Hysteresis in mode competion in the high power 170 GHz CW gyrotron for ITER

Summary form only given. In gyrotrons hysteresis is the phenomenon that causes the amplitude of oscillations to lag behind the magnetic field and the voltage, so that operation regions of modes for rising and falling magnetic field and voltage are not the same. Understanding of hysteresis is important in connection with mode competition, frequency tuning, voltage overshooting, amplitude modulation of the signal, etc. Measurements of hysteresis with respect to accelerating voltage in a multimode coaxial gyrotron have been performed at the Forschungszentrum Karlsruhe. The first devoted experimental verification of different hysteresis phenomena in CW low- power gyrotrons operating in a single mode has been done at the Research Center for Development of Far-Infrared Region of Fukui University. Just recently, researchers from the Japan Atomic Energy Agency have observed a clear hysteresis effect with respect to magnetic field in mode competition in the 170 GHz CW gyrotron for ITER. In the present work the single mode hysteresis calculations are generalized to multimode case with particular emphasis on mode competition. A hysteresis map is calculated in the plane "frequency mismatch - dimensionless current" for the operating TE_m,p mode and the parasitic TE_m-1,p mode .The theory is illustrated by a number of specific examples. In particular, it is shown how the hysteresis loop observed in experiment can be described theoretically.