Investigation of passive magnetic shielding methods combining bulk superconductors and coated conductors

A lot of engineering applications require the development of electrical devices with high power densities and involve large magnetic fields. These devices are generally character- ized by high values of stray flux densities potentially detrimental to their environment. In this context, it is necessary to screen these devices by means of effective magnetic shields. Large flux densities in the low frequency regime (typically < 100 Hz) cannot be efficiently shielded by conventional ferromagnetic or conducting materials and require the use of superconductors. The goal of this thesis is to investigate methods to combine the properties of bulk high-temperature superconductors and second generation coated conductors in order to design sizable passive magnetic shields.

In order to develop efficient magnetic shields, it is necessary to obtain information on the properties of the individual superconducting samples. Accordingly, the first part of this thesis consists in characterizing the electrical and magnetic properties of bulk sam- ples and coated conductors. In addition to providing critical information to interpret experimental results, these properties can also be used to implement numerical models. In this work, bulk superconductors are characterized in terms of their critical current den- sity and flux trapping ability. Some of the coated conductors are deposited on a slightly ferromagnetic substrate. An experimental set-up is thus developed to characterize the ferromagnetic properties of these substrates.

Then, hybrid magnetic shields (combining bulk superconductors and coated conduc- tors) are investigated experimentally. The study is performed in configurations of in- creasing complexity both in terms of the uniformity of the applied magnetic field and in terms of the symmetry of the shield structure itself. In particular, a cylindrical shield made of coated conductor annuli stacked together is characterized under different config- urations of external applied field. Methods to improve the efficiency of such a shield are also presented. Next, we demonstrate the ability to screen the stray flux density over a significant volume by using arrays of bulk superconducting samples. The influence of the non-superconducting joints between the different samples can be mitigated by combining these arrays with coated conductors in closed loop configuration.

Finally, an experimental prototype of hybrid magnetic screen is tested in the vicinity

of a large field superconducting magnet exhibiting a significant stray flux density. These conditions are commonly encountered in large scale engineering applications. This work is performed in the Department of Applied Science and Technology at Politecnico di Torino (Italy).

The various experimental results obtained throughout this study can be successfully reproduced by 2D and 3D numerical models. These models are useful to interpret the experimental data and estimate the influence of the superconducting parameters on the efficiency of the shield. Consequently, they allow for a deeper understanding of the shield- ing mechanisms. The results obtained in the framework of this thesis demonstrate the relevance of combining bulk superconductors and coated conductors for low frequency magnetic shielding. The intent is that these results can provide insights to design a variety of efficient magnetic shields combining both kinds of superconductors.