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.
