Indentation effects on strain sensitivity of critical current for internal-tin Nb3Sn strand

The cable-in-conduit conductor (CICC) technology has been widely used in large scale superconducting magnet. The Short-Twist-Pitch (STP) design was aimed to avoid the conductor degradation during electromagnetic and thermal cycling. But manufacture of STP CICC deforms the cabled strands significantly. Indentations appear on superconducting strands after cabling and compaction. The conductor used for China Fusion Engineering Test Reactor (CFETR) magnet will be subjected to much higher Lorentz force than ITER. The STP CICC will be the first choice for the conductor design, since it showed no observed current sharing temperature Tcs degradation after electromagnetic cycling. In order to estimate the effects of indentation on the superconducting and mechanical properties of Nb3Sn strands, series measurements have been done on an internal-tin (IT) Nb3Sn strand, fabricated by Western Superconducting Technologies Co., Ltd (WST). The strand was artificially indented and cut to several sections for different kinds of measurements, which mainly included the critical current Ic versus axial strain measurements with Pacman device and stress-strain tests. The results show that the indentation has less impact on Ic, but more impact on mechanical performance. The sample preparation, test results and analysis have been described in details.