We calculated the optimal parameters of a low-Q cavity of a millimeter-wavelength continuous-wave gyrotron which ensure that the maximum efficiency is reached for a limited heat load on the cavity wall. The influence of the cavity optimization on the efficiency of energy recovery of a collector electron beam is considered. Stability of the operating mode to self-excitation of other modes is studied. Gyrotrons with radiation power 1 MW, frequency range 140–170 GHz, and operating modes TE22.6 and TE25.10 are studied as the example. The obtained results are generalized to gyrotrons with other operating modes and frequencies.

An electron frequency tuning band of a conventional high-Q cavity gyrotron approximately coincides with the unloaded cavity bandpass which usually does not exceed 0.1%. To expand the frequency tuning band we introduce a gyrotron with an RF circuit that consists of two coupled circular cavities separated by a thin iris. A nonlinear self-consistent gyrotron theory predicts a saturated efficiency of 18% and a tuning band of 1.1%. An experimental study of an X-band tube operating in the TE0.11 mode was carried out to test the gyrotron with the coupled cavities concept. Measurements of gyrotron performance showed a 0.96% tuning band-a tenfold increase over that of an ordinary gyrotron. Simulation results are in a good agreement with the experimental data.

We study two versions of a gyrotron operating in the TE28.7 and TE31.8 modes for electron-cyclotron heating of plasma in the international thermonuclear reactor ITER. The gyrotron cavity parameters are optimized allowing for ohmic losses, dips in electron beam potential, and velocity spread. The influence of the ion compensation for the space charge, the setting of oscillations at when the gyrotron is switched on, and the competition between the operating and parasitic modes are discussed. The possibility of attaining an efficiency of 32 to 36% for a specific power of ohmic losses in the cavity of less than 2.5 kW/cm2 in the TE28.7 mode and 2 kW/cm2 in the TE31.8 mode is demonstrated.

The paper presents an advanced method for and results of calculating main parameters of CW 170 GHz/1 MW gyrotrons operating at the TE28.7 and TE31.8 modes for ITER. Parameters are optimized to achieve maximum efficiency of the gyrotron with an acceptable Ohmic load on the cavity. Numerical modeling of starting up a gyrotron with an optimized cavity and processes of mode interaction are discussed.

Potential applications of high-power tunable microwave sources to plasma diagnostics, nonlinear spectroscopy and technology have motivated the efforts to improve the performance characteristics of tunable gyrotrons. In our previous study it has been shown that the band of electron frequency tuning of a conventional high-0 cavity gyrotron coincides approximately with the unloaded cavity pass band which does not exceed 0.1 % usually. To expand frequency tuning band we propose gyrotron circuit consisted of two coupled circular cavities. Nonlinear self-consistent gyrotron theory predicted 15-20 % efficiency and 1.3 % bandwidth in this oscillator. An experimental study of an oscillator with coupled cavities operating in the TE 011 mode at X-band has been carried out. Experimental results of about.1 % bandwidth with an saturated efficiency of about 20 % were achieved and shown to be in good agreement with the theory.