Measurement of Electromagnetic and Thermal Stresses on Conduction-Cooled Joints of the SST-1 Spare TF Coil

Low-dc-resistance superconducting joints in toroidal- and poloidal-field (TF and PF, respectively) coils of the steady-state superconducting tokamak-1 (SST-1) at the Institute for Plasma Research (IPR) is under testing. The feasibility of conduction-cooled leak-tight joints made between two double pancakes in the winding pack of the TF coil is validated through experiments. The configuration of these conduction-cooled joints is comprised of a prefabricated SS304L oxygen-free high-conductivity copper leak-tight termination, into which the unconduited and soldered portion of the cable-in-conduit-conductor (CICC) is inserted. Once the cable space is inserted inside the prefabricated piece, solder filling is carried out, and the joints are realized by overlapping the mating ends and soldering them together. The supercritical helium flowing through the CICC exits prior to the termination length, and the joints are cooled by conduction. The joints are subjected to I × B-induced and bending-induced stresses during SST-1 operational scenarios. These stresses can lead to leaks in the joint region if they exceed the material strength or the brazing/welding strength. Both the thermal and electromagnetic stresses that developed at the copper-stainless steel prefabricated brazed region are measured on the SST-1 spare TF coil. These stresses are measured using the strain gauges during the cooldown and the charging of the spare TF coil up to its operational current of 10 kA at a conventional 4.5 K and 4 bar of supercritical helium forced flow. The electromagnetic-stress behavior at the time of quench that occurred accidently during the spare TF coil test at an 8 kA transport current was also studied. The signal-conditioning electronics required for this measurement are engineered and tested at the IPR before its implementation to the spare-TF-coil test campaign. The measured thermal and electromagnetic stresses are found to be in good agreement with the simulated finite-element Ansys results.