Bending-Strain Test Results of ITER Nb3Sn Wires and Preliminary Model Analysis

Bending effects on Nb 3 Sn wires have been investigated to understand the critical current degradation of large Nb 3 Sn superconducting cables, such as the ITER conductors. Characterizations of the critical currents of various Nb 3 Sn wires were carried out by a previously reported pure-bending strand test device using the 20 T, 195 mm Bitter magnet at the National High Magnetic Field Laboratory. The critical currents were measured over a large range of bending, up to 0.8% of the nominal bending strain at the wire surface. Irreversible degradations of the critical currents due to bending were also evaluated after straightening. Five different Nb 3 Sn wires were tested. Three of them were internal-tin wires of recent ITER TF US Oxford and Luvata wires and older EU EM-LMI wire, while two of them were a recent EU EAS and an older Furukawa bronze wire. The experimental data were evaluated with a newly developed integrated model that accounts for neutral axis shift, current transfer length, mechanical filament breakage and uniaxial strain release due to applying bending.