  A novel approach for positron injection and acceleration in laser driven
plasma wakefield is proposed. A theoretical model is developed and confirmed
through PIC simulation. One ring-shaped beam and one co-axially propagating
Gaussian beam drive wakefields in a preformed plasma volume filled with both
electrons and positrons. The laser's ponderomotive force as well as the charge
separation force in the front bucket of the first bubble are utilized to
provide the transverse momenta of injected positrons and those positrons can be
trapped by the focusing field and then accelerated by the wakefield. The
simulation shows that a relatively high-charge, quasi-monoenergetic positron
beams can be obtained.
