Narrow bandwidth, low-emittance positron beams from a laser-wakefield accelerator

Abstract Laser-wakefield accelerated electron beams have been demonstrated to be a viable alternative to those produced by radio-frequency systems, with unique characteristics including intrinsic femtosecond-scale duration and micron-scale source size. In addition, they present the practical advantage of a generally compact and cost-effective setup inherently synchronsed with a high-power laser. The rapid progress experienced by these accelerators is now posing the question as to whether they could be included in the design of the next generation of electron-positron colliders. However, the asymmetry of the accelerating wake-fields presents challenging complications on positron acceleration. Despite considerable numerical work on positron wakefield acceleration, this area of research has thus far experienced limited experimental progress due to the lack of positron beams suitable to seed a plasma accelerator. Here, we experimentally demonstrate that moderate power lasers ($\approx$ 100 TW peak power) can generate ultra-relativistic positron beams 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 with a 5\% bandwidth around 600 MeV, with femtosecond-scale duration and micron-scale normalised emittance. It is proposed that beams with these characteristics are readily suited for experimental studies of positron acceleration in a plasma wakefield.