Progress on Internal-Tin Route Nb₃Sn Superconducting Strand for ITER in China

Abstract

Multi-filamentary Nb3Sn strand for ITER has been successfully fabricated by a conventional internal-tin (IT) route at Western Superconducting Technologies, Co. Ltd. All the performances of the IT Nb3Sn strand is able tomeet the ITER specifications. In 2010, one hundred kilograms of IT Nb3Sn strand has been cabled, jacketed and integrated into conductor sample titled with TFCN2 by ASIPP in China and tested in CRPP, Switzerland. After 1200 cycles warm-up and cooling down, Tcs of the TFCN2 Sultan sample at 10.78 T, 4.5 K are 6.35 K for the left leg and 6.37 K for the right leg, respectively, which is larger than the ITER lower limit of 5.7 K. This is the first successful Sultan sample made of Chinese Nb3Sn strand. In 2011, the influence of applied axial strain on critical current (Ic) was investigated using TFCN2 strand in the University of Twente, the Netherland. The irreversible degradation of Ic starts around +0.23% tensile intrinsic strain. The heat treatment influence of the duration at 650 °C on microstructure and performance was also studied. It was found that the fraction of un-reacted Nb after heat treatment is dependent on the position in the cross section. The hysteresis loss increases much faster than Ic when the duration at 650 °C increased.