A theoretical and experimental study on a 94 GHz low-voltage, low-current gyrotron is presented in this paper. In order to obtain stable radiation and make great use of the equipment in the lab, the TE 6,2 mode is selected as the operating mode of the desired gyrotron. The efficiency of the electron beam interaction with RF fields under different conditions has been calculated by a code, which is based on the self-consistent nonlinear theory for gyrotrons. The gyrotron is equipped with a single-stage depressed collector and a quasi-optical mode converter. Then, a gyrotron with optimized parameters has been designed, constructed, and tested. An output power of 23 kW is obtained at an accelerating beam voltage of 31.5 kV, a beam current of 1.8 A, and a collector depression voltage of 7.4 kV, corresponding to an overall efficiency of 53%.

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Terahertz and sub-terahertz science and technology are promising topics today. However, it is difficult to obtain high power source of terahertz wave. In this paper, the mode competition and beam-wave interaction in a gradually tapered cavity are studied to achieve high efficiency of a 0.4THz second harmonic gyrotron in practice. In order to attain high power and stable radiation, the TE32,5 mode is selected as the operating mode of the desired gyrotron to realize single mode oscillation. The issues of studying on the high-order mode gyrotrons are solved effectively by transforming the generalized telegraphist's equations. The efficiency and output power of the gyrotron under different conditions have been calculated by the code, which is based on the transformed equations. Consequently, the results show that single mode second harmonic radiation with power of over 150 kW at frequency of 0.4 THz could be achieved.