Single-bunch Instability of Positron Beams in Electron Cloud

Single-bunch instability of a short and dense positron beam in a photo-electron plasma is studied numerically using code LCODE. The code was originally developed for studies of plasma wakefield acceleration. It is twodimensional and fully relativistic, with both the beam and electrons modelled by macro-particles. The instability is shown to affect the rear part of the beam, right after the arrival of nearby electrons to the axis. As the result, the emittance of the whole beam grows exponentially. The instability can be stabilized by an external longitudinal magnetic field. The field does not itself stabilize the instability, but prevents the electrons from going to axis once they are thrown to the wall by the previous bunch.