Development of Internal Tin Nb₃Sn Strand for CRAFT in WST

Western Superconducting Technologies Co. Ltd. (WST) has developed several types of internal tin (IT) process Nb₃Sn strands for the Comprehensive Research Facility for Fusion Technology (CRAFT) project. By increasing the number of niobium filaments and the tin content, the Nb₃Sn phase was more effectively formed after heat treatment, thereby enhancing the critical current of the strands. The distribution of the niobium filaments was also optimized to minimize hysteresis loss. To date, WST has prepared over twenty tons of strands with a diameter of 0.99 mm for toroidal field (TF) coils. The critical current ranges from 370 A to 450 A at 12 T and 4.2 K. Additionally, the hysteresis loss ranges from 272 mJ/cm³ to 567 mJ/cm³ at ± 3 T and 4.2 K. Compared to ITER strands, the average hysteresis loss of 432 mJ/cm³ was reduced by 10%. Furthermore, approximately eight tons of strands with a diameter of 0.82 mm have been produced for Superconducting Conductor Testing Facility. The critical current ranges from 293 A to 359 A at 12 T and 4.2 K, which is 20% higher on average than that of ITER strands. All the performances of the strands fabricated by WST meet the requirements of CRAFT project.