Methods for the Design and Analysis of Higher-Order Mode Couplers applied to Superconducting Accelerating Structures

Higher-order modes (HOMs) may affect beam stability and refrigeration requirements of superconducting proton linacs such as the SPL which is being studied at CERN as a driver for future neutrino programs. One option being considered to limit these effects is the use of coaxial HOM couplers mounted on the cutoff tubes of the five-cell cavities. In this work, potentially dangerous modes are analyzed and corresponding damping requirements defined, primarily on the basis of longitudinal beam dynamics and eigenmode simulations. A novel approach is worked out to characterize entire HOM spectra based on truncated wake potentials and an extended formulation of vector fitting. The design process of coaxial HOM couplers is examined under new aspects. The first contribution to systematically design coaxial microwave filters on the basis of filter functions is elaborated. Prior to this are empirical studies of certain microwave structures that have not yet been well understood. Furthermore, the problem of transmission zeros inherent to cutoff tubes is formulated. Finally, coaxial HOM couplers are discussed in a more general context. Besides the characteristic frequency response, the designs of several HOM couplers applied to SPL cavities are investigated and compared in terms of mode damping, mechanical tolerances, thermal loads, structural deformations, and multipacting. Some of the computational analyses are validated by prototype measurements.