Experimental study of the screening properties of quasi-bulk superconducting magnetic shields made of coated conductors

Abstract

This master thesis is dedicated to the experimental study of quasi-bulk superconducting magnetic shields made of coated conductors. The geometries studied are stacks of second generation GdBa2Cu3O7 coated conductors arranged in two quasi-bulk superconducting plates. One plate has an isotropic arrangement and the other plate has an anisotropic arrangement. First, the magnetic response of the plates is modelled by two theoretical models. One theoretical model discretizes coated conductors and the other theoretical model assumes infinite coated conductors. The two theoretical models are compared with each other. This shows that the discrete model is more accurate than the infinite model. The discrete model is used to compare experimental results while the infinite model allows to understand the dependence between the geometrical parameters and the magnetic field of the plates. Then, the magnetic field trapped by the plates is measured experimentally. The experimental results are compared to the discrete model. The coated conductors are assumed to be in the critical state and the critical current of the coated conductors is adjusted to have the same Bz component of the magnetic field at the centre of the plates. The critical currents of the coated conductors of the two plates are then obtained and the critical currents of the anisotropic plate is almost 3 times higher than that of the isotropic plate. To explain this difference, the effect of the geometrical parameters of the plates on the critical currents are analysed. In particular, the spatial distribution of the Bz component of the magnetic field is studied with the infinite model. Finally, the shielding properties of the two plates are studied as a function of the sweep rate of the applied field and the distance from the plates. The shielding properties are very different between the two plates. For the anisotropic plate, the shielding properties have two different behaviours depending on the applied field. For a low applied field, the anisotropic plate follows the behaviour of a type-I superconducting disk. For a high applied field, the anisotropic plate is in critical state. For the isotropic plate, the shielding properties are found to vary smoothly with the applied magnetic field. The isotropic plate is not in critical state and follow the behaviour of a type-I superconductor only for very low applied field. The screening properties of the two plates are strongly dependent on the arrangement of the coated conductors.