Experimental and numerical study of radial and axial HTS magnetic couplers

• Research is focused on superconducting couplers for flywheel energy storage. • Couplers in radial and axial configurations were studied. • A model for numerical calculation has been developed. • The load characteristics of the radial coupling prototype were measured. The paper presents the results of an experimental study and numerical calculations of torques in magnetic couplers of various configurations. A comparison was made for model couplers based on permanent magnets (PMG) and couplers based on high-temperature superconducting (HTS) tapes cooled with liquid nitrogen. In PMG-couplers, both coupler halves are made of permanent magnets, while in HTS-couplers, one coupler half is made of HTS-tapes stacks, and the second of permanent magnets. Both types of couplers are considered in both axial and radial configurations, with different clearances between the coupler halves. The results of experimental studies showed that in the configurations we considered, radial HTS-couplers are superior to their axial analogues in terms of the maximum torque between the coupler halves. In addition, in the considered configurations, PMG couplers exceeded their HTS analogues in torque by 10 times. Numerical finite element calculations using a combined A-T-H formulation were used to optimize the design of the HTS-coupler. The model was verified using the obtained experimental data for moments in model couplers. The model was used to calculate an optimized coupler with added HTS-windings, lower HTS temperature and stronger permanent magnets. The calculation results showed that HTS-couplers can be comparable with their PMG analogues, at the same time they are significantly superior to them in damping external disturbances. Due to the fact that the best characteristics were shown by model radial couplers with minimal gaps between the coupler halves, radial couplers were manufactured based on windings of 10 and 20 layers of HTS-tapes and the torques in them were measured in the FC and ZFC modes. It has been shown that in most operating conditions, 10-layer couplers outperform 20-layer couplers.