Variational-method-based higher order mode analysis extendible to realistic tapered disk-loaded structures

In order to obtain high luminosity and energy efficiency in future linear colliders, most designs for e+ and e− collisions in the TeV range will use multi-bunch operation. Therefore, the study of higher order modes excited by previous bunches in the train becomes very important for the optimal design of the accelerator components. Many designs have used tapered disk-loaded waveguides for acceleration. Various numerical methods have been used for the modal analysis of the structure. In this paper, a high-precision eigenmode-computation analysis based on a variational method will be discussed. It allows for rounding the edge of a disk hole without any approximation in shape treatment and calculates the exactly synchronous modes. It converges much faster than the mesh-based computer code SUPERFISH. Good agreement was observed between the results of the variational method and those of other methods.