NbTi superconductors with low ac loss: A review

Niobium titanium is still the superconductor of choice for magnets where the field changes rapidly with time. To keep the hysteretic ac losses within bounds, it must be finely subdivided. For practical handling and reliable performance in magnets, it is therefore made in the form of filamentary composite wires, with NbTi filaments embedded in a matrix of copper. Unfortunately, the copper matrix introduces eddy current coupling losses, which must be controlled by twisting and by resistive barriers. To date, the major applications have been in particle accelerators and fusion, where high operating currents are required. To achieve these high currents, the filamentary wires are made into cables. Eddy current coupling between wires within the cable can cause additional ac losses. Problems of non-uniform current distribution between the wires can prevent magnets from reaching their full current, particularly with the largest cables.