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Description
In this paper the successful fabrication and assembly of the Multistage Depressed Collector (MDC) prototype developed at KIT for megawatt-class gyrotrons is described. The collector design bases on the application of the E×B drift concept for electron trajectory separation, a method that has been presented in [1-3]. The MDC prototype has a cylindrical structure with a helical isolation cut [4-6]. In particular, the design is tailored for compatibility with the different prototype gyrotrons for fusion applications at KIT, such as for W7-X, ITER and DEMO.
The fabrication process involves meticulous steps, starting with the essential inner electrodes made of a copper-chromium-zirconium (CuCr1Zr) tube. The electrodes consist of two stages with a triple helix isolation design to achieve the desired compact geometry. The design of the electrodes also involves the creation of 30 identical modules to extend the helical surfaces. The vacuum housing, a critical component for reliable experiments, is divided into four parts, each playing a specific role in supporting the collector stages and providing electrical isolation. The lower and middle assemblies are made of stainless steel, using a careful combination of welding and machining processes. The other parts are the modular ceramic for isolation of the second depression potential and the top plate for support of the second electrode.
One aspect of the MDC prototype is the incorporation of external coils to collimate the spent electron beam within the E×B region. These coils, wound with copper wire on non-slotted aluminium bodies, are fitted with cooling jackets. The assembly process is detailed for two configurations: one for the 170 GHz 2 MW coaxial cavity gyrotron and the other for the W7-X upgrade short pulse gyrotron. The first assembly of the vacuum housing demonstrated that the manufacturing of such a modular collector can be achieved successfully with an excellent vacuum tightness. The second assembly, tailored for the W7-X gyrotron, involves the precise arrangement of the inner electrodes and all remaining components.
In conclusion, the fabrication and assembly of the MDC short pulse prototype has been carried out successfully. The prototype is ready for verification in the KIT FULGOR test stand. The work provides a valuable guideline for future modifications and improvements in this innovative field of research.
Acknowledgement
This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.
References
[1] I. Gr. Pagonakis, et al., IEEE Transactions on Plasma Science 36, 469–480 (2008).
[2] C. Wu, et al., EPJ Web Conf. 149, 04005 (2017).
[3] V. Manuilov, et al., Infrared Phys. Technol. 91, 46–54 (2018).
[4] C. Wu, et al., Physics of Plasmas 25, 033108 (2018).
[5] C. Wu, et al., Physics of Plasmas 26, 013108 (2019).
[6] B. Ell, et al., IEEE Transactions on Electron Devices 70, 1299–1305 (2023).
