High average power and high peak power gyrotrons: present capabilities and future prospects

We survey work in the U.S.A. High average power gyrotrons (1 MW at 100-150GHz) are required for ECRH of magnetic fusion plasmas. Even though gyrotrons have superior mode stability compared with other tubes, they must be so grossly overmoded to handle 1 MW CW that mode control does become an issue. Studies of gyrotron-oscillator cavity designs with enhanced mode stability properties are described, including complex and Fabry-Pérot cavities. In the complex cavity concept, two cavities each operating in a different mode are strongly coupled. Exploitation of this concept in a 60 GHz TE01/TE02 gyrotron has already resulted in power greater than 200 kW CW. Designs of TE03/TE04 and TE03/TE06 gyrotrons which promise to extend performance are discussed. A 115 GHz gyrotron using a Fabry-Pérot cavity has been successfully demonstrated at power levels above 50 kW and prospects of extending performance are considered. Study of high peak power gyrotrons is stimulated by the perceived requirement for 300 MW RF sources at λ ∼ 3 cm to drive very high energy (1 TeV) linear electron accelerators. For this application, the phase of the microwave signal must be precisely controllable. These demanding requirements have led to renewed interest in gyro-klystron amplifiers, and studies of relatively low power gyro-klystrons will be reviewed. Finally, a design of a 300 MW gyro-klystron is presented.