Design considerations for a TE28,12-mode 2-MW, 140-GHz gyrotron for the ECRH system at W7-X

Following the recent upgrade of the Electron Cyclotron Resonance Heating (ECRH) system at the stellarator Wendelstein 7-X (W7-X) with a 1.5-MW, 140-GHz gyrotron (TH1507U), efforts are now focused on further advancing gyrotron technology by developing a 2-MW, 140-GHz Continuous Wave (CW) prototype. This paper proposes two RF and electron beam optics designs of a conventional-cavity 2-MW, 140-GHz gyrotron, operating with the TE28,12 mode. Each design corresponds to different operating conditions, namely Low-Voltage High-Current (LVHC) and High-Voltage Low-Current (HVLC). The primary objective of the work is to leverage the existing infrastructure at W7-X while minimizing design modifications to the existing 1.5-MW gyrotron, therefore ensuring cost efficiency and increasing the possibility for a rapid implementation of the proposed designs. Given the significant (and challenging) space-charge depression associated with the TE28,12 mode, several gyrotron startup scenarios are investigated thoroughly using the existing TH1507U diode Magnetron Injection Gun (MIG) as well as a new triode-type MIG design, which is based on the diode design. The findings of this study offer key insights into the design and operational challenges of future 2 MW-class gyrotrons operating at 140 GHz and beyond.