Characterization of Current Stability in an HTS NMR System Energized by an HTS Flux Pump

HTS magnets are operated in driven mode due to non-existent persistent joint technology. The power supply (PS) current leads place a large heat load on the system cryogenics. Applications such as nuclear magnetic resonance (NMR) require the magnet to be stable to the sub-PPM range, which necessitates an ultra-stable PS. We have investigated the use of a mechanical HTS flux pump (FP) as an alternative to the PS. The FP is integrated into the magnet´s cryogenic environment and used as a current source, which significantly reduces the heat leak to the cold mass. Earlier work showed a reduction in heat load by a factor of 4.5 when using an HTS-based FP in a small cryogen-free HTS magnet. We extend the use of the FP to a 2-T NMR relaxometry magnet of substantial inductance and investigate the system´s temporal stability. The 0.4-H iron-yoked dipole magnet was ramped to a field of 2 T (at 110 A) in 2.5 h. A temperature-compensated Hall sensor was used as a feedback element in a PID control loop to actively control the magnet current through modulation of the rotational speed of the FP. We report the stability achieved using proton NMR measurements.