The gradient of the phase of superconducting order parameter diverges at the center of a quantized flux (fluxon). Hence, the superconducting order parameter should have a zero value at the center to avoid the divergence of the superconducting current density at that point. This is the physical reason for the normal core in the fluxon. Energy is dissipated when fluxons are forced to move. The microscopic model of Bardeen and Stephen shows that the energy dissipation occurs mostly inside the normal core due to the motion of normal electrons driven by the electric field induced by the flux motion. However, the normal core causes the flux pinning interaction with inhomogeneities through which the energy dissipation can be avoided by stopping the flux motion.
Although high-Tc superconductors (HTS) are very promising for high-field generation over 25 T, it is difficult to apply them to NMR magnets because of their low index values and the difficulty caused by superconducting joints. Therefore, high-field NMR magnets including HTS coils will be operated in the driven-mode. In order to evaluate magnetic filed stability in the driven-mode, we remodeled a 14 T (600 MHz) NMR magnet. Regarding the magnet, persistent switches for axial shim coils (z0, z1, z2) as well as for the main coil were removed for constant operation with a power supply. In addition, a 1 W GM cryocooler at 4 K and HTS current leads were installed in the cryostat to re-condense the boiled helium gas. As a result, there was a long-term change of about 6 ppm in the magnetic field stability, but in the short-term (a few hours), a change of about 2 ppm was observed. In order to reduce the field fluctuation, we tested the damper coil by making a closed-loop circuit using Bi2223 tapes. The damper coil was cooled to about 23 K lower than Tc by a single-stage GM cryocooler in the magnetic field. The magnetic field fluctuation was reduced to less than 0.07 ppm by the damper coil.