Mode generator for the cold test of step-tunable gyrotrons

To verify the performance of broadband quasi-optical mode converters of step-tunable gyrotrons in cold-test experiments, several high order cavity modes at different frequencies have to be excited at low power levels. In order to avoid the huge effort of building a mode generator for each single mode a broadband quasioptical method was chosen. Here a Gaussian beam is focused onto a mode specific caustic in a coaxial cavity through a translucent cavity wall. The mode generator can be tuned to different resonances by eventually exchanging the inner conductor of the cavity and by slightly adjusting the position of its quasi-optical components.