Numerical investigations of parasitic oscillations in gyrotrons

Summary form only given. At present, gyrotron oscillators, as sources of high power millimeter waves, play a central role in fusion energy research. Recently, spurious oscillations in high power gyrotrons gained attention as an influencing factor on the overall efficiency: in the up-taper/launcher section, a region exists where the decreasing magnetic field is alleged to excite spurious oscillations¹. Numerical investigations are a main pillar for a deeper understanding of this phenomenon. The KIT in-house code SELFT was modified to accommodate the influence of an axial variation of the external magnetic field on the cavity dynamics; recently, varying (w.r.t B=B(z)) the code internal reference frequency for the slow phase variable has also been considered in order to enable the simulation of large magnetic field tapers. The new capabilities of the code have been applied to the calculation of the W7-X 140 GHz TE−28.8 mode gyrotron²: results confirm that an undesired interaction in the up-taper/launcher region can result in additional parasitic oscillations. Spurious oscillations have been observed also in the spectral plot in presence of a magnetic field with significant variation towards the simulation domain output³.