Abstract :
[en] Hybrid magnetic screens combining a disk-shaped high-temperature superconducting bulk with closed-loop coated conductors have been recently demonstrated to significantly overcome the screening properties of a single bulk. The purpose of the present work is to investigate how to scale up these hybrid screens, i.e. further increase the field attenuation and widen efficiently the screened region using more closed-loop coated conductors. First of all, an axisymmetric finite element model using the Gmsh and GetDP environments with the H-Φ formulation is implemented and used to study the influence of physical and geometrical parameters of the screens. The results show that the low-field screening factor SF and the screened surface area are strongly dependent on the geometry while having large Jc materials is mandatory to maintain the screening properties up to high fields. An important design rule is that the spacing between the concentric loops should be sufficiently small, which we explain physically by analysing the different paths followed by the flux lines. Then, experiments are carried out in liquid nitrogen (77 K) with GdBa2Cu3O7 samples. Mapping the three components of the flux density above the screens subjected to an inhomogeneous applied field gives the following key results. First, the combination of the bulk with four loops spaced as recommended by the numerical model allows the screened surface area (SF > 2) to be multiplied by ~ 9 with respect to the bulk alone. Second, reducing the asymmetry of the practical structure by flipping over some of the loops is highly beneficial for improving the screening properties: the maximum SF at 5.7 mm above the bulk is measured to be multiplied by 1.6 without any additional superconductor. Importantly, the design rules obtained from both the numerical model and the experiments can be further extended and applied to various geometries or sizes of hybrid superconducting screens.
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