  The rapid progress that plasma wakefield accelerators are experiencing is now
posing the question as to whether they could be included in the design of the
next generation of high-energy electron-positron colliders. However, the
typical structure of the accelerating wakefields presents challenging
complications for positron acceleration. Research in plasma-based acceleration
of positrons has thus far experienced limited experimental progress due to the
lack of positron beams suitable to seed a plasma accelerator. Here, we report
on the first experimental demonstration of a laser-driven source of
ultra-relativistic positrons with sufficient spectral and spatial quality to be
injected in a plasma accelerator. Our results indicate, in agreement with
numerical simulations, selection and transport of positron beamlets containing
$N_{e+}\geq10^5$ positrons in a 5\% bandwidth around 600 MeV, with
femtosecond-scale duration and micron-scale normalised emittance.
Particle-in-cell simulations show that positron beams of this kind can be
efficiently guided and accelerated in a laser-driven plasma accelerator, with
favourable scalings to further increase overall charge and energy using
PW-scale lasers. The results presented here demonstrate the possibility of
performing experimental studies of positron acceleration in a plasma wakefield.
