Magnetic screening using superconducting bulks and coated conductors: the road to scalability (invited)

Recent developments in large-scale engineering superconducting devices (e.g. rotating machines) involve magnetic flux densities of the order of 1 T and above. These devices often have a stray field of high magnitude, at low frequency and extending over wide surfaces. This stray field may disturb neighbouring equipment and should therefore be reduced, which we call “magnetic screening”. At these field levels, flux lines are no longer channelled in a ferromagnetic material and only superconductors provide an efficient magnetic screening thanks to the current loops induced when a time-varying magnetic field is applied to the superconductors. Nowadays, one of the greatest challenges is to make superconducting screens scalable, i.e. such that their size is compatible with that of large-scale applications. The goal of this communication is to describe different solutions that have been proposed to tackle this challenge and to present ways to improve the potential of superconducting screens.

Among high-temperature superconductors, both bulk materials and coated conductors have been considered as solutions for magnetic screening. High-quality superconducting bulks exhibit excellent screening properties but they are hardly scalable over ~20 cm². Hence, a possibility explored in the literature was to juxtapose and stack several bulk superconductors. The challenge in this case was to mitigate the magnetic leakage occurring through the gaps between the bulks. On the other hand, coated conductors can be used as large magnetic screens, either by juxtaposing several wide (~ 40 mm) tape segments with a certain overlap, or by creating closed superconducting loops. These closed-loop coated conductors can easily be made larger than 20 cm² but provide a limited magnetic screening effect at their centre.

More recently [1], we have demonstrated that hybrid screens combining a disk-shaped bulk with closed-loop coated conductors significantly surpass both bulks and coated conductors alone in terms of screening efficiency and extension of the screened region. In this communication, we focus on explaining physically why such improvements are obtained. Then, we present some experimental results highlighting the scalability of hybrid superconducting screens. For example, using a 30 mm-diameter bulk and 4 concentric closed-loop coated conductors, the screened surface, for which the flux density at 4.7 mm above the screen is attenuated by a factor 2 or more, is roughly a 94 mm-diameter disk (~ 69 cm²).

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