Vacuum compatability of welded joints for NSTX-U

The high vacuum environment of the National Spherical Torus Experiment (NSTX) cannot tolerate a leak rate of greater than 1 × 10-5 Torr-liters per second or an out-gassing rate of more than 2 × 10-12 Torr-liters per second per cm². This is maintained with an austenitic 316 stainless steel vacuum vessel. The NSTX Upgrade (NSTX-U) will require large weldments attached to the current vacuum vessel that will become the new vacuum boundary. Due to the lack of superstructure, all loads are passed through the vacuum vessel. This means the welds must carry a substantial load as well as provide vacuum integrity. Distortion of the vessel must also be minimized in order to accommodate precisely-aligned diagnostics as well as to mate to the new second neutral beam. The ideal candidate for these welds is Flux Cored Arc Welding (FCAW). FCAW can deposit a great amount of weld material quickly with minimal heat input. This paper will discuss the vacuum compatibility of FCAW and compare it to other standard welding processes used on NSTX.