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The context of this master thesis is the understanding of the magnetic properties of superconducting “trapped field magnets”, i.e. a bulk superconducting element of ~1cm3 size in which lossless current loops are induced and trapped permanently. When a trapped field magnet is subjected to a field non-parallel to its magnetization, the distribution of the current loops is affected and may lead to a self-demagnetization. Although the behaviour of an isolated superconductor can now be understood when the applied field is generated by a stable field source, a clear understanding of the behaviour of a system where the crossed field component is generated by a second neighbouring trapped field magnet is missing. In this context, my master thesis consists in elaborating an experimental system to study the magnetic properties of two trapped field magnets in close proximity. These measurements constitute the first step towards the design of applications where higher fields are generated by exploiting the interaction between trapped field magnets. The scientific approach is divided in two steps. 

The first step consists in a set of experiments performed at the Bulk Superconductivity Group of the University of Cambridge (UK). The measurements consist in analysing the magnetic behaviour of a pair of cylindrical bulk YBa2Cu3O7 superconductors when their symmetry axis is not aligned with one another. Two major points have been investigated: (i) the magnetization of a single trapped field magnet when magnetized by a field misaligned with its symmetry axis; (ii) the effect of adding a second sample, aligned with the magnetizing field. Experiments were carried out at 65 K and 77 K both in field cooled under 2.5 T and in zero field cooled under 7 T. The sample holders allowing two superconducting magnets (typically 16 mm in diameter and 8 mm in height) to be placed at different angles were designed. A set of cryogenic Hall sensors have been calibrated and a Printed Circuit Boards for holding them have been fabricated. An analytical model was elaborated and used to analyse the results. For a centre-to-centre distance equal to twice the sample height, the presence of the second sample is found not to alter the current distribution inside the first. Consequently, the contribution of both samples adds up, thus increasing the magnetic flux density between them.

The second step consists in analysing the magnetic field distribution around a trapped field magnet when a second one is approached through a computer-controlled translational motion. The experimental system was designed to accommodate and displace two cylindrical samples. The samples were magnetized in field cooled condition under 1.2 T before they were moved, either face to face or sideways. These experiments were performed at Uliege. If the separating distance is decreased below than the diameter of the samples, an irreversible decay of the trapped field was measured when the samples were separated again. Repeated approach cycles showed that the irreversible loss of trapped field is more significant for the first approach.

Trapped field magnets --- Applied Superconductivity --- Bulk superconductors --- Ingénierie, informatique & technologie > Multidisciplinaire, généralités & autres