Crossing Phenomena and Flux Balance in the Force–Displacement Hysteresis Curve of Superconductor–Magnet System

Abstract

Three YBCO bulk superconductors with different critical current densities are used to investigate the magnetic force of a simple superconductor–magnet system under different relative speed values between YBCO and a magnet. The initial cooling processes for superconductors include zero field cooled (ZFC) and field cooled (FC). Crossing phenomena can be observed both in force–displacement hysteresis curves of superconductors with high and very low critical current densities. The maximum repulsive force under different relative speed values is analyzed. For the high-performance superconductor, the ZFC maximum repulsive force increases at first and reaches a peak value and then decreases with the increase of relative speed. When the magnet reaches 0.1 mm above the minimum clearance and its speed is changed to 0.1 mm/s for 1 s, the levitation force increases a lot at first and then decreases during this very slow approaching process to the minimum clearance. Based on the experimental results and analysis, the crossing phenomena reflect the effect of vortex dynamic and are directly related to the flux balance between trapped magnetic field and flux creep.