Effect of mode launcher geometry on the performance of a mode-selective input coupler for a gyrotron amplifier

In the process of developing a wideband input coupler for a gyrotron amplifier, the authors have measured the scalar reflection coefficient for three mode launching geometries, each mounted coaxially in an overmoded circular waveguide. A helical structure is fed directly from a 50 Omega TEM coaxial line. The impedance match is significantly affected by changes in the helix and helix support structure. A bandwidth from 16.5 to 18GHz is demonstrated from a TE₀₂ mode input coupler for one of the geometries under study.