Observation of Mode Competition in Operation of a 420 GHz, TE17.4 Second Harmonic Gyrotron With Complex Cavity

The lack of terahertz sources, especially the high-power ones, is one of the main obstacles in the developments of terahertz science and technology. A gyrotron, based on the electron cyclotron masers, is one of the most promising sources to generate high-power terahertz radiation. Therefore, a 0.42-THz gyrotron with a gradually tapered complex cavity operating at the second harmonic is designed and experimentally tested in this paper. Experimental results show that the designed gyrotron can generate high-power THz radiation of 19.2 kW at 0.42 THz with the beam voltage V
₀
of 51 kV, beam current I
₀
of 4.4 A and the external magnetic field B
₀
of 8.03 T, corresponding to an efficiency of 8.6%. By comparison with theoretical results, it can be shown that the operating mode in the designed gyrotron is confirmed as the TE
_17.4
mode. At the same time, the excitation of the fundamental TE
_0.5
mode is also observed in the experiment when the beam current is increased to 7.3 A, where the operating frequency is 216.143 GHz. Through extensive experimental efforts, it is found that the designed gyrotron can operate stably at the desired TE
_17.4
mode when the beam current is in the range of 3-7 A.