Design considerations on coaxial gyrotron cavities for MW-class continuous-wave dual-harmonic operation

In the need for development of megawatt-class gyrotrons capable of ultra-fast switching between two widely separated operating frequencies, which will allow the same device to deliver power both for Electron Cyclotron Resonance Heating (ECRH) and rapid Radiative Instability Suppression in fusion reactors, essential considerations are presented. They refer to the operating-mode selection and to the design of a coaxial-cavity for stable Continuous-Wave (CW) operation at both the fundamental cyclotron frequency and the second cyclotron harmonic. Tools for the selection of the appropriate operating mode pair are developed, taking into account window reflections, compatibility with the quasi-optical output coupler, Ohmic wall loading, and the need for rapid switching between the two cyclotron harmonics. The required ultra-fast switching (in the range of milliseconds) between the two widely separated operating frequencies is possible by changing only the voltage values in a triode-type electron gun configuration. On this basis, and using the developed tools, a coaxial cavity that supports dual-harmonic CW operation and generates an RF power of around 1.3 MW both at 170 GHz and at 85 GHz is designed and validated by multi-mode simulations. An ultra-fast switching scenario is also demonstrated. The presented results point towards the feasibility of dual-harmonic MW-class gyrotrons, which are expected to reduce the overall cost and space requirements of the ECRH installations.