Laser Generation of Near-GeV Low Emittance Positron Beams

We report on the first spatial and spectral characterisation of near-GeV positron beams generated in a fully laser-driven configuration. The energy-resolved geometric emittance, source size and spectrum were simultaneously measured for electrons and positrons generated from a laser-wakefield accelerated electron beam impacting on a thin high-Z converter. More than $10^5$ positrons were observed within 5\% of 600 MeV, with a source size smaller than 100 $\mu$m and sub-micron geometric emittance, in agreement with numerical modelling. We conclude that the positron emittance was dominated by the transverse size of the primary electron beam at the converter. Minimising the drift distance between the electron source and the converter would allow for the generation of GeV-scale positron beams with micron-scale source size and normalised emittance of a few microns, using a 150 TW laser system. It is proposed that beams with these characteristics are suited for experimental studies of positron acceleration in a plasma wakefield.