The efficiency and output power of our 140 GHz MW gyrotron are greatly improved using a two-section cavity. In both TE15,2 and TE16,2 modes the gyrotron can produce more than 1 MW power at the efficiency of about 30%. The maximum power of 1.2 MW at 32% efficiency is produced at 148 GHz with TE16,2 mode. All these experimental results are quite reproducible. New designs of high efficiency cavities are ongoing, including the design of coaxial cavities.
K. Y. Xu,
"Efficient operation of a megawatt gyrotron", Proc. SPIE 1514, 15th International Conference on Infrared and Millimeter Waves, 15142J (1 December 1990); doi: 10.1117/12.2301501; https://doi.org/10.1117/12.2301501