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.
Accurate measurement of electromagnetic and thermal stresses in cryogenic environments on superconductors is quite challenging, since the environment involves cryogenics and large time-varying as well as dc transport currents through the superconductor. In off-normal cases, such as the transition of a superconducting state to a normal state, which is often referred to as the ‘quench’ of the magnet, a high voltage is developed across the superconductor and the strain gages including their signal conditioning electronics are necessarily required to be isolated. Cryogenics brings in a large drifting thermal offset in the measurement, whereas the presence of a magnetic field around the strain gage introduces a magneto-resistance error. An external bridge completion technique has been developed and effectively extended to measuring the stresses on the joint region of the superconductors with noisy environment and space constraints. This paper describes the stress measurement techniques in the space-restricted joint regions of the superconductors and validates the techniques in a repeatable fashion on measurements carried out on the inter-connecting joints of large superconducting magnet winding packs of the steady state superconducting tokamak. This technique is advantageous in situations where in situ full active bridge configuration is not feasible due to practical limitations.
Current transfer length (CTL) and current sharing with the normal paths for a metal sheathed MgB 2 superconductor are studied by analytical and numerical methods. The study was aimed at wire geometries with short sample lengths. The computations show low CTL values for Fe sheathed MgB 2 and significantly higher CTL for Cu sheathed MgB 2 . For short length samples a considerable part of the applied current is not transferred into the superconducting layer and flows through the sheath in Cu sheathed conductors. Close to the current contacts the heat generated due to current sharing with normal paths is significantly higher for the Fe sheath than for the Cu sheath.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.