High-quality positrons from a multi-proton bunch driven hollow plasma wakefield accelerator

By means of a hollow plasma, multiple proton bunches work well in driving nonlinear plasma wakefields and accelerate electrons to the energy frontier with preserved beam quality. However, the acceleration of positrons is different because the accelerating structure is strongly charge dependent. There is a trade-off between keeping a small normalized emittance and a small energy spread. This results from the conflict that the plasma electrons, used to provide focusing to the multiple proton bunches, dilute the positron bunch. By loading an extra electron bunch to repel the plasma electrons and meanwhile reducing the plasma density slightly to shift the accelerating phase with a conducive slope to the positron bunch, the positron bunch can be accelerated to 400 GeV (40% of the driver energy) with an energy spread as low as 1% and well preserved normalized emittance. The successful generation of high quality and high energy positrons paves the way to the future energy frontier lepton colliders.