Calculation of Rf Eigenmodes Using S-parameters of Resonator Parts

Eigenmode analysis is a fundamental method to characterize the interaction of beam and accelerator components. Several codes are able to calculate eigenmodes but all of them act on the complete structure which is under consideration. Thus they are restricted to a certain complexity of the resonator depending on the computational capabilities. We present a method that overcomes this restriction utilizing a description of structure components in terms of their wave scattering properties. The eigenmodes are found as the solutions of an eigenproblem of small dimension, once the S-parameters of each structure section are known. Their calculation can be performed in parallel. Sections of identical shape have to be calculated only once. Furthermore it is possible to specify a frequency range to search for eigenmodes without the knowledge of all modes with lower frequency. The method is explained in detail in the paper; its capabilities are illustrated using a prototype example which allows to compare with the direct eigenmode calculation.