Characterization of Japan's DEMO Candidate Reinforced Nb₃Sn Wires Under Crossover Contact Stress

The size of Japan's demonstration power plant (JA DEMO) reactor is planned to be approximately 1.4 times larger than that of ITER magnet. Mechanical behavior and the reinforcement of the Nb ₃ Sn strands under a complex distribution of electromagnetic force in the cable-in-conduit conductor (CICC) are of great concern. In this study, an experimental setup for I _c measurement under crossover contact stress in external magnetic fields, which is regarded as more realistic situation in CICC, was constructed. Then, we studied superconducting properties and microstructures of JA DEMO candidate reinforced Nb ₃ Sn wires under crossover contact stress. CuNb-reinforced bronze-route Nb ₃ Sn wires made by Furukawa Electric and distributed-tin (DT) Nb ₃ Sn wires reinforced with brass matrix made by Kobe Steel were tested. The reinforced wires have shown better I _c properties against contact stress, compared with conventional Cu matrix wires. Under contact stress, the outer copper sheath was predominantly deformed, which is believed to contribute to relaxation of the stress concentration. The brass matrix seems to be significantly effective in suppressing distortion in the filament region. The use of reinforced Nb ₃ Sn wires increases the reliability of the TF magnet operation.