Proton acceleration from microtube targets at the ALEPH laser

A super-intense laser pulse, incident on a microtube target, can accelerate protons to tens of MeV. Microtube targets have an advantage over flat foils because additional hot electrons are accelerated from the tube surface, strengthening the accelerating sheath field. The ALEPH laser at Colorado State University (40 fs, 3\texttimes 10$^{\mathrm{21\thinspace }}$W/cm$^{\mathrm{2}} \quad \lambda =$400 nm) was used to accelerate ions from 3D-printed microtube targets. At best performance, the microtube targets increase the proton cutoff energy relative to flat foils by \textasciitilde 65{\%}, and increase the proton yield by \textasciitilde 50{\%}. A wide parameter scan of microtube targets, varying tube dimensions, determined an optimum microtube size for accelerating protons on ALEPH. 2D particle-in-cell simulations show that electrons from the tube surface are accelerated to higher energy than the ponderomotive scaling, and are collimated to the center of the tube target. For the simulated optimum tube case, this process doubles the maximum proton energy relative to flat foils. \textit{This work is supported by the DOE National Nuclear Security Administration under Award Number DE-NA0003842; and by the DOE Office of Science, Fusion Energy Sciences under Contract No. DE-SC0019076.}