Impact of porosity on the magnitude of trapped magnetic field and mechanical stress in HTS bulks during PFM

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.