Approximate method for electromagnetic analysis of a dielectric-loaded coupler used in accelerating structures

In this paper, a theoretical analysis of a dielectric-loaded coupler based on the transmission line model is proposed. One of the major challenges in the design of dielectric-loaded accelerating (DLA) structures is the proper coupling of high-power RF into the dielectric-loaded tube. The considered X-band coupler consists of a tapered dielectric-loaded tube converting TM${}_{01}$ mode in a conventional circular waveguide to the one in a uniform dielectric-loaded tube. To analyze the coupler with an approximation based on the transmission line model, the coupler is divided into a large number of sections in the propagation direction. Every section is considered as a short-length uniform transmission line with its own characteristic impedance and propagation constant. Then, scattering matrix parameters of the coupler are calculated by the total transmission matrix of the structure. To investigate the accuracy of the proposed method, two coupler prototypes with different dielectric materials are considered and their scattering matrix parameters are obtained by the proposed method and full-wave simulation. Overall, it is shown that the analytical results are sufficiently close to the simulation and optimizations performed by the proposed method can be used as the starting point for full-wave or numerical optimization.