Field propagation effects and related multibunch instability in multicell capture cavities

Field propagation effects during the filling and re-filling of the cavity are not harmful for a relativistic beam accelerated in a multicell standing wave cavity. On the opposite the beam dynamics of a non relativistic beam injected into a capture cavity can be significantly affected by the interaction with other modes of the fundamental passband, under beam-loading conditions. In fact, the dominant beating provided by the passband mode nearest to the accelerating n-mode introduces fluctuations on the accelerating voltage. The phase slippage occurring in the first cells, between the non relativistic beam and the lower modes, produces an effective enhancement of the shunt impedances, which are usually negligible for a relativistic beam in a well tuned cavity. In some cases simulations show a significant oscillation of the energy spread and transverse normalized emittance along the bunch train. As an example, we have applied our computational models to the beam-loading problem in the Tesla Test Facility (TTF) superconducting capture cavity.