Status of Internal-Tin Nb3Sn Strand for Divertor Tokamak Test Facility

Divertor Tokamak Test facility (DTT) is going to be built in Italy, and is expected to be operated along with ITER to explore the potential solution for the power exhaust to be eventually implemented in DEMO. 77 tons of Nb3Sn strand in two types are required by DTT. For type one, the weight is 55 tons. The dominant performance requirements are I-c >285 A (4.2 K, 12 T) and hysteresis loss <1000 mJ/cm(3) (4.2 K, +/- 3 T). For type two, the weight is 22 tons. The dominant performance requirements are Ic >260 A (4.2 K, 12 T) and hysteresis loss <400 mJ/cm(3) (4.2 K, +/- 3 T). These performance requirements are quite challenging in the respect of performance stability in Nb3Sn strand production. WST has successfully developed three types of Nb3Sn strand to meet these requirements especially for DTT. For DTT type one, considering the ultra-high level of I-c, Nb and Sn volume was increased based on the structure of WST Nb3Sn strand for ITER. I-c >300 A (4.2 K, 12 T) and hysteresis loss <700 mJ/cm(3) (4.2 K, +/- 3 T) have been obtained from both 164-filament structure and larger-filament structure. For DTT type two, Cu split structure was selected to guarantee low hysteresis level. For the structure with 160 filaments and one Cu split, I-c >260 A (4.2 K, 12 T) and hysteresis loss <400 mJ/cm(3) (4.2 K, +/- 3 T) have been achieved. Finally, all the performance of DTT Nb3Sn strand has been accomplished by WST.