Bulk, high temperature superconductors have significant potential for use as powerful permanent magnets in a variety of practical applications due to their ability to trap record magnetic fields. In this paper, soft ferromagnetic sections are combined with a bulk, large grain Y-Ba-Cu-O (YBCO) high temperature superconductor to form superconductor/ferromagnet (SC/FM) hybrid structures. We study how the ferromagnetic sections influence the shape of the profile of the trapped magnetic induction at the surface of each structure and report the surface magnetic flux density measured by Hall probe mapping. These configurations have been modelled using a 2D axisymmetric finite element method based on the H-formulation and the results show excellent qualitative and quantitative agreement with the experimental measurements. The model has also been used to study the magnetic flux distribution and predict the behaviour for other constitutive laws and geometries. The results show that the ferromagnetic material acts as a magnetic shield, but the flux density and its gradient are enhanced on the face opposite to the ferromagnet. The thickness and saturation magnetization of the ferromagnetic material are important and a characteristic ferromagnet thickness d* is derived: below d*, saturation of the ferromagnet occurs, and above d*, a weak thicknessdependence is observed. The influence of the ferromagnet is observed even if its saturation magnetization is lower than the trapped flux density of the superconductor. Conversely, thin ferromagnetic discs can be driven to full saturation even though the outer magnetic field is much smaller than their saturation magnetization. [5,6]) is well beyond the saturation magnetization of conventional ferromagnets. This makes them extremely promising as a competing technology for traditional permanent magnets in various applications [7][8][9][10]. The combination of ferromagnetic and superconducting materials can enhance the performance of the superconductor [11,12] and even lead to new applications [13]. Large grain, bulk superconductors are often used in applications that incorporate ferromagnetic materials, such as in motors and generators [14,15]. Ferromagnets can also increase the force in levitation systems [16,17] and close the magnetic circuit, which improves the available flux produced by bulk superconductors [18]. Similarly, ferromagnetic materials are used as sheaths around multifilament wires and tapes [19,20] or as magnetic flux diverters to modify the flux distribution around tapes and superconducting coil magnets [21][22][23][24][25][26][27][28]; thereby improving their electrical properties (i.e. increasing the critical current and reducing AC losses).

YBCO tubes of ~ 10 mm diameter closed at one extremity were engineered by a Buffer-Aided Top Seeded Melt Growth fabrication process (BA-TSMG). These tubes can act as efficient "dc" magnetic shields and are observed to reduce axial flux densities of 1.5 T by a factor of 100 at 20 K. Such performances are comparable in magnitude to the record threshold inductions reported for bulk MgB2 and Bi-2212 materials at lower temperatures. Magnetic shielding measurements for open and closed tubes at 77 K also show that the presence of the cap improves substantially the shielding performance at the closed extremity since it reduces the penetration through the open end. This fabrication technique is extremely promising for shielding "dc" stray fields generated by HTS magnets operated in a temperature range obtained by cryocoolers, liquid hydrogen (20 K) or liquid neon (27 K).

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In this paper we investigate the magnetic shielding of hollow and semi-closed bulk superconducting tubes at 77 K. We first consider the properties of a commercial Bi-2223 tube closed by a disk-shaped cap placed against its extremity. The results are compared to those obtained on a bulk large grain Y-Ba-Cu-O (YBCO) tube produced by buffer-aided top seeded melt growth. In this process, the disk-shaped pellet and the tubular sample are grown together, resulting in a tube naturally closed at one extremity. The field to be shielded is either parallel or perpendicular to the main axis of the tube. The experimental results are compared with the results of 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 the shielded volume can be enhanced easily by increasing the thickness of the cap. In the transverse configuration, the results show the critical role played by the superconducting current loops flowing between the tube and the cap for magnetic shielding. If the tube and the cap are separated by a non-superconducting joint or air gap, the presence of a cap leads to only a small improvement of the transverse shielding factor, even for a configuration where the gap between the cap and the tube contains a 90° bend. The cap leads to a significant increase in the transverse shielding when the cap and the tube are naturally grown in the same process, i.e. made of a continuous superconducting material. The experimental results can be reproduced qualitatively by 3D numerical modelling.

Persistent current loops of arbitrary size can be made from currently manufactured RE123 coated conductors. Our previous work has shown that an assembly of such loops is able to shield effectively quasi static axial magnetic field due to the absence of resistive joint. The shielding effectiveness depends on the aspect ratio and the number of layers. In the present work we study experimentally the detailed magnetic response of two different configurations of the magnetic shields for various orientations of the applied field. Using a 3-axis Hall probe we determine the amplitude of magnetic field generated by the induced persistent currents and its direction with respect to the applied field. The effectiveness of the uniaxial shield decreases strongly when the applied field is not collinear with its axis. In the triaxial (Polywell type) structure comprised of three pairs of coils whose axes are mutually orthogonal, the field attenuation is shown to be only weakly dependent on the direction of the magnetic field. We discuss the properties of the triaxial shield and the ways to improve its screening performance.