Influência da geometria de anéis para geração de campos magnéticos inomogêneos em dispositivos supercondutores

Abstract

Conducting materials transport electric currents very effectively due to their low resistivity. A conductor's electric current distribution induces a magnetic field around it, which the Ampère-Maxwell Law can describe. On the other hand, a superconducting material transports current even more efficiently, as it has no resistance to the passage of electric current. In particular, a type-II superconductor admits magnetic flux penetration into its interior through magnetic flux quanta surrounded by supercurrents, named vortices, preserving the superconducting state. The organization of vortices in the sample depends on several factors, such as geometry and defects present in the sample. The so-called critical state models can describe the macroscopic relationship between the magnetic flux density inside the material and its critical current density. A commonly used strategy for increasing the critical current density of superconductors is to include artificial defect networks that anchor vortices. However, recent studies show that the same result can be obtained by cooling the superconductor under inhomogeneous magnetic fields. In this work, we investigate magnetic fields generated by transport currents in thin films, conductors, and superconductors, varying their geometries. The first part of this work is theoretical, containing simulations of thin films in the presence of applied field and transport currents. For that, the so-called TA formulation was used to deal with the thin film geometry for both cases. The constitutive relation E−J and the well-known Bean critical state model were also considered for superconductors. The simulations agreed with what was expected by the analytical results. They allowed us to conclude that, near the tape, fields generated by moderate currents carried by superconductors are more intense than those generated by conductors. Furthermore, considering critical current data for typical superconducting thin films prepared in our research group, we found that wider superconducting tapes generate more intense fields in their surroundings than thicker tapes. In the second part, aluminum strips with different widths were prepared and investigated to validate part of the simulations. Using the experimental technique of Magneto-Optical Imaging, the magnetic flux density distributions of these tapes were compared with the theoretical results, showing that it is possible to use the simulation techniques employed to describe realistic situations.