High temperature superconductors (HTSs) are enablers of extensive electrification for aircraft propulsion. Indeed, if used in electrical machines, HTS materials can drastically improve their performance in terms of the power-to-weight ratio. Among the different topologies of superconducting electrical machines, a flux modulation machine based on HTS bulks is of interest for its compactness and light weight. Such a machine is proposed in the FROST (Flux-barrier Rotating Superconducting Topology) project led by Airbus to develop new technologies as part of their decarbonization goals driven by international policies. The rotor of the machine will house large ring-segment-shaped HTS bulks in order to increase the output power. However, the properties of those bulks are scarcely known and have barely been investigated in the literature. In this context, the present work aims to fill out partially this scarcity within the framework of FROST. Thus, a thorough characterisation of the performances and homogeneity of 11 large REBaCuO bulks was carried out. Ten of the bulks are to be utilized in the machine prototype, originally keeping the eleventh bulk as a spare. A first set of characterisation was conducted on the eleven bulks. For this set, the trapped field mapping and the critical current were estimated. Then, a series of in-depth characterisations on the eleventh bulk followed. It included critical current measurement, X-ray diffraction, and scanning electron microscopy on different millimetre-size samples cut out from the bulk at various locations. The X-ray diffraction and scanning electron microscopy showed weakly oxygenated regions inside the bulk explaining the local drop or loss in superconducting properties. The objective was to determine the causes of the inhomogeneities found in the trapped field measured on all the bulks, sacrificing one of them, here the spare one. To help obtain a clearer picture, a numerical model was then elaborated to reproduce the field map of the eleventh bulk using the experimental data obtained from the characterisation of its various small samples. It is concluded that further characterisations, including the statistics on various bulks, are still needed to understand the underlying reasons for inhomogeneity in the trapped field. Nonetheless, all the bulks presented enough current density to be usable in the construction of the proposed machine.

During pulsed-field magnetization (PFM) processes, high-temperature superconducting (HTS) bulks are subjected to significant mechanical stress due to rapid changes in magnetic flux (Lorentz force) and temperature (thermal stress), potentially leading to mechanical failures. This is particularly emphasized by the inherent fabrication-induced defects such as cracks, grain boundaries, and pores. These defects often result in non-uniform trapped magnetic fields across the bulk, which can strongly impact its trapping performance. The present study focuses on the impact of porosity on the trapped magnetic field, temperature, and mechanical stress of REBCO disk-shaped bulks through a 2D transient multiphysics finite element model using COMSOL Multiphysics^®. The results show that porosity causes significant losses by locally densifying the current, leading to high local temperatures, which in turn reduces the critical current density, infusing more local losses. As a result, lightning-like thermal pathways passing through the pores radiate from the edge of the bulk to its center as the applied magnetic field increases, therefore limiting the magnitude of the trapped field. Finally, porosity causes two distinct peaks in maximum tensile stress during PFM: one attributed to thermal effects and the other to electromagnetic forces. Higher porosity often results in the increase of both peaks of tensile stresses, potentially compromising the bulk’s physical integrity. It highlights the importance of controlling porosity to improve the trapping performance of HTS bulks. One mitigating action is to fill the pores with silver, which is shown to remarkably reduce both the temperature rise and stress.

High-temperature superconductors, especially rare earth barium copper oxide (REBCO) bulks, are promising for superseding permanent magnets in power applications. However, being essentially ceramics, their practical use is limited by their brittleness that turns into mechanical failures during high-field magnetization. These failures often arise from defects that appear during the fabrication process, such as pores and cracks. This fragility may be mitigated by including artificial holes in the bulks. The number, size and distributions of these holes may not just improve cooling, but they may also reduce porosity, and enhance both mechanical strength and flux pinning. Modelling the impact of such holes on the trapping performance of the bulks and their thermo-mechanical response to magnetization, particularly the pulsed field magnetization (PFM) that is considered practical for actual applications, is challenging and requires a multiphysics approach. In that context, the present work introduces a 2D finite element model (FEM) that includes coupled electromagnetic, thermal and mechanical physics to simulate thin-wall REBCO bulks with ring reinforcement. The numerical results provide insight on how to maximize trapped magnetic fields while minimizing mechanical loads with thin-wall reinforced REBCO bulks.

The present paper deals with the development of a costumed-made system and techniques to address the challenges of the magnetization and characterization of bulk superconduc tors as large as 100 mm in diameter. The system is composed of a dry superconducting magnet capable of producing a field variation up to 18 T and a measurement setup relying on an advanced field mapping arrangement made of a 10 × 10 Hall sensor matrix for rapid magnetic field acquisition under 0.5 s. To achieve the desired accuracy, the experimental setup incorporates precise thermal management and calibration. Its purpose is to investigate strategies to enhance the stability, structural integrity, and overall performance of superconducting bulk materials magnetized in high magnetic fields. The goal is to promote scientific collaborations through a dedicated experimental facility for forthcoming magnetization and fast field-mapping studies, thereby supporting the development of bulk superconductors.

In the present work, the objective is to quantify via modeling the impact of the variability of the critical parameters (critical current density and critical temperature) as well as the presence of midsize to large defects on the thermo-magnetic response of REBCO bulks under pulse field magnetization. A particular interest is given to the trapped magnetic field as a figure of merit of the strength of the trapping mechanism. For this study, a statistical methodology is proposed that can be extended by adding experimental data to numerical results. The statistics are generated using a Monte Carlo method running a thermo-electromagnetic 3D finite element model (TM-FEM). The TM-FEM solves the heat balance equation in conjunction with the H - ϕ mixed formulation of the Maxwell equations. The results are analyzed using cluster analysis and multivariate analysis of variance to identify dominant regimes of variability and statistically significant parameter interactions. In addition to the physical parameters, numerical parameters such as mesh quality and computation time are incorporated into the analysis to account for possible biases arising from modeling parameters. A response surface model is obtained providing a reduced-order predictive representation of trapped magnetic field. The results confirm that the variability of J c is the primary physical parameter that defines the magnitude of the trapped magnetic field. Nevertheless, defects introduce nonlinear interactions that can indirectly degrade the trapping. From response surfaces, nonlinear regression models are provided for the case at hand. The proposed statistical methodology yields a systematic approach for uncertainty quantification to help defining guidelines to improve the fabrication and optimal use of REBCO bulks as pseudo-magnets.