Quasi-Optical Input Mode Coupler for a Ka-Band Multimegawatt Gyroklystron

We report a design of a new quasi-optical input coupler developed for the 35-GHz two-cavity gyroklystron. The coupler is a section of longitudinally corrugated waveguide placed between two cutoff irises. It operates with an eigenmode being a superposition of the rotating on azimuth TE 7,4 and TE 13,2 modes in the input cavity. The corrugated cavity is designed with holes in the walls to ensure necessary mode selection, because in a broad frequency band the operating eigenmode only does not have fields at the holes. In experiments, the gyroklystron equipped with this coupler has shown a peak output power of 4.2 MW with a nearly 1- \mu \text{s} pulse duration, a power gain of 24 dB, and an efficiency of about 15%.