AC losses of NbTi superconducting wires with fine filament

An investigation was conducted of the AC losses in coils with NbTi superconducting wires. The authors used the multiple-stacking procedure to prepare the wires, which varied in filament size from 10 μm to 0.07 μm. Three kinds of wires with 0.07-μm filament were specially fabricated to estimate the proximity effect on AC losses. The distances between the filaments of these wires were 21 nm, 42 nm, and 70 nm, respectively. AC losses were measured for small coils at 60 Hz. The proximity effect was observed for the wires with 0.07-μm filament when the distance was 21 nm. Conversely, when the distance was 42 nm, the loss decreased to the small value of 11 kW/m³ at 0.5 T and 4.2 K