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

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 nonplanar 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 MoS2 coatings have been developed and tested at room and cryogenic temperature (77 K) to allow relative sliding of adjacent coils under load without stick-slip 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. After deposition and after testing/exposure the coatings have been characterized in detail to obtain information for the industrial production of the coatings. (C) 2007 Elsevier B.V. All rights reserved.