Possibilities for Dual-Harmonic High-Power Operation of a Coaxial Gyrotron Cavity

In the need of development of gyrotrons capable of fast switching between two widely separated operating frequencies, allowing the same device to deliver power for both Electron Cyclotron Resonance Heating (ECRH) and rapid Radiative Instability (RI) suppression in fusion reactors, the existing 2nd harmonic coaxial cavity at KIT, designed to provide an output power of 1.55 MW at 170 GHz, is studied to assess whether it allows room also for efficient MW-class operation at the fundamental cyclotron frequency. The feasibility of dual-harmonic operation at 170/85 GHz and at 170/88 GHz is theoretically demonstrated, together with a novel concept for fast switching (in the range of milliseconds) between the two harmonics, using a triode-type electron gun. The results build a foundation for future coaxial-cavity designs for MW - class continuous-wave dual-harmonic gyrotron operation.