Resonant emittance mixing of flat beams in plasma accelerators

Plasma accelerators produce large accelerating 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 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 other schemes may avoid the resonance and mitigate emittance deterioration. An analytical model describing the emittance mixing process will be presented.