Ring-like spatial distribution of laser accelerated protons in the ultra-high-contrast TNSA-regime

The spatial distribution of protons accelerated from submicron-thick plastic foil targets using multi-terawatt, frequency-doubled laser pulses with ultra-high temporal contrast has been investigated experimentally. A very stable, ring-like beam profile of the accelerated protons, oriented around the target's normal direction has been observed. The ring's opening angle has been found to decrease with increasing foil thicknesses. Two-dimensional particle-in-cell simulations reproduce our results indicating that the ring is formed during the expansion of the proton density distribution into the vacuum as described by the mechanism of target-normal sheath acceleration. Here - in addition to the longitudinal electric fields responsible for the forward acceleration of the protons - a lateral charge separation leads to transverse field components accelerating the protons in the lateral direction.