Experimental studies of transverse stress effects on the critical current of a sub-sized Nb₃Sn superconducting cable

Large superconducting magnets will play a central role in the success of the International Thermonuclear Experimental Reactor (ITER) and for the future of fusion energy. Cable-in-conduit conductors (CICC) will be used for the ITER magnets. As a CICC Is energized, electromagnetic forces accumulate across the conductor, pressing strands transversely against one side of the conduit. We have developed a device to study the effect of transverse stress on a sub-sized cable using a mechanical load that simulates the Lorentz loads in the ITER Central Solenoid conductor. The test sample is a single turn (about 110 mm diameter) circular cable composed of 36 superconducting strands (cabling pattern of 3 × 3 × 4). The transverse stress is applied to the cable using a conical wedge that converts a vertical force into a radial (transverse) force. The vertical force is provided by a linear actuator. The stress on the cable is measured using two independent techniques: (1) strain gages applied directly to the ring where the cable is located and (2) a load cell located outside the cryostat. The device was successfully tested using a 20 T, 190 mm bore Bitter magnet at National High Magnetic Field Laboratory (NHMFL), FSU, and a first series of tests were carried out showing a degradation of the critical current as a function of transverse stress. The maximum transverse stress was 100 MPa, for which the cable critical current degraded to roughly 30% of its initial value.