Influence of soft ferromagnetic sections on the magnetic flux density profile of a large grain, bulk Y–Ba–Cu–O superconductor

Abstract

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).

Comparison Between Modeling and Experimental Results of Magnetic Flux Trapped in YBCO Bulks

“…For the following studies, n-values were computed on the basis of the expression provided in [19]: 30 at 77 K, 60 at 30 K, 180 at 12 K, at which values the trapped magnetic flux density is found insensitive. At a temperature of 77 K, for which measurements are available, the expected n-values for HTS bulks range from 18 to 45 according to [20]- [22], being consistent with the proposed value.…”

Trapped field properties of a Y–Ba–Cu–O bulk by pulsed field magnetization using a split coil inserted by iron yokes with various geometries and electromagnetic properties

“…In recent years, a ferromagnetic yoke has been inserted in the magnetizing fixture to increase the trapped magnetic field [12,13], in which permendur (50Fe+50Co alloy) and soft iron were used experimentally as a yoke material. We have also achieved a trapped field of over 3 T at 40 K on Gd-Ba-Cu-O disk bulk by PFM, employing a split coil with a pair of soft iron yokes [14], in which a symmetric trapped field profile can be also realized [15].…”

Numerical Simulation of the Performance of High-Temperature Superconducting Coils

“…However, the formulation also allows the introduction of magnetic sub-domains into the model by introducing a relative permeability μ r = 1, which may be not constant but varies with magnetic field, i.e. μ r (H ) [15][16][17][18][19][20]. The electrical behaviour of the superconductor is modelled using the E-J power law, where E is proportional to J n , and when n > 20, it becomes a good approximation of Bean's critical-state model (for which, n → ∞) [9].…”