Abstract Body

Many large-scale engineering applications involve low-frequency magnetic fields of high amplitude extending over wide surfaces. When equipment sensitive to magnetic fields (e.g., a cryocooler) is placed near such devices, the large stray magnetic field must be screened. Superconductors can act as excellent screens thanks to the current loops appearing when they are subjected to a time-varying applied field. Unlike ferromagnetic materials, superconductors are able to screen efficiently well above the tesla range: they are thus well suited for high-field applications.

Among high-temperature superconductors (HTS), bulk materials and coated conductors (CCs) can be considered for magnetic screening. High-quality HTS bulks exhibit excellent screening properties but they are hardly scalable over ~10 cm². By contrast, structures made of CCs can easily be made larger than 10 cm² but provide limited screening at their centre. In a previous work [1], we demonstrated that hybrid screens, combining a disk-shaped bulk with closed-loop CCs, significantly surpass HTS bulks in terms of screening efficiency and extension of the screened region. In order to create a closed loop, a slit is milled in the middle of a CC segment and both halves are separated around a cylindrical holder. The resulting “eye-shaped” structure is asymmetric and there exists a vertical offset between both halves. The typical geometry of a hybrid screen is shown in Fig. 1.

In this work, we describe and demonstrate experimentally how to extend the size of such hybrid HTS screens in practice so that they can be scaled up efficiently while using a bulk superconductor of typical size.

Experiments are carried out using a 30 mm-diameter disk-shaped bulk made of GdBCO/Ag from CAN Superconductors and closed-loop CCs made out of second-generation GdBCO tapes from Shanghai Superconductor Technology. The screens are cooled down using liquid nitrogen. They are subjected to a DC inhomogeneous field reaching ~100 mT at the bottom surface of the bulk. A bespoke 3-axis cryogenic Hall probe [2] attached to a 3D displacement system is used to measure B_x,B_y,B_z above the screen.

Understanding physically the behaviour of hybrid screens is the first step towards obtaining large screened regions in practice. As explained in [1], provided that the superconductors are weakly penetrated, the flux lines need to meander around them to reach the region above the screen. The flux lines are therefore diverted either all around the loops or are forced to go near the superconducting loops. The point is that loops can oppose at best the flux lines passing close to them because the opposite field generated by the induced currents in the loops is maximal close to their surface. Hence, superconducting screens acting over wide surfaces can be designed by adding several closed-loop CCs of increasing sizes around the bulk. If the spacing between the loops is sufficiently small, increasing the number of loops enables less flux lines to meander all around the screen. This process of adding larger and larger loops can be extended easily.

We build a wide screen experimentally by adding 45, 60, 75 and 90 mm-diameter loops around the 30 mm-diameter disk-shaped bulk. In this case, the screened surface, for which the flux density at 3.7 mm above the screen is attenuated by a factor 2 or more, is roughly a 92 mm-diameter circle. In comparison, the screened surface, at the same height, for the bulk alone is equivalent to a 32 mm-diameter circle.

The second part of this work focuses on the detrimental effect of the asymmetry of the loops on their screening ability. This asymmetry induces a transverse component which strongly limits the maximum attenuation of the field. Experimentally, this limitation can be solved by creating a hybrid screen whose one loop out of two is reversed. In other words, the upper part of the loops is alternatively on one side of the bulk then on the other side. For a screen with 45 and 60 mm-diameter loops around the bulk, both configurations are illustrated in Fig. 2. Measurements show that the maximum screening factor SF = at 3.7 mm above the screen increases from 38 to 66. Additionally, the spatial distribution of SF is much less asymmetric as shown in Fig. 3. In comparison, the maximum measured SF for the bulk alone at this height is 18.

In conclusion, this work presents and explains two important design rules that should be applied to build practical hybrid superconducting screens for large surfaces. First, adding loops of increasing size around the bulk allows the screened region to be significantly extended while keeping the dimensions of the bulk unchanged. Second, symmetrizing the design by alternating the orientation of neighbouring loops largely improves the field attenuation.

References

[1] Rotheudt N et al (2024) Supercond. Sci. Technol. 37 065008

[2] Rotheudt N et al (2023) Cryogenics 133 103693

Acknowledgment

N. Rotheudt is recipient of a research grant from F.R.S-FNRS. This work is supported by F.R.S.- FNRS grant CDR J.0184.23.

Keywords: Magnetic screening, hybrid superconducting screens, HTS, bulk, coated conductors, magnetic measurements.