National Spherical Torus Experiment (NSTX) center stack design and assembly

The National Spherical Torus Experiment (NSTX) is a low aspect ratio, spherical torus (ST) configuration device under construction at PPPL. One of the most complex components of this device is the Center Stack Assembly (CSA). The CSA measures 5.7 meters (18.6 feet) in length and 34.2 cm. (13.476 in.) in diameter. It consists of the inner legs of the Toroidal Field (TF) winding, the Ohmic Heating (OH) solenoid and its associated tension cylinder, three inner Poloidal Field (PF) coils, thermal insulation, diagnostics and an Inconel casing which forms the inner wall of the vacuum vessel boundary. Some of the engineering issues associated with the CSA design and which are included in this paper are loads, load paths and stresses on the CSA, material selection including conductor, insulation, and manufacturing/assembly issues. An elaborate support structure is required for reacting loads in the CSA. This structure includes a tension tube for supporting the OH coil and reacting the axial loads and hub assemblies at each end of the CSA to react electromagnetic loads on the bolted TF flags to act together as a monolithic structure. High insulation shear stresses of 16.6 MPa (2.4 ksi) were anticipated in the inner TF coil legs. It was necessary to establish a R&D program to determine the maximum shear strengths and fatigue life of the selected insulation. These R&D test results are included in the paper. The CSA is designed to be disassembled from the vacuum vessel in the event that future repairs or upgrades are required. The dimensional restraints on the outside diameter of the CSA has resulted in reduced clearances between components. This along with the excessive lengths combine to make the assembly of the CSA components a critical phase of the program. This paper provides a description of the NSTX CSA and reviews those engineering issues associated with the design.