HTS Magnetic Field Damper for Short-Term Fluctuations in the Driven-Mode

Although high-temperature superconductors (HTS) are very promising for high-field generation over 25 Tesla, it is difficult to apply them to an NMR magnet because of their low index values and the difficulty caused by superconducting joints. The properties of HTS appear to cause poor magnetic field stability in the persistent-mode operation. Therefore, in this study, a high-field NMR magnet including HTS coils will be operated in the driven-mode. We evaluated the magnetic field drift of a 14 Tesla (600 MHz) NMR magnet in the driven-mode. The magnetic field stability in several hours was about 2 ppm. However, in a shorter-term, magnetic field fluctuations with a main period of approximately 0.01 Hz remained. These fluctuations are unfavorable for a precise NMR measurement. In order to reduce the field fluctuation, we tested the damper coil by making a closed-loop circuit using Bi-2223 tapes. The damper coil was cooled by a single-stage GM cryocooler in the magnetic field. The inside magnetic field of the damper coil was measured with an NMR Teslameter. The shielding current in the damper coil canceled out the field fluctuation, which indicates that the damper coil worked effectively.