Possible variants of a future 2-MW, 140-GHz gyrotron for the ECRH system at W7-X

The scheduled upgrade of the Electron Cyclotron Resonance Heating (ECRH) system at the stellarator Wendelstein 7-X (W7-X) has now focused studies from the available 1.5-MW, 140-GHz gyrotron (TH1507U), towards a future 2-MW, 140-GHz gyrotron. To advance in 2-MW operation a higher-order operating mode than the TE28,10 mode used in the 1.5-MW gyrotron is required and its selection depends on various criteria. This paper explores operation with three TE mode candidates by taking into account the compatibility with the existing infrastructure at W7-X as well as the possibilities of similarity to existing successful tubes. Using either the conventional or the coaxial-cavity concept was also investigated.