@article {16209, title = {The design of a multi-harmonic step-tunable gyrotron}, journal = {Physics of Plasmas }, volume = {24}, year = {2017}, pages = {033101}, abstract = {The\ theoretical\ study of a step-tunable\ gyrotron\ controlled\ by successive excitation of multi-harmonic modes is presented in this paper. An axis-encircling\ electron beam\ is employed to eliminate the harmonic mode competition. Physics images are depicted to elaborate the multi-harmonic interaction mechanism in determining the operating parameters at which arbitrary harmonic tuning can be realized by\ magnetic field\ sweeping to achieve\ controlled\ multiband frequencies{\textquoteright} radiation. An important principle is revealed that a weak coupling coefficient under a high-harmonic interaction can be compensated by a high\ Q-factor. To some extent, the complementation between the high Q-factor and weak coupling coefficient makes the high-harmonic mode potential to achieve high efficiency. Based on a previous optimized\ magnetic cusp\ gun, the multi-harmonic step-tunable\ gyrotron\ is feasible by using harmonic tuning of first-to-fourth harmonic modes. Multimode simulation shows that the multi-harmonic\ gyrotron\ can operate on the 34 GHz first-harmonic TE11\ mode, 54 GHz second-harmonic TE21\ mode, 74 GHz third-harmonic TE31\ mode, and 94 GHz fourth-harmonic TE41\ mode, corresponding to peak efficiencies of 28.6\%, 35.7\%, 17.1\%, and 11.4\%, respectively. The multi-harmonic step-tunable\ gyrotron\ provides new possibilities in millimeter{\textendash}terahertz source development especially for advanced terahertz applications.}, url = {http://aip.scitation.org/doi/abs/10.1063/1.4977452}, author = {Xiang-Bo Qi and Chao-Hai Du and Juan-Feng Zhu and Shi Pan and Pu-Kun Liu} }