  A new scheme for accelerating positively charged particles in a plasma
wakefield accelerator is proposed. If the proton drive beam propagates in a
hollow plasma channel, and the beam radius is of order of the channel width,
the space charge force of the driver causes charge separation at the channel
wall, which helps to focus the positively charged witness bunch propagating
along the beam axis. In the channel, the acceleration buckets for positively
charged particles are much larger than in the blowout regime of the uniform
plasma, and stable acceleration over long distances is possible. In addition,
phasing of the witness with respect to the wave can be tuned by changing the
radius of the channel to ensure the acceleration is optimal. Two dimensional
simulations suggest that, for proton drivers likely available in future,
positively charged particles can be stably accelerated over 1 km with the
average acceleration gradient of 1.3 GeV/m.
