Influence of ion motion in a resonantly driven wakefield accelerator
Several different schemes for plasma wakefield acceleration using a train of drivers have been pursued, based on the resonant excitation of a plasma wave. Since these schemes rely on the plasma electron wave surviving for many periods, the motion of the plasma ions can have a significant impact on the beam--plasma interaction. In this work, simulations are used to study the impact of this ion motion on the development of the self-modulation of a long beam, directly applicable to recent experiments. It is shown that two related but distinct effects contribute to the suppression of the wakefield excitation: the loss of resonance between the drive beam and the plasma wave it excites, and phase mixing due to transverse wavebreaking. Although only the latter has previously been investigated, we show that the two effects follow the same scaling with ion mass.