Critical current asymmetry in HTS switches using iron-core electromagnets

Iron-core electromagnets are widely used as applied-field switches in high-temperature superconducting (HTS) flux pumps. It was recently reported that the critical current ( Ic ) of the HTS wire is suppressed by up to 45% in the air gap of the iron core. Here, it is reported that the self-field Ic of a single tape (unifilar) is suppressed from 357 A to 195 A while in the air gap of an iron core, agreeing closely with the previous study. This is extended by showing that the Ic response to a perpendicular magnetic field is asymmetric, with a peak Ic of 249 A found to be at 0.16 T of applied magnetic field. This effect can be eliminated using an anti-parallel current configuration (bifilar), which is shown experimentally to recover close to the no-core self-field Ic of 357 A with no observed asymmetry. The results of both types of switch configurations are closely replicated in finite-element modelling using COMSOL Multiphysics®. This is caused by the asymmetric interaction between the self-induced magnetic field of the HTS switch element and the magnetic circuit of the iron-core electromagnet.