  Plasmas are a compelling medium for particle acceleration owing to their
natural ability to sustain electric fields that are orders of magnitude larger
than those available in conventional radio-frequency accelerators. Plasmas are
also unique amongst accelerator technologies in that they respond differently
to beams of opposite charge. The asymmetric response of a plasma to
highly-relativistic electron and positron beams arises from the fact that
plasmas are composed of light, mobile electrons and heavy, stationary ions.
Hollow channel plasma acceleration is a technique for symmetrizing the response
of the plasma, such that it works equally well for high-energy electron and
positron beams. In the experiment described here, we demonstrate the generation
of a positron beam-driven wake in an extended, annular plasma channel, and
acceleration of a second trailing witness positron bunch by the wake. The
leading bunch excites the plasma wakefield and loses energy to the plasma,
while the witness bunch experiences an accelerating field and gains energy,
thus providing a proof-of-concept for hollow channel acceleration of positron
beams. At a bunch separation of 330 um, the accelerating gradient is 70 MV/m,
the transformer ratio is 0.55, and the energy transfer efficiency is 18% for a
drive-to-witness beam charge ratio of 5:1.
