Indentation Effects on Critical Performance of the New Developed WST High J_c Nb₃Sn Strand

To provide candidate materials for the high-field winding-package (WP) of the toroidal field coil (TF) in next-generation Chinese compact burning plasma tokamak, Western Superconductor Technology Co., Ltd (WST) has newly developed a type of high J_c Nb₃Sn wire with restacked-rod process (RRP) of the distributed barrier layout. To avoid the conductor degradation during electromagnetic and thermal cycling, the high-field WP is designed as a short-twist-pitch (STP) cable. After cabling and compaction of conductor, the STP design can cause severe indentation damage to the strands. The indentation not only impact the transport performance of the strand, in the meantime impact the thermo-magnetic stability. To investigate the effect of indentation on the performance for the WST high J_c Nb₃Sn strand, series measurements were carried out. In this paper, the critical current (I_c) measurement, residual resistance ratio (RRR) measurement, magnetization measurements, and V-I tests were performed on indented samples, and the results were compared with other two types of high J_c Nb₃Sn strands used in high-field WP. It was shown that the critical performance of the samples decreased significantly at an indentation depth of 0.4 mm, and the RRR value decreases with increasing indentation depth.