Differences in plasma focusing of short, relativistic electron and positron bunches

Summary form only given. Short, relativistic particle bunches propagating in plasmas drive large amplitude wakefields. These wakefields have longitudinal, accelerating and decelerating components, as well as transverse, focusing and defocusing components. When the particle bunch length is comparable to the electron plasma wavelength of the relativistic plasma wake, only plasma electrons respond to the bunch fields, and the plasma ions are immobile. Plasma electrons are expelled from an electron bunch, while they are attracted toward a positron bunch. This asymmetry leads to important differences in the focusing of these bunches in the plasma. In particular, in the case of bunches with densities larger than the plasma density, an electron bunch creates a pure, uniform density ion column with focusing properties free of geometric aberrations. Therefore, the incoming emittance of electron bunch with a narrow energy spread can be preserved upon acceleration by the plasma wake. On the contrary, the plasma exerts a nonuniform focusing force on a positron bunch. The focusing force is dependent on the incoming bunch parameters, and the plasma electron density inside the positron bunch can exceed the bunch density. As a result, the positron bunch develops a charge halo, and its emittance grows along the plasma. These differences are observed in experiments as well as in numerical simulations, and could have important implications for the applications of plasma-based acceleration to a future electron/positron linear collider. Experimental and simulation results will be presented.