National Spherical Torus Experiment (NSTX) torus design, fabrication and assembly

The National Spherical Torus Experiment (NSTX) is a low aspect ratio spherical torus (ST) located at Princeton Plasma Physics Laboratory (PPPL), Fabrication, assembly, and initial power tests were completed in February of 1999. The majority of the design and construction efforts were constructed on the Torus system components. The Torus system includes the centerstack assembly, external poloidal and toroidal coil systems, vacuum vessel, torus support structure and plasma facing components (PFCs). NSTX's low aspect ratio required that the centerstack be made with the smallest radius possible. This, and the need to bake NSTX's carbon-carbon composite plasma facing components at 350 degrees C, was major drivers in the design of NSTX. The centerstack assembly consists of the inner legs of the toroidal field (TF) windings, 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. It took approximately nine months to complete the assembly of the centerstack. The tight radial clearances and the extreme length of the major components added complexity to the assembly of the centerstack components. The vacuum vessel was constructed of 304-stainless steel and required approximately seven months to complete and deliver to the Test Cell. Several of the issues associated with the construction of the vacuum vessel were control of dimensional stability following welding and controlling the permeability of the welds. A great deal of time and effort was devoted to defining the correct weld process and material selection to meet our design requirements. The PFCs will be baked out at 350 degrees C while the vessel is maintained at 150 degrees C. This required care in designing the supports so they can accommodate the high electromagnetic loads resulting from plasma disruptions and the resulting relative thermal expansions between the PFC's and the vacuum vessel on which supports are attached. This paper will provide a brief review of the issues associated with the design, fabrication and assembly of the NSTX Torus system including those outlined above.