Design of the plasma facing components for the National Spherical Tokamak Experiment (NSTX)

The NSTX device plasma facing components [PFC] consist of inboard divertors, outboard divertors, primary passive plates, secondary passive plates, a center stack casing (CSC), and the heating/cooling fluid distribution system. The PFC surfaces are protected by 3584 individually mounted carbon tiles. Surfaces exposed to high heat flux and/or high loads utilize composite C-C graphite and the remainder utilize less costly ATJ graphite. A variety of diagnostics are incorporated into the PFCs including thermocouples, Langmuir probes, Mirnov coils, and Rogowski coils. The NSTX device is designed to be operated in a pulse mode of five seconds on followed by five minutes off. Its PFC components are also required to be baked out to 350 °C. During operation the PFC tiles are permitted to ramp up thermally and then cool sufficiently between shots to prevent ratcheting during subsequent shots. The CSC tiles are required to be thermally isolated from the CSC so that the primary heat loss is radiation to the other PFC components. The other PFCs are thermally coupled to water cooled plates by conductive gaskets. Special mounts are required which permit thermal expansion and can withstand disruption loads while maintaining thermal contact. The tiles and mounts for the CSC are required to fall within a total radial space allotment of only 14 mm. A unique design for mounting graphite tiles to the CSC was developed which utilizes drift (shear) pins and Inconel brackets. Installation is accomplished via hidden fasteners accessed through very small holes in the tile faces. Analyses of the CSC mounting structure were performed and pull tests were performed on assemblies which simulated the attachment geometry in an attempt to determine the ultimate strength of the configuration and the mechanism of failure