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

Abstract

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 regime (∼1 μ m) down to below the onset of relativistic laser-transparency (<40 nm). This is done with a single target material in the form of freely adjustable films of liquid crystals along with high contrast (via plasma mirror) laser interaction (∼2.65 J, 30 fs, $I\gt 1\times {10}^{21}$ W cm ^−2 ). Thickness dependent maximum proton energies scale well with TNSA models down to the thinnest targets, while those under ∼40 nm indicate the influence of relativistic transparency on TNSA, observed via differences in light transmission, maximum proton energy, and proton beam spatial profile. Oblique laser incidence (45°) allowed the fielding of numerous diagnostics to determine the interaction quality and details: ion energy and spatial distribution was measured along the laser axis and both front and rear target normal directions; these along with reflected and transmitted light measurements on-shot verify TNSA as dominant during high contrast interaction, even for ultra-thin targets. Additionally, 3D particle-in-cell simulations qualitatively support the experimental observations of target-normal-directed proton acceleration from ultra-thin films.

Keywords

• plasma physics
• ion acceleration
• ultra-intense laser
• liquid crystal
• 52.38.-r
• 64.70.M-