Progress in the Development of a 1.5 MW, 140 GHz Gyrotron for the Upgrade of the ECRH System at W7-X

Following the outstanding results from the first experimental period (OP1) of the stellarator Wendelstein 7-X [1-6], and in view of the second experimental period (OP2; planned to begin in 2021), an upgrade of the Electron Cyclotron Resonance Heating (ECRH) installation is underway. A major target of the upgrade is to increase the installed microwave power. To this end, a prototype Continuous-Wave (CW) 1.5 MW, 140 GHz gyrotron is under development, which is planned to be followed by two additional series tubes. In order to keep the risk and cost of the development as low as possible, the design of the 1.5 MW gyrotron is regarded as an upgrade of the successful design of the existing 1 MW, 140 GHz gyrotrons at W7-X [7]. The scientific design of the 1.5 MW tube has been realized by KIT with contributions from IPP and NKUA. The design will be provided to the industrial partner, in order to fully develop and manufacture the prototype. In parallel, a short-pulse mock-up gyrotron, which has an identical scientific design to that of the 1.5 MW CW prototype, is also under development at KIT. This gyrotron is planned to support the development of the prototype and to mitigate the risks by providing the means for the experimental validation of the scientific design of all critical components. The paper reports on the complete scientific design of the 1.5 MW CW prototype and on the progress in the construction of the short-pulse gyrotron at KIT. Possible exchange: A demand for increased gyrotron unit power in ECRH installations is present worldwide, both in existing as well as in future fusion reactors. Although several successful 1 MW CW tubes at 140 or 170 GHz already exist, the number of CW tubes at higher power is still limited. Since EU, Japan, and US are developers of MW-class gyrotrons, the exchange of ideas and experience on advanced tubes, reliably performing well-above the 1 MW power level, can be very fruitful.