Increased laser-accelerated proton energies via direct laser-light-pressure acceleration of electrons in microcone targetsa)

We present experimental results showing a laser-accelerated proton beam maximum energy cutoff of 67.5 MeV, with more than 5 × 106 protons per MeV at that energy, using flat-top hollow microcone targets. This result was obtained with a modest laser energy of ∼80 J, on the high-contrast Tridentlaser at Los Alamos National Laboratory. From 2D particle-in- cellsimulations, we attribute the source of these enhanced proton energies to direct laser-light-pressure acceleration of electrons along the inner cone wall surface, where the laser light wave accelerates electrons just outside the surface critical density, in a potential wellcreated by a shift of the electrostatic field maximum with respect to that of the magnetic field maximum. Simulations show that for an increasing acceleration length, the continuous loading of electrons into the accelerating phaseof the laser field yields an increase in high-energy electrons.