Finite Element Simulations of Twisted NbTi Conductors

Low-temperature superconducting wires, NbTi and Nb
₃
Sn, designed for ac applications, such as CERN and ITER magnets, are composed of twisted multifilament structures. Under time-varying applied magnetic field, twisting decreases the induced electromotive forces between the filaments and is therefore an effective method to reduce interfilamentary coupling. In order to study coupling losses computationally with high precision, 3-D numerical models are needed. In this work, we use 3-D finite-element method simulations to study hysteresis and coupling losses in NbTi superconductors. We investigate the effect of twist pitch on ac losses. In practice, NbTi wires cannot be studied at the filament level due to the extremely complex geometries. However, the manufacturing of these wires is done by using filament bundles. Therefore, we consider wires that consist of homogenized filament bundles embedded in normal conducting matrix. In particular, we consider the effect of barriers around filament bundles and how filaments should be arranged in bundles to minimize the losses. The simulations show that the qualitative behavior of the model is consistent with the analytical results and it can be used, e.g., in optimization processes, where a comparison of wire geometries is needed. Additionally, when considering the coupling of filaments, the barrier plays a very important role in minimizing the losses.