Enhanced Suppression of First Harmonic Competing Modes in Harmonic Coaxial Gyrotron Cavities with Tapered Corrugation Depth

Abstract The international race toward realizing the first economically viable fusion power plant is in full progress. Start-ups, in particular, are proposing compact high-field tokamak experiments that require electron cyclotron heating at frequencies well above 200 GHz. This demand drives the intensive pursuit of high-power gyrotrons operating beyond 200 GHz. Second harmonic operation offers a promising route to reach such frequencies without necessitating stronger magnetic fields. However, the intrinsically lower interaction efficiency and strong competition from first harmonic modes pose significant challenges. To enhance mode selectivity, we propose a cavity design with a novel scheme of profiled impedance corrugations, changing the surface impedance on the inner conductor along the axis of a coaxial gyrotron cavity. This design achieves unprecedented suppression of competing first harmonic modes while maintaining ohmic wall loading levels compatible with continuous-wave (CW) operation. As a result, this novel corrugation scheme enables higher output power, broadens the range of stable gyrotron operation, and substantially reduces sensitivity to electron beam quality. These advances establish tapered impedance corrugations as a powerful tool for realizing robust, efficient, and scalable harmonic gyrotrons at sub-terahertz frequencies.