Magnetic shielding of open and semi-closed tubes made of bulk superconductors : the role of a cap

Amongst the range of prospective applications for bulk superconductors, the shielding of high magnetic fields at low frequency is extremely promising. Unlike conventional high permeability ferromagnetic materials which exhibit a relatively low saturation magnetization (e.g. µ0Msat ~ 0.75 T for mumetal), superconductors do not suffer from this drawback. They are therefore well suited to shield magnetic flux densities above 1 tesla. This can be achieved by hollow bulk superconductors of cylindrical geometry. The shielding efficiency and shielded volume are both strongly improved when the hollowed tube is closed at one extremity. As an example, we showed recently [1] that YBa2Cu3O7 (YBCO) tubes of ~ 10 mm inner diameter closed at one extremity and engineered by a buffer-aided top seeded melt growth fabrication process were able to attenuate axial flux densities of 1.5 T by a factor of 100 at 20 K. Such record performances are partly due to the presence of a cap at one extremity. The aim of the present work is to examine further and understand the influence of a cap on the shielding properties for various configurations.

In this communication we investigate the shielding properties of hollow and semi-closed bulk superconducting tubes at 77 K. For the semi-closed configuration, we compare situations the cap is either placed against the extremity (with a small controllable gap between them) or welded naturally. The field to be shielded is either parallel or perpendicular to the main axis of the tube. Both YBCO tubes produced by buffer-aided top seeded melt growth and commercial Bi-2223 tubes are studied. The experimental results are compared to finite element numerical modelling carried out either in 2D (for the axial configuration) or 3D (for the transverse configuration). In the axial configuration, the results show that a small gap between the tube and the cap is not detrimental to the shielding properties. We also show that the shielded volume can be easily enhanced by increasing the thickness of the cap or its critical current density. In the transverse configuration, the results point out the crucial role of the superconducting joint between the tube and the cap. The need for a superconducting joint depends therefore on the main direction of the magnetic field to be shielded.

References:
[1] L. Wéra. et al., IEEE. Trans. Appl. Supercond. 27 (2017) 6800305