Study of Higher Order Modes in Superconducting Accelerating Structures for Linac Applications

Higher Order Modes (HOMs) can severely limit the operation of superconducting cavities in a linear accelerator with high beam current, high duty factor and complex pulse structure. Therefore, the full HOM spectrum has to be analysed in detail to identify potentially dangerous modes already during the design phase and to define their damping requirements. For this purpose a dedicated beam dynamics simulation code, Simulation of higher order Mode Dynamics (SMD), focusing on beam-HOM interaction, has been developed in the frame of this project. SMD allows to analyse the beam behaviour under the presence of HOMs, taking into account many important effects, such as for example the HOM frequency spread, beam input jitter, different chopping patterns, as well as klystron and alignment errors. SMD is used to investigate in detail into the effects of HOMs in the Superconducting Proton Linac (SPL) at CERN and in particular their potential to drive beam instabili- ties in the longitudinal and transverse direction. Based on these results, HOM damping requirements for the HOM coupler design are then defined. In addition, the linear ac- celerators of the European Spallation Source (ESS) and the Spallation Neutron Source (SNS) are analysed with respect to HOM impact and the results are compared with the SPL simulations.