Azimuthal Mode Coupling in Coaxial Waveguides and Cavities With Longitudinally Corrugated Insert

Coaxial resonant cavities with longitudinal corrugations on the inner conductor are used in high-frequency high-power gyrotrons as means to reduce the number of possible competing modes. For a sufficiently large number of corrugations, the analytical approach usually treats the surface corrugation as a homogeneous surface impedance to obtain simple formulas for the characteristic equation and field components. These formulas can be introduced to interaction codes in a quite straightforward way. Full-wave approaches that account for the azimuthal periodicity of the structure and consider azimuthal spatial harmonics to describe the field distributions have been also employed, increasing though the complexity of the solution and the effort given in numerical calculations. In this paper, a full-wave code is used in an attempt to identify the way that the azimuthal spatial terms contribute to the reformation of the eigenvalue spectrum and propose a criterion for the selection of the spatial terms that should be taken into account for accurate enough calculations.