Reduction of AC losses in ultra-fine multifilamentary NbTi wires

The authors present AC loss measurements for a series of wires in which the proximity effect has been greatly reduced by increasing the distance between the filaments while retaining high values of the overall critical current density, typically 3000 A/mm² at 0.5 T. For the best performing wires, 50-Hz AC losses in W/(Am) (=(losses in W/m³)/(overall J_c in A/m²)) for small test coils are a factor of ten lower for 0.5-T peak field and a factor of three lower at 1-T peak field. For wires with negligible proximity coupling, losses at low fields are reduced compared to that calculated by the Bean model, due to the small number of flux lines that thread across the width of each filament