Abstract
The lack of high-power terahertz sources is a major impediment to the development of terahertz science and technology. The gyrotron has great potential to generate high-power terahertz radiation, and so attracts extensive attention. In order to alleviate the problems of limited power capacity and wall loading, the high-order modes are often chosen to be the operating modes. Due to the dense spectrum of the modes, it will cause intense mode competition, which is harmful for the stable operation of a gyrotron. Currently, a 0.68-THz second harmonic gyrotron is under development in the Terahertz Research Center of University of Electronic Science and Technology (UESTC). The experiment shows that the measured output power is 2.75 kW with the efficiency of 9.05%. In order to determine whether the operating mode is the desired mode TE−15,2, the calculation of the starting current should be more accurate. In this article, we generalize the existing linear theory for the starting current, considering the effects of beam thickness and velocity spread. The starting currents of the operating mode and its competing modes are calculated using our designed gyrotron. We find out that the velocity spread and the beam thickness have significant influence on the starting current. The calculations also show that the desired mode TE−15,2 can be triggered first at the operating point. Besides, the measured output power from the experiment is much lower than that of the simulation result. Therefore, the effects of the velocity spread and beam thickness on beam–wave interactions are also analyzed; our results show that the efficiency is sensitive to the beam quality.
Published Version
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