  Relativistic positron beams are required for fundamental research in
nonlinear strong field QED, plasma physics, and laboratory astrophysics.
Positrons are difficult to create and manipulate due to their short lifetime,
and their energy gain is limited by the accelerator size in conventional
facilities. Alternative compact accelerator concepts in plasmas are becoming
more and more mature for electrons, but positron generation and acceleration
remain an outstanding challenge. Here we propose a new setup where we can
generate, inject and accelerate them in a single stage during the propagation
of an intense laser in a plasma channel. The positrons are created from a
laser-electron collision at 90 degrees, where the injection and guiding are
made possible by an 800 nC electron beam loading which reverses the sign of the
background electrostatic field. We obtain a 20 fC positron beam, with GeV-level
central energy within 0.5 mm of plasma.
