Analysis of cylindrical open resonator mode selection for 140GHz gyrotron oscillator

With the increase of the working frequency of gyrotron, higher order mode is used as operating mode, which makes the frequency spacing between the different modes become smaller and smaller and results in mode competition. It is very important to select an appropriate mode as operating mode for keeping stable operation. In this paper, based on the linear theory of gyrotron oscillator, a slowly varying cross-section cylindrical open cavity is used to calculate the coupling coefficient variation with guide center, frequency spectrum density and starting current versus magnet field intensity and the effect of ohm loss endurance capacity of the cavity surface on mode starting oscillation and stable operation. Finally, the two modes of $\mathbf{TE}_{28,8}$ and $\mathbf{TE}_{25,9}$ are considered as operating modes through comparing the effect of their operating frequencies, starting oscillation currents, mode spacing, coupling coefficients on stable operation.