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At the Wendelstein 7-X stellarator, a 174 GHz Collective Thomson Scattering (CTS) diagnostic will be implemented. One of the 140 GHz Electron Cyclotron Resonance Heating (ECRH) gyrotrons will be operated at around 174 GHz in a higher cavity mode, using it as source for the CTS mm-wave probing beam. To prevent any damage to the CTS receiver, a notch filter cuts out the high-power gyrotron signal at the entrance of the receiver. The bandwidth of the gyrotron signal determines the notch filter bandwidth. First proof-of-principle experiments on frequency stabilization were conducted on W7-X ECRH gyrotrons employing Phase-Locked Loop techniques. The gyrotron output frequency was controlled with the accelerating voltage, which is applied between the anode and cathode of the gyrotron diode-type Magnetron Injection Gun. Frequency stabilization experiments with 10 ms pulses were conducted at the gyrotron nominal frequency of 140 GHz as well as at 174 GHz. It is concluded that the gyrotron frequency could be stabilized for at least 3 ms at 140 GHz and 8 ms at 174 GHz. In the frequency spectrum, a clear main peak of the gyrotron frequency at 140 GHz with a full -15 dB linewidth of below 500 Hz was achieved.

The existing ECRH system at W7-X consists of 10 gyrotrons, with output power levels ranging from 0.6 MW up to 1.0 MW each at a frequency of 140 GHz, quasi-optical transmission lines and microwave launchers at the plasma vessel. Compared to other large fusion experiments, W7-X has a relatively low power-to-volume ratio. However high heating power is particularly necessary for achieving high plasma beta values, where the improved confinement of fast ions, one of the optimization criteria of W7-X, can be examined. It is therefore necessary to expand the ECRH systems in several consecutive steps. It is planned to increase the number of gyrotron positions from 10 to 12 and at the same time to evolve the gyrotron output power in several development steps from 1 MW to nominal 1.5 MW and, finally, up to 2 MW. At the same time, the transmission lines will also be upgraded for 2 MW operation. A special effort is also made to improve the reliability of the system by the fast control system.

For the required upgrades of the Electron Cyclotron Resonance Heating system at the stellarator Wendelstein 7-X, the development of a 1.5 MW 140 GHz Continuous Wave (CW) prototype gyrotron has started. KIT has been responsible to deliver the scientific design of the tube (i.e. the electron optics design and the RF design), with contributions from NKUA and IPP. The prototype gyrotron has been ordered at the industrial partner, Thales, France, and is expected to be delivered in 2021. In parallel, a short-pulse pre-prototype gyrotron has been developed at KIT, to provide the means for a first experimental validation of the scientific design in ms pulses, prior to the construction of the CW prototype. This paper reports on the status of the 1.5 MW CW gyrotron development, focusing on the scientific design and its numerical and experimental validation. © 2021 Konstantinos A. Avramidis, Zisis C. Ioannidis, Gaetano Aiello, Patrick Bénin, Ioannis Chelis, Andreas Dinklage, Gerd Gantenbein, Stefan Illy, John Jelonnek, Jianbo Jin, Heinrich P. Laqua, Alberto Leggieri, François Legrand, Alexander Marek, Stefan Marsen, Ioannis Gr. Pagonakis, Tobias Ruess, Tomasz Rzesnicki, Theo Scherer, Dirk Strauss, Manfred Thumm, Ioannis Tigelis, Dietmar Wagner, Jörg Weggen, Robert C. Wolf, the Wendelstein 7-X Team

Gyrotron R&D within the EUROfusion Work Package Heating and Current Drive (WPHCD) is addressing the challenging requirements posed on gyrotrons by the European concept for a demonstration fusion power plant (EU DEMO). These requirements, as specified within WPHCD, ask for highly reliable and robust long-pulse operation of the gyrotron, delivering 2 MW of microwave power at frequencies above 200 GHz with a high overall efficiency above 60% and the option for fast frequency step-tunability. To meet these targets, which are clearly beyond today's state-of-the-art, the R&D activities within WPHCD are organized in five main branches: these are the experimental verification of the advanced coaxial gyrotron technology at long pulses, the development of a coaxial gyrotron meeting the EU DEMO requirements, the development of multi-stage depressed collectors for enhanced energy recovery, the development of large broadband diamond windows to allow fast frequency tunability of the gyrotron, and the studies on further innovations and improvement of critical gyrotron components, in view of optimization of performance, reliability, and industrialization. The paper presents the progress made on these activities, the recent results, and the near-term planning. The major recent achievements include (i) the experimental validation of the fabrication of the new, water-cooled components for the longer-pulse coaxial gyrotron at Karlsruhe Institute of Technology, by demonstrating 2.2 MW at 170 GHz with 33% efficiency without depressed collector, (ii) the design of key components for a 2 MW, 170/204 GHz dual-frequency coaxial gyrotron, (iii) the design of a two-stage depressed collector with 77% efficiency, (iv) the first-ever production, in an industrial plasma reactor, of large chemical vapor deposition diamond wafers of 180 mm diameter, (v) the procurement of an advanced electron gun with coated emitter edges, and (vi) advances in the theoretical and numerical modelling for investigating improved concepts for the gyrotron beam tunnel and cavity. © EURATOM 2019.