Effective generation of collimated ion beams by relativistic laser pulse using 2D microstructured foils: 3D PIC simulations

High power laser systems allow us to obtain collimated laser beams by irradiating thin foils with short intense laser pulses. In this work we performed several numerical simulations to study influence of target structure on ion beam acceleration efficiency. New target design was proposed to enhance ion acceleration mechanisms. For numerical simulations we used Mandor2 package: 3D Particle-in-cell code designed for analysis of interactions between short powerful laser pulses and plasma. The target proposed in the work was a disc (diameter 15 μ m, width 3 μ m) with 6 symmetrical holes in it (diameter 3 μ m). This target was irradiated by a laser pulse with the following properties: intensity 1019 W/cm2, pulse duration 200 fs, wavelength 1 μ m, focal spot size 10 μ m. For comparison similar simulations were performed for a solid target. Comparisons showed that the target with holes produced more energetic ion beams than the solid one (maximum energy 10 MeV for target with holes vs. 4 MeV for solid one). However the number of accelerated protons was 2 times lower in case of the target with holes. The mechanism of ion acceleration from such targets was also investigated in this work. It was discovered that the ion beam consists of several streams, each originating from the particular hole in the target. We assumed the following mechanism of formation of such streams: after the laser-plasma interaction, hot electrons appear, which then flow to the hole area, creating an uncompensated negative electric charge. This charge attracts ions, which focus in the center of the hole. Focused ions are then accelerated via TNSA mechanism, forming observed streams. A possibility of ion acceleration by longitudinal components of a Gaussian laser pulse is also investigated in this work. The optimal configuration for acceleration by longitudinal fields is shown to match the optimal configuration of the proposed target (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)