Use of interleaved bulks and coated conductors to screen the stray magnetic field of a large superconducting coil from a high-torque rotating machine

Large superconducting magnetic devices often produce an inhomogeneous stray magnetic field extending over wide surface areas which must be attenuated. In this work, we introduce high-temperature superconducting magnetic screens (or shields) where four 60 mm-diameter disk-shaped bulks are arranged side-by-side, combined with interleaved closed-loop coated conductors and short stacked tapes. We subject these hybrid screens to the inhomogeneous stray magnetic field of a large superconducting coil designed to demonstrate a rotating machine pole of a direct-drive wind turbine. Experiments carried out in liquid nitrogen and for an applied magnetic field up to ∼ 150 mT demonstrate that adding the coated conductors around the bulks mitigates the presence of the gaps between the bulks and provides a lightweight and effective solution to increase the surface area over which the applied field is attenuated. Suitable field attenuations (screening factor SF ≥ 1.5 at low field) are obtained over at least 138 mm. Then, we explain the measurements by analysing how the axial and transverse components of the field are affected by the presence of the screen. Apprehending how the magnetic flux lines are diverted allows us to explain the influence of the inhomogeneity of the applied field on the screening (or shielding) properties. Finally, we use trapped field measurements to confirm the role of the different superconducting parts of the screen.