Simultaneous observation of angularly separated laser-driven proton beams accelerated via two different mechanisms

Abstract

We present experimental data showing an angular separation of laser accelerated proton beams. Using flat plastic targets with thicknesses ranging from 200 nm to 1200 nm, a laser intensity of 6×1020 W cm−2 incident with an angle of 10°, we observe accelerated protons in target normal direction with cutoff energies around 30 MeV independent from the target thickness. For the best match of laser and target conditions, an additional proton signature is detected along the laser axis with a maximum energy of 65 MeV. These different beams can be attributed to two acceleration mechanisms acting simultaneously, i.e., target normal sheath acceleration and acceleration based on relativistic transparency, e.g., laser breakout afterburner, respectively.