Investigation of methods for combining several trapped field superconducting magnets to obtain large magnetic field gradients

The unique magnetic and electric properties of type II superconductors make these materials extremely attractive in applications where large magnetic flux density gradients are desired. Indeed, these gradients can be exploited to exert remotely a magnetic force which is of interest for magnetic levitation systems, magnetic drug delivery systems or brake systems to enumerate a few. The ability of type II superconductors to trap permanent superconducting current loops allows them to act as permanent magnets with the significant advantage that they can generate magnetic flux densities up to one order of magnitude larger than the saturation magnetization of conventional ferromagnetic materials. This thesis aims to determine how several trapped-field magnets with non-parallel magnetization directions can be combined efficiently to generate large magnetic field gradients and surpass the limits of ferromagnetic materials. Starting from configurations inspired by those involving conventional magnets, the work elucidates the
advantages and potential limitations associated with superconducting assemblies. Then, several modifications, either in the geometry of the assembly or in the combination procedure, are proposed and explored to enhance the performance of the final structure. To this aim, two cryogenic experimental setups are designed, assembled, calibrated and used extensively.
The first setup allows the assembly of up to five pre-magnetized samples arranged linearly
through a precisely controlled translation, at liquid nitrogen temperature. This system is
used to investigate how superconductors magnetized independently can be efficiently arranged in a Halbach array configuration. Trapped-field measurements show that a superconducting Halbach array of three YBaCuO samples produces, at a distance of 20 mm, a magnetic flux density gradient 30% higher than an isolated sample. Since the magnetizations of neighbouring trapped-field magnets are perpendicular in such arrangements, each superconductor of the array inevitably experiences a time-varying field component perpendicular to its main magnetization direction during the assembly process. Considering that magnetized superconductors are prone to partial demagnetization under these circumstances, a specific emphasis is placed on developing
methods to mitigate the detrimental impact of such partial demagnetization. To this aim, the use of superconductors with triangular cross-sections as peripheral samples in a three-sample configuration is shown to significantly reduce the demagnetization and to maintain better performances than stand-alone superconductors. Alternatively, using the trapped magnetic field of an additional sample positioned above the central superconductor of the array, and extracted after the assembly process, is found to be an effective re-magnetization technique.
The second setup is an insertion tool compatible with the sample chamber of a Physical Property Measurement System. It allows for the in-situ magnetization of two samples and for the controlled rotation of one sample by an angle of up to 190°. With this system, the magnetic flux density gradient achieved with either a single or a pair of magnetized YBaCuO superconductors in the presence of a uniform background DC magnetic field is investigated at 59 K, 65 K and 77 K. Although the background field reduces the trapped field ability of individual samples, it is shown that substantial gradients can still be generated in such conditions. This investigation reveals that the distance between the samples is sufficient to avoid any mutual demagnetization effect during the rotational motion. As a result, the combined contribution of each sample generates a higher magnetic flux density gradient in comparison to the one produced with a stand-alone superconductor.