Reduction of AC losses in ultra-fine multifilamentary NbTi wires

The use of superconducting multifilamentary wires in 50/60 Hz AC applications is now possible due to advances in the new technology of ultra-fine filamentary wires (filaments 100 nm). While previous wires have shown very low losses, 1.5 10/sup -5/ W/(Am) and 5 10/sup -5/ W/(Am) at peak 50 Hz inductions of 0.5 T and 1 T respectively, (the unity W/(Am) = (losses in W/m/sup 3/)/(overall Jc in A/m/sup 2/) being a measure of the wire performance), the authors have shown that such wires are susceptible to proximity effects between neighboring filaments. In this article, they present AC loss measurements for a new series of wires where this proximity effect has been greatly reduced by increasing the distance between the filaments whilst retaining high values of the overall critical current density, typically 3000 A/mm/sup 2/ at 0.5 T. For the most performant wires 50 Hz AC losses in W/(Am) for small test coils are a factor of 10 lower for 0.5 T peak field and a factor of 3 lower at 1 T peak field. For wires with negligible proximity coupling there is a benefit from a reduction of losses at low fields as compared to that calculated by the Bean model, due to the small number of flux lines that thread across the width of each filament (increased entry field, surface currents, and reversible flux line motion)