Calculation and measurement of higher order mode losses in ITER ECH transmission lines

The ITER transmission lines (TLs) must be designed to deliver 20 MW from a 24 MW, 170 GHz gyrotron system. Miter bends are the main source of loss for these highly overmoded, corrugated, cylindrical waveguide TLs. Previous estimates have used only a pure HE₁₁ mode for the analysis of the loss due to a miter bend, however higher order modes (HOMs) must be considered for a practical analysis. For the linearly-polarized, Gaussian-like beam from a gyrotron, the LP_mn mode basis set should be used to describe the fields in the corrugated waveguide. The HOM content greatly affects the propagation of HE₁₁ content in a miter bend, with a large emphasis placed on the percentage of HOMs and the phase difference between HE₁₁ and each HOM. By considering LP modes, a complete basis set is used to investigate the HOM effects on HE₁₁ loss in a miter bend. We also present a new conservation theorem relating the power centroid offset and propagation angle due to any two LP_mn modes propagating in the corrugated waveguide.