Spurious Fundamental-Harmonic Oscillations in the Horn Section of a High-Harmonic Gyrotron

In the experiments with a 30 keV/0.7 A sub-terahertz (THz) large-orbit gyrotron operating at the third cyclotron harmonic in the continuous-wave regime, it was found that the presence of reflections of the output signal from an external load leads to the excitation of parasitic oscillations at the fundamental cyclotron harmonic. Theoretical analysis shows that the excitation of parasitic oscillations occurs in the smooth-irregular output horn section of the microwave system, where the parasitic wave is close to the cyclotron resonance with the electron beam. Moreover, the fundamental-harmonic parasitic wave excited first in the output horn section can “penetrate” into the operating cavity, which can lead to suppression of the operating oscillation excited there at a high cyclotron harmonic.