Robust energy enhancement of ultrashort pulse laser accelerated protons from reduced mass targets

This paper reports on a systematic investigation of the ultrashort pulse laser driven acceleration of protons from thin targets of finite size, so-called reduced mass targets (RMTs). Reproducible series of targets, manufactured with lithographic techniques, and varying in size, thickness, and mounting geometry, were irradiated with ultrashort (30 fs) laser pulses of intensities of about 8 x 10(20) W cm(-2). A robust maximum energy enhancement of almost a factor of two was found when comparing gold RMTs to reference irradiations of plain gold foils of the same thickness. Furthermore, a change of the thickness of these targets has less influence on the measured maximum proton energy when compared to standard foils, which, based on detailed particle-in-cell simulations, can be explained by the influence of the RMT geometry on the electron sheath. The performance gain was, however, restricted to lateral target sizes of greater than 50 mu m, which can be attributed to edge and mounting structure influences.