MoS2 coatings for the narrow support elements of the W-7X nonplanar coils

Abstract

WENDELSTEIN 7-X (W7-X) is a stellarator presently under construction and assembly in Greifswald, Germany. The superconducting magnet system of W7-X consists of planar and non planar coils. The electromagnetic forces on the superconducting magnet are supported by backing of all coils against a central ring and by wedging among coil casings through "Narrow Support Elements" (NSE) that have to satisfy very specific requirements. Anti friction MoS 2 coatings have been developed and tested at room and cryogenic temperature (77K) to allow relative sliding of adjacent coils under load without stickslip to avoid coil quenching. The influence of thickness on the coating endurance has been demonstrated. To prevent ageing both constructive methods and protective top coats have been investigated. This paper describes the atomic sputter deposition technique, the reduced and full-scale tribological tests, ageing tests and arrangements to protect the coatings from humidity and oxidation which have been carried on so far.

Tribological Performance of MoS2 Coatings in Various Environments

“…As an example, Figure 5 shows a friction plot for a PVD-MoS 2 coating in high vacuum. The coating is similar to the one used in the Wendelstein 7-X fusion experiment [5]. The peaks to negative direction are caused by test stops and restarts.…”

“…Because frictional heat and mechanical shocks are able to initiate a normal conducting zone, a lubricant must be employed that guarantees low friction sliding without stick-slip at these stick-slip at these supports. Tests have shown that only MoS2 coatings without any additions are able to meet such requirements [5].…”

“…A similar concept has been used by Koch et al [5] for coatings developed for support elements in the superconducting magnet system of the WENDELSTEIN W7-X fusion experiment. They are able to withstand extreme operational conditions such as temperatures as low as 5 K in combination with high vacuum.…”

“…By using a PVD process, deposition parameters can be optimized to gain coatings with a more textured structure orienting the (002) basal planes parallel to the substrate surface [9]. A similar concept has been used by Koch et al [5] for coatings developed for support elements in the superconducting magnet system of the WENDELSTEIN W7-X fusion experiment. They are able to withstand extreme operational conditions such as temperatures as low as 5 K in combination with high vacuum.…”

Full-scale friction test on tilted sliding bearings for Wendelstein 7-X coils

“…The main part of a NSE is an aluminium bronze pad, PVD coated on its spherical surface with MoS 2 [3]. During energization of the magnet system it slides against the opposite plane stainless-steel coil housing which is sprayed with MoS 2 .…”