Design requirement, qualification tests and integration of a thin solid lubricant film of MoS2 for the cold mass support structure of the steady state superconducting tokamak SST-1

Abstract

The SST-1 is a superconducting tokamak, which is in the commissioning phase and will soon be ready for plasma operation. The superconducting magnet system of SST-1 comprises toroidal field (TF) and poloidal field (PF) coils. The 16 TF coils are nosed and clamped towards the in-board side and are supported toroidally with the inter-coil structure at the out-board side, forming a rigid body system. The 9 PF coils are clamped on the TF coils structure. The integrated system of TF coils and PF coils forms the cold mass of @50 ton weight. This cold mass system (CMS) is freely supported on the rigid support ring at 16 locations and the support ring in turn is supported on the 8 columns of the machine support structure. This CMS is accommodated inside the high vacuum chamber (cryostat). During the operation this cold mass attains a cryogenic temperature of 4.2 K in the hostile environment of high vacuum. During the cool down, the thermal excursion of cold mass and its supporting structure generates severe frictional forces at the sliding surfaces of the support. There is a design requirement of introducing a thin layer of solid lubricant film of molybdenum disulfide having coefficient of friction 0.05 between the sliding surfaces to control the stress contribution due to the friction. To ascertain the compatibility of molybdenum disulphide (MoS2) as a solid lubricant in a high vacuum and very low temperature environment, we have carried out qualification tests on various samples and measured the coefficient of friction in both room temperature conditions and at high vacuum and after thermal shocking at a temperature of 4.2 K in a high vacuum environment to simulate the actual working condition. After successful qualification tests and process establishment, the actual components are fabricated and integrated in the cold mass support structure assembly. The design requirement and qualification tests performed at 4.2 K and room temperature as well as details about the integration of a thin solid lubricant film of MoS2 with the SST-1 machine are discussed in this paper.