Fabrication and Investigation of Low AC Loss Cu0.5Mn-based NbTi Superconducting Strands

The Cu-0.5 wt.% Mn alloy is selected as the inter-filament matrix in NbTi superconducting strand with low AC Loss. Three types of NbTi strand with the filament number of 10 800, 12 960 and 33 696 were prepared based on high homogeneity Nb47Ti alloy, pure Nb sheet, high purity oxygen free copper and Cu-0.5 wt.% Mn alloy. Heat treatment with 3-5 intermediate annealings has been applied to increase the Jc of the strands. When the number of annealings increases from 3 to 5, Jc raises from 2295 to 2958 A/mm2 for the 10 800 filament strand, and from ∼2500 to ∼2800 A/mm2 for the 12 960 filament and 33696 filament strands. As the filament number increases from 10800 to 33696, the ‘ n ’ values keep the same level, demonstrating that no “sausage like” or “nodular like” filaments exist in the final strands. When the filament diameter of the strand is reduced from 4.6 μm to 2.6 μm, the hysteresis loss decreased from 42.8 mJ/cm3 to 17.3 mJ/cm3 at ± 3 T&4.2 K. Based on this research, WST has obtained a stable mass production technology of the strands. Using heat treatment with 4 annealings, the different batches of the strands with Jc ranged from 2600 to 2800 A/mm2 have been produced with piece length level of 1-3 kilometer.