Trapped Flux Dependence of Bulk High-Temperature Superconductors Between 77 and 30 K under a Limited Excitation Field

The trapped-flux dependence of bulk high-temperature superconductors between two typical working temperatures of 77 and 30 K was investigated by a field-cooling magnetization (FCM) method under a limited excitation field of 5 T to approach actual application conditions. An exponential trapped-flux dependence between the two temperatures was obtained by fitting the experimental data. Its effectiveness was checked from the theoretical temperature relationship of the critical current density and trapped-field data in the literature. It was found that the achievable trapped flux at 30 K is closely dependent on the excitation field. The bulk samples with moderate performance around the trapped-field range of 1.0–1.5 T at 77 K can be efficiently activated to reach a trapped field over 4.6 T at 30 K. Bulk samples with better performance at 77 K will get saturation and cannot be fully excited by the 5 T excitation field. The presented results are useful for the appropriate estimation of trapped-flux performance from 77 to 30 K and may support some basic references for the applications of bulk HTS magnets.