Experiment of a High-Power Sub-THz Gyrotron Operating in High-Order Axial Modes

To explore the approach for generating high-power frequency-tunable subterahertz (sub-THz) to THz radiation, a 220-GHz high-order axial modes (HOAMs) gyrotron is developed, and the proof-of-principle experiment is presented in this paper. Experimental results demonstrate that driven by an electron beam of high voltage and high current, HOAMs are successfully excited in a long cylindrical cavity. By tuning the operating magnetic field, a mode transition between the first four axial modes was experimentally observed with a stepwise frequency tuning range of 0.79 GHz around 219 GHz, while a 0.58-GHz frequency tuning range via beam voltage tuning by operating in the first three axial modes. During the process of frequency tuning, the output power was kept not less than 0.45 kW, while the maximum output power was 3.80 kW with an output efficiency of 6%. These experimental results of mode transition and mode competition between axial modes in high-power millimeter-wave gyrotron could be beneficial for the development of frequency-tunable gyrotron toward some up-and-coming novel THz applications.