Magnetization of high temperature superconducting trapped-field magnets

High temperature superconducting (HTS) bulks and stacks of coated conductors can be magnetized to become trapped-field magnets that provide much stronger magnetic fields than those reachable with conventional permanent magnets. The trapped-field magnets are promising for a variety of electrical applications that use permanent magnets. The pulsed field magnetization (PFM) method is attracting attention as it can provide costeffective, compact and flexible in situ magnetization, which is critical for realizing practical applications of trapped-field magnets. The key challenge of PFM is that the produced trapped fields are generally lower than those produced by field cooling or zero field cooling methods due to the temperature increase caused by fast flux motions. The motivation of this work is to investigate and understand the flux dynamics during PFM and to propose possible strategies to improve the trapped field produced by PFM. For HTS bulks, 2D electromagnetic-thermal coupled models are used to simulate the PFM process and relevant experiments in the literature are reviewed. The findings are that the simulation results are highly parameter dependent and 2D models cannot properly describe HTS bulks due to inhomogeneity of HTS bulks. Stacks of HTS coated conductors are simulated with newly developed 2D and 3D models based on the finite element method. By means of 2D electromagnetic-thermal coupled modelling, two strategies are proposed to improve the trapped field produced by PFM. One is to use controlled magnetic density distribution coils to magnetize the stack. The other is to apply an optimal pulse sequence with a large enough initial pulse and successive pulses of descending amplitudes with infinitesimal intervals. The second strategy is qualitatively validated by experiments. With 3D modelling, a square flat stack and a curved stack are investigated. The curved stack is of interest because of its geometrical applicability for electrical machines. The magnetic field distributions of flat and curved stacks magnetized by field cooling are calculated with 3D static models and the results quantitatively agree with experimental measurements. Using 2D and 3D electromagnetic-thermal coupled models, the flat and curved stacks magnetized by PFM are simulated. Homogenization and mesh techniques are used to speed-up the simulations. It is found that 3D and 2D models obtain close trapped fields for a flat square stack and the results quantitatively agree with experiments reported in the literature. The curved and flat stacks magnetized by PFM show similar behaviors. The magnetic field distribution of a curved stack is changed due to geometrical deformation, which is qualitatively validated by experiments. The numerical models developed and used in this thesis work constitute the state-of-the-art simulation tools for investigating the magnetization of HTS coated conductor stacks: for the first time, the electromagnetic and thermal behavior of such stacks was simulated in 3D, and without simplifying assumptions on the geometrical structure in 2D.