Resonant emittance mixing of flat beams in plasma accelerators

Plasma accelerators sustain large field gradients and could enable future compact linear colliders. To achieve the required high luminosity, linear colliders rely on flat beams to avoid potentially deleterious beamstrahlung effects. Here, we show that flat beams in plasma accelerators can be subject to beam quality degradation due to emittance mixing caused by transverse coupling in the wakefields. When there is a resonance between the betatron oscillations in the horizontal and vertical planes for the beam particles in a coupled wakefield, the transverse emittances fully exchange, leading to a round beam. Depending on the mechanism causing the resonance, the use of laser drivers, flat particle beam drivers, or hollow plasma channels can avoid the resonance and mitigate the emittance deterioration.