Mode Switching in a Gyrotron with Azimuthally Corrugated Resonator

Abstract

Summary form only given. When in a gyrotron the resonator wall is azimuthally corrugated with the number of azimuthal variations twice the azimuthal index of the operating mode, such corrugation provides the coupling between the waves rotating azimuthally in opposite directions and leads to formation of standing wave patterns. Such cosine and sine modes have different frequencies and their frequency separation is proportional to the corrugation amplitude. The coupling between two such standing modes depends on the radius of an electron beam. In particular, it is possible to choose such beam radius that the coupling is strong (that means that single-mode oscillations of either mode are stable), but, when the gyrotron operates near the boundary between the strong and weak coupling, it can be easily switched from one mode to another with the use of low-power driver. This sort of operation can be used for dynamic suppression of neoclassical tearing modes in large tokamaks and stellarators (a so-called AC method of NTM stabilization'). For this application, the frequency of switching from the first mode to the second and back should correspond to the rotational frequency of magnetic islands. When the frequency separation of standing modes is much larger than the width of mode resonances, omega/Q , such gyrotron can improve reliability of the narrow-band frequency diplexer proposed " for directing the gyrotron wave beam to one of two output channels providing the NTM stabilization.