Nonlinear dynamics of multimode gyrotron under the influence of external quasi-monochromatic signal
The possibility of controlling the radiation frequency of a powerful multimode gyrotron under the influence of an external signal with a harmonically varying frequency or amplitude, as well as two monochromatic signals, has been theoretically studied. These tasks are important for various practical applications, such as suppressing plasma instabilities in tokamaks and stellarators, spectroscopy, as well as creating a complex of coherently emitting gyrotrons in the future. The conditions are found when the frequency and phase of the operating mode are locked by an external signal. In this case, generation occurs at the operating mode with a periodically changing frequency and amplitude. The regimes observed at the exit from the locking zones are investigated. The calculations were carried out on the example of a megawatt power level gyrotron developed at IAP at a frequency of 170 GHz with an operating mode TE28,12. When the gyrotron is exposed to an external signal with a harmonically changing frequency close to the frequency of the operating mode, the frequency of this mode can be locked. The frequency control are possible if the changes in the frequency of the external signal are quite slow compared to the locking time, and the instantaneous frequency of the external signal lies within the locking band at all times. If the frequency of the external signal leaves the locking band, beating of the operating mode or excitation of spurious modes occur. Under the influence of an amplitude-modulated signal with a harmonically changing amplitude or two monochromatic external signals, periodic changes in the frequency and amplitude of the operating mode occur with a period of signal amplitude modulation. If the beat of the amplitude of the external signal is rather deep and slow in the locking time scale, and if there are no free-running single-mode operation for the specified gyrotron parameters, decay into spurious modes takes place. If the amplitude fluctuations are sufficiently small, the parasitic modes are not excited. When changes in the frequency and amplitude of the external signal are rapid in the locking time scale, parasitic modes are not excited, because the amplitude and frequency of radiation do not have time to track changes in the parameters of an external signal and fluctuate in a small range of values.