Development of 14 GHz 1 MW and 28 GHz 0.4 MW gyrotrons for low-field fusion devices

Megawatt-scale gyrotrons operating in the relatively low frequency range of 14–35 GHz are required for electron Bernstein wave heating, electron cyclotron heating, and electron cyclotron current drive experiments in specific low-magnetic field fusion devices. In this study, a 14 GHz 1 MW gyrotron was developed based on a design concept that incorporates direct RF beam coupling through a built-in corrugated waveguide. This approach minimizes the RF transmission path and enhances the transmission efficiency. In the initial experimental test, an output power of 1.05 MW was achieved with a pulse width of 2 ms at 14.018 GHz. This represents the first instance of a 1.05 MW output in the 14 GHz gyrotron, demonstrating its potential use in fusion reactor and devices. The study also details the development of a novel 28 GHz 0.4 MW continuous wave gyrotron designed for the Q-shu University Experiments with Steady-state Spherical Tokamak. This gyrotron utilizes a double-disk sapphire window and a depressed collector. Experimental results at the output window demonstrated a maximum power of 1.24 MW with a pulse width of 2 ms as well as a maximum total efficiency with a collector potential depression of 53.1% with a output power of 0.52 MW and a pulse width of 8 ms. Additionally, a 28/35 GHz dual-frequency gyrotron was employed to evaluate the cooling performance of the double-disk sapphire window—an essential component for high-power gyrotrons at low frequencies. By comparing the experimental data, including an output power of 0.13 MW at a pulse width of 30 s at 28 GHz, with simulation results, the feasibility of achieving 0.4 MW CW operation at 28 GHz was confirmed.