Operational Experience with NSTX Demountable TF Joint

A key feature of the spherical torus (ST) is the demountable toroidal field (TF) coil, which requires a high current electrical joint in a high magnetic field. On NSTX a fixed joint relying on high pressure contact between silver plated copper conductors is deployed at the radius of the center bundle of conductors. In 2003 the original coil failed at a lower joint after 7200 pulses. A new coil was designed and fabricated. Operation commenced in 2004 at field levels up to 4.5kG. While the new coil performed better than its predecessor, measurements revealed that the structural support system was still not working as intended, so the operating level was reduced to 3kG midway through the run. Following conclusion of the 2004 campaign after 2800 pulses the coil was disassembled, and a major defect in the structural support system was corrected. The 2005 campaign consisted of 2500 pulses with routine operation at 4.5kG, and a limited number of pulses up to 5.75kG. Improvement in performance over 2004 was dramatic. This paper describes the engineering of the joint, its physical behavior, and the measurements and observations made thus far