Cryogenic vacuum tests of scaled down narrow support elements for the W7-X coil system

The narrow support elements (NSEs) between the casings of the superconducting coils of the Wendelstein 7-X magnet system consist of pads made of Al–bronze, which can slide and tilt against stainless-steel counter-faces. Static friction peaks or stick–slip events during sliding may cause disturbances to the magnet system and have to be avoided. Because the NSEs are operated at a temperature of ≈4 K and in vacuum at a residual pressure of p < 10−6 mbar, only solid lubricants, in particular MoS2, are applicable. Therefore, the static and dynamic friction behaviour of MoS2-coatings was tested in liquid helium at 4.2 K and in vacuum at room temperature under high contact pressures and very low sliding velocities. In model tests with small Al–bronze pins, sliding in reciprocating motion against a PVD–MoS2-coating on a steel substrate, very stable sliding behaviour without coating failure, and a friction coefficient below 0.1 was observed. However, tests with downscaled samples in original geometry showed stable sliding in liquid nitrogen, but unexpected stick–slip behaviour in liquid helium at 4.2 K. For further investigations and because in actual service the NSEs are not immersed into a liquid cryogen, tests under vacuum at low temperatures are being prepared.