Increase of Gyrotron Output Power at High-Order Axial Mode Through an After-Cavity Excitation of the Next Transverse Mode

In the experimental study on the frequency tuning of a double-beam gyrotron, an unexpected abrupt increase of the output power was observed for some magnetic field values corresponding to the excitation of high-order axial modes (HOAM). This effect can be explained through an after-cavity interaction of the spent but bunched electron beam with the neighboring transverse mode which has the same azimuthal but the next (i.e., greater by one) radial index with respect to the operating mode. The calculations show that under certain conditions the cyclotron synchronism between the electron beam and the next radial mode occurs in the extended region of an output cone, while the bunching of the spent electron beam is responsible for the excitation of this mode. The influence of some additional factors such as reflections of both modes from the output window, electron velocity spread, and magnetic field profile was studied numerically. It was shown that reflections, mode transformation, and velocity spread can enhance the effect of an abrupt power increase and reduce the starting current of the operating mode. The discovered after-cavity interaction can be either useful for a power increase at the HOAM operation of the frequency-tunable gyrotron or harmful for high cyclotron harmonic operation due to an expansion of the zone of fundamental cyclotron resonance mode excitation.