Flux penetration of an HTS coated-conductor tape by an approaching permanent magnet

Interactions between inhomogeneous magnetic fields and a superconductor lie at the heart of several high-temperature superconducting (HTS) devices. An example is the situation where the magnetic field from a small permanent magnet (PM) is applied to a coated-conductor HTS tape, such that flux penetrates the superconductor. The PM field is highly spatially inhomogeneous, such that there is significant variation in the applied field magnitude and direction across the tape. This leads to a varying local critical current density, J(c)(B, theta), across the tape, and consequently different 'shielding' (magnetic flux penetration) behaviour than is the case for a uniform applied field. Here, we report results from measurements and numerical simulations of the penetrating magnetic field within an HTS coated-conductor tape which occur as a PM dipole approaches from a distance. Electromagnetic simulations were performed using a finite-element model based on the H-formulation, and which incorporated experimentally-measured anisotropic J(c)(B, theta) properties from a coated-conductor tape. This showed good agreement with experimental measurements. The effects of varying key modelling assumptions and parameters were then studied, including changing the widths of the PM and HTS tape and the magnitude of J(c)(B, theta). The inhomogeneity of the PM field leads to a characteristic gull-wing distribution of magnetic flux across the superconductor at elevated applied fields. Close approach of the PM to the tape suppresses J(c) in the centre of the tape, and this results in the observation of a characteristic maxima for the total shielding currents circulating in the tape. A figure of merit is introduced which uses this effect to provide a threshold definition for 'full flux penetration' of an HTS tape by an inhomogeneous dipole field.