Abstract
The ac losses induced by an alternating transport current in type-II superconductors is a well-known phenomenon, which still attracts much attention because of its intrinsic relevance for the proper development of practical applications. In the case of single core superconducting cables of cylindrical cross-section, it is possible to find exact analytical solutions at self field conditions, and it has been believed for nearly two decades that the use of an ideal soft ferromagnetic sheath with negligible magnetization losses will not affect the electromagnetic properties of the superconducting wire, and on the contrary due to the shielding magnetic properties of the ferromagnet, the total ac losses of the SC wire have to be reduced or as maximum they must be equal to the one for the bare superconductor at self-field conditions, what contraries the experimental evidences that show a non-negligible increase on the ac losses. In this paper, we explain the physical nature of this mysterious increase in the ac losses for rounded superconducting/ferromagnetic heterostrutures, which for the sake of generality, has been solved within the critical state theory, and a magnetic multipolar expansion that enables the direct coupling of the magnetostatic properties of the superconductor and an ideal soft ferromagnet. A significant increase in the transient electric field during excitation period has been observed, which might have utter implications on the adequate selection of insulation materials for superconducting/ferromagnetic heterostructures.
Highlights
M ACROSCOPIC Superconducting/Ferromagnetic heterostructures have been the focus of considerable attention in recent years due to the novel phenomena and applications that can be envisaged with their use [1]–[9]
The electromagnetic response of a SCFM heterostructure composed by a type-II SC of cylindrical cross section and radius R1, it embedded within an ideal soft ferromagnetic sheath (FM) of relative magnetic permeability μr = 46 and radius R2 = 1.5R1, has been presented
Our model is based onto the variational formulation of the generalized critical state (CS) model introduced in [28], together with an exact analytical calculation of the magnetostatic coupling between the SC and the ferromagnetic material (FM) in cylindrical topologies (figure 1 (c))
Summary
M ACROSCOPIC Superconducting/Ferromagnetic heterostructures have been the focus of considerable attention in recent years due to the novel phenomena and applications that can be envisaged with their use [1]–[9]. A non negligible increase in the AC losses of these systems has been already reported for multiple SCFM configurations either under external magnetic field conditions (no transport current) [23], or in the absence of an applied magnetic field but with the presence of a transport current [23]–[25], which does not match with the losses provided by the bare FM at low field frequencies comparable with the self-field magnitude [26]. Tion III we show some of our obtained results when an ideal FM sheath of relative magnetic permeability μr = 46 (a common value for Fe-based FMs) and outer radius R2 = 1.5R1 is considered, providing a solid physical explanation to the origin of the increment in the AC losses of SCFM rounded heterostructures at self-field conditions.
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