Solder development and fabrication techniques for coolant tube bonding in toroidal field conductors for the National Spherical Torus experiment

The National Spherical Torus Experiment (NSTX) is being enhanced to significantly expand the operational space for plasma conditions that are essential for next step Spherical Torus (ST) engineering and scientific development. Improvements to this leading, high performance ST include doubling the magnetic toroidal field (TF) and plasma current with a tenfold increase to the plasma discharge flat top duration. This requires a Center Stack Upgrade (CSU) with almost double the current capability of the TF inner conductors. Bonding water cooling tubes in these new TF Conductors required developing nonionic solder flux and a controlled, repeatable heating technique.