  An all-optical centimeter-scale laser-plasma positron accelerator is modeled
to produce quasi-monoenergetic beams with tunable ultra-relativistic energies.
A new principle elucidated here describes the trapping of divergent positrons
that are part of a laser-driven electromagnetic shower with a large energy
spread and their acceleration into a quasi-monoenergetic positron beam in a
laser-driven plasma wave. Proof of this principle using analysis and
Particle-In-Cell simulations demonstrates that, under limits defined here,
existing lasers can accelerate hundreds of MeV pC quasi-monoenergetic positron
bunches. By providing an affordable alternative to kilometer-scale
radio-frequency accelerators, this compact positron accelerator opens up new
avenues of research.
