Laser-driven ion acceleration via target normal sheath acceleration in the relativistic transparency regime

We present an experimental study investigating laser-driven proton acceleration via Target Normal Sheath Acceleration (TNSA) over a target thickness range spanning the typical TNSA-dominant region (1 μm) down to below the relativistic laser-transparency regime (l 40 nm), enabled by freely adjustable target film thickness using liquid crystals along with high contrast (via plasma mirror) laser interaction ( 2.65 J, 30 fs, I g 1 × 10lsupg21l/supg W/cmlsupg2l/supg). Thickness dependent maximum proton energies scale well with TNSA models down to the thinnest targets, while those under 40 nm indicate transparency-enhanced TNSA via differences in light transmission, maximum proton energy, and proton beam spatial profile. Oblique laser incidence (45°) allowed additional diagnostics to be fielded to diagnose the interaction quality: a suite of ion energy and spatial distribution diagnostics in the laser axis and both front and rear target normal directions as well as reflected and transmitted light measurements on-shot collectively verify the dominant acceleration mechanism as TNSA from high contrast interaction, even for ultra-thin targets. Additionally, 3D particle-in-cell simulations support the experimental observations of target-normal-directed proton acceleration from ultra-thin films.