Heat Treatment and Structure Optimizations of Internal Tin Nb3Sn Superconducting Strands in WST

Western Superconducting Technologies produced approximately 35 tons of Nb
₃
Sn strands for international thermonuclear experiment reactor (ITER) Toroidal field coils using the internal tin process. All performances of the strands met the ITER requirement well. The delivery of Nb
₃
Sn strands was finished in 2015. At the present, several kinds of Nb
₃
Sn strands with different diameters and structures have been developed. In this study, the influence of heat treatment on different strand diameters was investigated. The results showed that extending the heat treatment time was effective for increasing Jc in strands with larger diameters. A new layout of Nb
₃
Sn strand with more subelements and gap between the filaments was also fabricated at Western Superconducting Technologies Corporation, Ltd. After a final heat treatment step for 250 h at 650 °C, the critical current density in the non-Cu areas of the improved 1.2 mm diameter strand was 1110 A/mm
²
at 12 T and 4.2 K, which was approximately 10% higher than the ITER strand. The hysteresis loss of the strand can be significantly reduced to 288 mJ/cm
³
at ±3 T and 4.2 K. Approximately 57 kg of this improved strand was contributed to the prototype conductor for the Toroidal field coils of the EURO fusion DEMO.