Design and test of mode generators for high order rotating gyrotron modes

In order to cold test the performance of quasi-optical mode converters of gyrotrons, a high order gyrotron cavity mode has to be excited at low power levels. There are several approaches to generate such modes. One is to use combined sections of periodically-perturbed-waveguide-wall converters, which step-wise convert from fundamental to high order waveguide mode. Since recent gyrotrons use very high order modes such converters would become extremely long and therefore narrow band, as well as difficult to build mechanically. The most promising and compact solutions are mode generators where the resonant cavity is directly excited through coupling holes in the waveguide wall. One such solution is to use coupled cavities where the fundamental TE/sub 11/ input mode is coupled through an iris to an intermediate TE/sub 1n/ cavity which is finally coupled to the high order mode cavity mode through a fine electroformed screen. This paper focuses on an even simpler quasi-optical solution where a free space Gaussian beam is directly coupled to the high order gyrotron mode through a translucent cavity wall.