Comment on "Table-Top Laser-Based Source of Femtosecond, Collimated, Ultrarelativistic Positron Beams"
Sarri et al. have reported the generation of low divergence (~3 mrad), high-density (10 14 cm –3 ) positron beams using millimeter-scale converter targets and a 50 pC, 200 MeV laser-wakefield accelerated (LWFA) electron source. It was argued that the positron divergence was dominated by the pair-production birth cone angle $θ_{e+} ≈ 1/γ_{e–}$. The small, energy-independent divergence value was used to infer a positron beam density of 2 × 10 14 cm –3 from a 4.2 mm Ta converter target where the divergence and yield measurements agreed with their Monte Carlo (MC) simulations. We have repeated these simulations using experimental conditions and disagree with the reported density, divergence, and yield values by up to factors > 50. In our work, we find that a divergence on the order of milliradian is not physical and can only be achieved if inelastic particle scattering is omitted in the calculations.