  Plasma wakefield acceleration in the blowout regime is particularly promising
for high-energy acceleration of electron beams because of its large
acceleration gradients, high energy transfer efficiency and excellent beam
quality. However, no equivalent regime for positron acceleration in plasma
wakes has been discovered to-date. We show that after a short propagation
distance, an asymmetric electron beam drives a stable wakefield in a hollow
plasma channel that can be both accelerating and focusing for a positron beam.
A high charge positron bunch placed at a suitable distance behind the drive
bunch can beam-load or flatten the longitudinal wakefield and enhance the
transverse focusing force, leading to high-efficiency and narrow energy spread
acceleration of the positrons. Three-dimensional quasi-static particle-in-cell
(PIC) simulations show that over 30% energy extraction efficiency from the wake
to the positrons and 1% level energy spread can be simultaneously obtained, and
further optimization is feasible.
