Eigenmode computation for single and multicell cavities using perturbative methods

The design of accelerator cavities is a challenging task since it implies the manipulation of various shape parameters regarding different optimization goals, resulting in extensive geometric parameter studies. Computing the electromagnetic characteristics of only a single design may already involve high expenses, forcing an enormous total computational effort for studies. In most cases, this limits the observed frequency range and the number of computation passes. For the same reason, the effects of geometric imperfections are usually excluded from optimization processes, even though they may be of particular importance for the final design. Perturbative methods (PMs) offer an efficient approach to tackle this issue. They allow the computation of the eigenmodes and the derived cavity characteristics of a multitude of varied cavity designs based on one initial design. So, PMs are highly suited for carrying out parameter studies. In this contribution, we present the application of PMs to real life examples of single and multicell cavities by investigating the achievable accuracy and required computational effort depending on the observed mode order, nature and extent of the applied perturbation and complexity of the investigated structure.