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

Abstract

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 highconductivity 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.

SST-1 Toroidal Field Magnet Tests: Some Results and Lessons Learnt

“…9. It is evident from the statistics that the modified joint resistances are on average an order lower compared to the design value of 5 . This was one of the significant achievements of SST-1 Mission Team apart from the fact that the joints regions were never experimentally found to be the quench initiating zones.…”

“…3 for TF # 8. The cool-down was carried out in a controlled manner keeping a delta 'T' of 50 K and has established the resulting thermal stresses to be within the acceptable limit [5]. Fig.…”

“…Later on, with appropriate parallel planning, the average campaign duration could be reduced to eleven days. Each of these coils were equipped with nearly forty temperature sensors, twelve redundant voltage taps, four to eight strain gages, two hall probes, four flow meters, four pressure transducers [5]. Sensor signals were measured with in-house developed signal conditioning electronics [6].…”

Cryogenic Acceptance Tests of SST-1 Superconducting Coils

“…10 [15]. In addition, the displacement transducer, which was not certified by its vendor for use in cryogenic, magnetic, and vacuum environments, was tested in one such campaign.…”

SST-1 Status and Plans

“…Guaranteeing leak-tight magnet winding packs during the cooldown, warm-up, and current charging, a dc joint resistance was measured below 5 nΩ in operating conditions, with joints and termination region not being "initial quench zones" at any instances of the magnet operations. The joints of SST-1 interpancake and intercoil joints in SST-1 TF magnet winding packs have been designed and validated afresh keeping the aforementioned critical criteria [3], [4] through extensive experimental validations in their design, test protocols, and instrumentations. Subsequently, these joints have been extended onto the SST-1 TF and PF magnet winding packs and have been tested along with the coils.…”