^{2}$) deposits its energy inside the cone. These protons are accelerated from the contaminant layer on the wire surface, and are measured in the radial direction, i.e., in a direction transverse to the wire length. Simulations show that the radial electric field, responsible for the proton acceleration, is excited by three factors, viz., (i) transverse momentum of the relativistic fast electrons beam entering into the wire, (ii) scattering of electrons inside the wire, and (iii) refluxing of escaped electrons by ``fountain effect'' at the end of the wire. The underlying physics of radial electric field and acceleration of protons is discussed.