Center Stack Casing Bakeout BUS Design for the NSTX-Upgrade Fusion Device

The National Spherical Torus eXperiment (NSTX) has undergone a major upgrade to NSTX-U at the Princeton Plasma Physics Laboratory. NSTX-U will double the toroidal field, plasma current, and neutral beam injection heating power, as well as significantly increase the pulse duration. The plasma-facing components (PFCs) in the NSTX-U vacuum vessel are mainly graphite, which has a total surface area of about 41 m². To achieve high vacuum and reduce impurity from PFCs during operation, it is important to bake the graphite parts and remove most of the moisture absorbed by graphite during installation. Typical bakeout for NSTX-U lasts about 3 to 4 weeks. The NSTX-U inner vacuum vessel, i.e., the center stack casing, will be heated to about 450°C by passing 8 KA direct current through it during bakeout. The design of the bakeout bus directly attached to the casing flanges at vessel top and bottom are covered in detail in this paper. At the vessel top, the water-cooled bus terminal is subject to high thermal growth (about 18 mm in the vertical direction and 3 mm in the radial direction). At the vessel bottom, the bakeout bus must withstand 120 KA of halo current during disruption, as well as dislocation from thermal growth. This paper covers the design to address all these challenges. A machined CuCrZr terminal with internal water-cooling channels was used to prevent any brazing work in high stress areas. Detailed analysis will also be covered to show that the proposed design can satisfy thermal, structural, and fatigue requirements during bakeout and operation.