Laser proton acceleration from liquid crystal films of different thicknesses with ultra-high laser contrast

We present an experimental study which shows a possible pathway towards the robust and reliable generation of applicable, energetic, high-quality laser-driven proton beams at high repetition rates. This is enabled through the combination of ultra-high contrast laser pulses and liquid crystal film targets, which can be generated and characterized in situ within a thickness range from 10 μm to 10 nm at low cost.
Pulses from our in-house Ti:Sa laser (~3 J, 30 fs, I~10^20 W/cm^2) incident obliquely on target in order to distinguish between ion acceleration mechanisms. Increasing target normal proton energy cut-offs are observed for decreasing thicknesses, with > 25 MeV protons recorded for ultra-thin (< 50 nm) targets. The results are in agreement with the dominance of target normal sheath acceleration (TNSA) down to ultra-thin target thicknesses, in line with the observed robustness of the acceleration performance.