Reduced divergence of laser-accelerated proton beams due to a low-density plasma surrounding the target

Protons accelerated by target normal sheath acceleration typically have a high divergence, in the order of tens of degrees. However, for many applications lower divergence proton beams are beneficial, especially where the beam needs to be captured and transported or where a high proton flux density is required. Proton beams with divergences of 1-2 degrees have been observed from the interaction of a high-intensity laser pulse with a planar liquid sheet target; particle-in-cell simulations indicated that the low divergence may be attributed to interaction of the proton beam with a low-density plasma formed from the vapour surrounding the target [Streeter et.al. in submission]. The influence of a low-density background plasma has been studied further using gas around a foil target, providing independent control of the proton source and the background plasma. Measurements indicate that the divergence of the beam reduced when the background gas was present, with little impact on the maximum proton energy, presenting a potential method for generating low divergence proton beams compatible with multi-Hz repetition rates.