Investigations of the behaviour of superconducting magnetic levitating set-ups including a large MgB2 disc and a smaller NdFeB magnet

Abstract

We report the levitation and guidance forces of two set-ups including a 120 mm diameter MgB₂ disc cooled down to 20 K. The first one included the disc and a 30 mm thick and 100 mm diameter NdFeB magnet and is labelled as PM100 set-up. The second one called PM70 set-up included a 70 mm diameter and 35 mm thick magnet. All the measurements were carried out after cooling down the superconductor with a 30 mm separation between the magnet and the superconductor. Some remarkable results were obtained. It was reported by Xing et al (2024 Supercond. Sci. Technol. 37 02LT01) that the levitation force measured with the PM100 set-up has reached the strong value of 678 N at 5 mm separation between the magnet and the superconductor. In this new report, in addition to a comparison of the levitation forces measured on both set-ups, we report guidance force measurements. We show that this force decreases for the PM100 set-up as the magnet-superconductor distance decreases, while the guidance force of the PM70 set-up increases, as was reported for set-ups including Halbach guideways sections. Reproducing the measured levitation forces with analytical models, we show, on the one hand, that the large levitation forces measured with the PM100 set-up are due to the non-homogeneity of the magnetic field above the PM100 magnet. On the other hand, we explain the behaviour of the PM70 set-up guidance force by the low modulation of the magnetic moment of the superconductor resulting from the magnet motion.