Distinguishing between ion acceleration regimes at oblique incidence with particle-in-cell simulations

A recent high-contrast experiment using the Draco laser (~3 J, 10²¹ W/cm²) at 45 degrees angle of incidence on liquid crystal targets showed predominantly target normal directed ions for all target thicknesses from >1 μm down to 10 nm. Target normal ions are often considered to be an indication of the target normal sheath acceleration (TNSA) mechanism, but the thinner targets in this experiment are well under the predicted theoretical transition to radiation pressure acceleration (RPA). We present 3D particle-in-cell simulations which reproduce the dominance of target normal acceleration as well as the transparency onset as a function of target thickness. Particle tracking shows that high energy ions from the thinnest targets are accelerated volumetrically, in contrast to originating at the rear surface as in thicker targets. We show that target normally directed ions are produced by TNSA for thick targets, but by RPA for thin targets. A simple analytical model for RPA-driven target deformation is presented, which agrees well with 2D and 3D simulation results.