JT-60SA NbTi Wire Characterization After Thermal Shock Due to Helium Inlet Welding

As a part of the He inlet process's qualification and validation for the JT-60SA Toroidal Field Coil Magnets, TIG welding of the nozzle to the conductor jacket is a delicate process: in fact, NbTi/Cu wires may be exposed to high temperatures, thus resulting in a degradation of their critical currents. During the He inlet TIG welding process, temperatures have been accurately recorded by using several thermocouples placed in contact with the superconducting cable. After welding, several samples have been extracted from the thermally affected zone (TAZ), as well as far from TAZ (virgin state samples). Moreover, other samples have been heat treated according to the acceptable welding limit conditions allowed in the technical specification (i.e., 500°C for 30 s). Their critical current densities have been evaluated through detailed magnetic characterization measurements (isothermal hysteresis loops) as a function of an applied field (up to 7 T) and at two different temperatures (4.4 and 6.2 K). We found that none of the extracted wires are affected by welding. This findings implies that the margins defined in the technical specifications of the Nb-Ti/Cu conductors' welding process should be taken in due consideration to take all necessary precautions to stay well below that limit. However, it is worth noting that in the welding process only a limited number of wires in the cable may be affected thus assuring an adequate safety margin.