High efficiency quasi-optical mode converter for a 140 GHz 1 MW gyrotron

Current high power gyrotrons employ an internal quasi-optical mode converter to extract the rotating high-order cavity modes into a linearly polarized fundamental Gaussian beam. Conversion efficiencies close to unity are required since trapped RF energy jeopardizes the stable operation regime and may lead to heating of the tube structure. For the 1 MW continuous wave (CW) gyrotron, operated in the TE_28,8 mode at 140 GHz, in order to achieve a usable fundamental Gaussian distribution of the output beam with low diffraction losses, a high efficiency quasi-optical mode converter has been optimized and designed at Forschungszentrum Karlsruhe (FZK). It consists of an advanced dimple-wall antenna as a launcher, one quasi-elliptical mirror and two toroidal mirrors as the beam-forming system.