Superconductors - Wire structure and behavior

A superconductor is a priori the ideal material for electrical engineers, since it carries high current densities without being dissipative at all, at least when its electromagnetic environment remains constant over time. However, this non-dissipative state is limited by three quantities: the critical temperature (T _c ), the critical current density (J _c ) and the irreversibility field (H*). These three quantities form a surface, known as the critical surface, in the space of temperature, current density and magnetic field. For some superconductors, this surface can be modified by mechanical stress. The thermal limit is the most restrictive for the user, at least for low-temperature superconductors, which remain by far the most widely used materials. This document therefore focuses on the multifilament structure of "low T _c " superconducting wires. High-temperature superconductors, however, are often mentioned. The rise in operating temperature and the consequences for characteristic quantities are analyzed.

In this document, the superconductor is considered macroscopically with a simple but representative model: the critical state model and its simplified version, the Bean model, the physics of which is covered in the article Superconductors. Theoretical basis .

After presenting this model, we apply it to the magnetization of a superconductor, one of its fundamental characteristics. We then explain how the twisted multifilament structure stabilizes the superconducting state and enables safe, satisfactory operation, taking into account the specific features of superconductors and material properties at low temperatures. Alternating current (ac) losses are discussed, before concluding with the transition and protection of the superconducting wire in a superconducting device.