Validation of Homogenized Filament Bundle Model in AC Loss Computations

In high current ac applications, such as CERN and ITER magnets, superconducting cables are used. These cables typically consist of several individual strands and therefore have very complex geometries. Even a single NbTi strand designed for ac applications may consist of tens of thousands of filaments embedded in a normal conducting matrix. It is not practical to numerically model the cable or the strand at the filament level in the ac loss analysis due to extremely long computation times. However, an approximation for the ac losses can be computed if the filament zones or bundles are considered as a homogeneous mixture of superconducting filaments and matrix metal. This means, for example, substantial savings in the computation times of optimization procedures where hundreds of geometries are analysed. In this paper we compare the ac losses computed with both a homogenized bundle model and a detailed model (all or some of the bundles are modeled at the filament level) for different NbTi strand geometries to benchmark the accuracy of the bundle approximation. The results show that the bundle model is a suitable approximation, especially for certain bundle geometries.