Field penetration into hard type-II superconducting tubes: effects of a cap, a non-superconducting joint, and non-uniform superconducting properties

Abstract

Using the numerical method of Brandt (1998 Phys. Rev. B 58 6506), we study the penetration of a uniform magnetic field that is gradually applied parallel to the axis of finite type-II superconducting tubular samples with strong pinning. This study is carried out in view of designing low-frequency magnetic shields by exploiting the diamagnetic properties of type-II superconductors. First, we compare the field penetration into open and closed tubes. For long tubes (length larger than three times the outer diameter), we show that a cap weakly affects the maximum magnetic induction that can be shielded, but greatly increases the region over which the field is nearly uniform. When the length of the tube is shorter, both the maximum shieldable magnetic induction and the uniformity of the field attenuation are enhanced by closing the tube. We also show that making a hole in the cap, which is often necessary for applications, does not greatly affect the shielding properties provided the diameter of the hole is small compared to that of the tube (hole diameter smaller than a quarter of the outer tube diameter). In view of designing large size magnetic shields, superconducting tubes of finite size need to be joined together. We therefore examine in a second part how the presence of a non-superconducting joint between the tubes affects the shielding efficiency. It is shown that the effect of a joint depends upon its position along the tube axis and strongly increases with its size. Third, we study how non-uniform superconducting properties affect the shielding capabilities.

DC and AC Shielding Properties of Bulk High-Tc Superconducting Tubes

“…Above , the shielding currents cannot prevent the magnetic field from entering in the internal volume of the cylinder and the internal induction increases up to some value . On reversal of the applied magnetic field starting from the maximum value , the internal magnetic flux density remains constant until a value that is called is reached and then it follows hysteretic curve similar to what can be expected for an irreversible type-II superconductor [1], [2], [6].…”

“…cooling with liquid nitrogen at 77 K), high-temperature superconductors (HTS) can compete with soft ferromagnetic materials, both in terms of shielding factor (ratio of the applied field over the magnetic field inside the shield) and of threshold induction , i.e. the maximum induction that can be strongly attenuated by the shield [1], [2]. Manuscript A number of results can be found in the literature on the magnetic shielding properties of bulk polycrystalline tubes [5], [6], [8].…”

“…Manuscript A number of results can be found in the literature on the magnetic shielding properties of bulk polycrystalline tubes [5], [6], [8]. In our previous work in particular [1], [2], we have studied the magnetic shielding of a Bi-2223 polycrystalline tube subjected to an axial field. Experimental data have been confronted with a computer modeling based on the numerical model developed by Brandt [10].…”

Numerical Study of the Shielding Properties of Macroscopic Hybrid Ferromagnetic/Superconductor Hollow Cylinders

“…These structures have promising applications in bulk HTS motors. 3) Type-II superconducting tubes are able to shield lowfrequency magnetic fields in a more efficient way than a ferromagnetic cylinder would do [5], [6]. The axial and transverse magnetic shielding properties of HTS cylinders may even be improved by covering the HTS tube with a ferromagnetic sheath [19]- [24].…”

“…At the macroscopic scale, superconductors and ferromagnets also exhibit competing magnetic properties: In a given magnetic environment, the superconductor tends to expel the magnetic field lines from its volume by developing induced supercurrents [4]- [6]. By contrast, the ferromagnet, whose magnetic domains align with the external field, tends to concentrate the field lines inside its volume [7].…”