In this thesis, we investigate in details the magnetic shielding processes in high temperature superconductors. We aim at predicting and measuring the influence of the parameters governing the superconducting behaviour (e.g., the critical current density Jc and its field dependence) on the magnetic flux penetration inside hollow cylinders. Three objectives are pursued. The first objective is to characterize the performances of cylindrical magnetic shields made of various high temperature superconductors (Bi-2223, Bi-2212, Y-123) for several external parameters that can be directly controlled in the experiments. These include the temperature, the geometry of the magnetic screen, the amplitude of the applied magnetic field, its orientation (H // or ⊥ cylinder axis) and its frequency (in the case of an AC excitation). The focus has been put on the effect of the sweep rate of the applied magnetic field, dBapp/dt, on the threshold magnetic flux density, Blim, above which the magnetic shielding is no longer efficient. We show that the curve of dBapp/dt vs. Blim can be directly related to the E(J) constitutive law and we explain how the field dependence of the critical current density affects the relationship between both curves. A wide range of electric field levels is investigated in this thesis thanks to the use of a number of experimental techniques. The experimental set-ups required for these investigations are described in a separate chapter where the performances and limitations of each set-up is put forward. The second objective of the thesis is to confront experimental data and numerical simulations using models of increasing complexity. We present two numerical models that take into account the strongly non-linear E(J) relationship characterizing the superconducting behaviour. The first numerical model is based on the Brandt algorithm. This model is able to describe magnetic shielding experiments which involve time-varying magnetic fields on (i) infinite samples subjected to transverse magnetic fields or on (ii) samples of finite size in axisymmetric geometries subjected to axial magnetic fields. The second model is based on a finite element method (using the GetDP software). This model can be used for both axisymmetric 2D modelling and full 3D modelling. In the latter case, it provides extremely useful information for understanding configurations where the applied magnetic field and the hollow cylinder do not present a particular symmetry. Both 2D and 3D models are able to provide current and field distributions in the superconductor. The confrontation between experimental and modelling results allows us to better understand how physical properties (critical temperature Tc, critical current density Jc, ...) and geometry affect the magnetic shielding performances (shielding factor SF, threshold induction Blim). Conversely, we present three different practical procedures using either the analytical Bean model or a 2D numerical model in order to determine the constitutive law parameters of the superconducting materials from data measured on hollow cylinders. As a third objective, we aim at investigating how the different types of HTS magnetic screens of moderate dimensions (a few cubic centimetres) could be scaled up in order to provide larger shielded volumes. Three methods are explored. The first method to build larger magnetic screens (typically ten(s) of centimetres) is to find means to assemble several (pieces of) superconducting hollow cylinders to obtain a large shielding enclosure. Experiments are carried out on superconducting cylinders cut either along or perpendicular to their axis. The influence of the cut width and the angle of the applied magnetic field with respect to the cut plane are studied. Modellings are carried out on shorter cylinders in order to understand how the presence of the cut modifies the current distribution and thus the magnetic shielding properties. The second method consist in characterizing the magnetic shielding of several hollow cylinders made with the same superconducting material but having different sizes. This helps us in determining whether the manufacturing process is able to provide large cylinders with good magnetic shielding performances. In a last method, we investigate an architecture of superconducting magnetic screens that differs from those based on bulk hollow cylinders. The idea is to exploit the high Jc-performances of superconducting tapes (based on thin film technology) to build magnetic shielding structures which can be easily scalable. The measurement results obtained on such structures are promising as the actual limitation seems to be geometrical. Using more superconducting tapes in order to achieve a higher aspect ratio should give magnetic shields which are as efficient as bulk cylinders.
| Date of Award | 23 Nov 2011 |
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| Original language | English |
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| Awarding Institution | - Royal Military Academy
- Université de Liège
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| Supervisor | Michel Dirickx (Supervisor) & Ph Vanderbemden (Supervisor) |
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