Higher Order Mode Analysis of Tapered Disc-loaded Waveguides Using the Mode Matching Technique

Several designs for linear colliders for e+e-collisions in the TeV range are currently under discussion. Most of them will use multi bunch operation. Therefore the study of higher order modes excited by previous bunches in the train becomes even more important for the optimal design of the accelerator components. Many of the designs use long tapered disc-loaded waveguides for acceleration. Numerical reasons hinder-the mode analysis by discretization methods. In this paper the electromagnetic waves in tapered multi-cell structures with circular cross section are calculated by a modal field matching method. The multi-cell structure is subdivided into subregions of constant circular cross section. In each subregion the electromagnetic fields are expanded in orthogonal series over discrete modes. The field solutions are obtained by field matching at the transverse interfaces between the subregions. The scattering matrix formulation is used to calculate the amplitudes of the field expansion. Some convergence investigations are presented as well as comparisons with a grid-oriented numerical method and an equivalent circuit model. The field distribution of deflecting dipole-modes is analyzed for a 180-cell tapered discloaded S-band-structure of a 500 GeV Linear Collider Study.