Experimental Study on Hysteresis of Screening-Current-Induced Field in an HTS Magnet for NMR Applications

This paper presents a research on magnetic hysteresis of an HTS magnet comprising a stack of pancake coils wound with GdBCO-coated conductors. The hysteresis during a typical charging and discharging process of an HTS magnet occurs mainly due to the screening-current-induced field (SCF). To investigate the hysteresis, an HTS magnet, comprising 16 single-pancake coils with the respective winding i.d., o.d., and height of 100, 124, and 4 mm, was constructed. Magnetic field distributions in a 10-mm cold bore of the HTS magnet were measured by a three-dimensional field mapper, and their spatial harmonics, axial and radial, were obtained using associated Legendre equations. Field mappings were conducted at every 10 A during charging up to 90% of the magnet critical current, and repeated at every 10 A during discharging. The hysteresis loops were obtained in terms of major field gradients, axial of Z1 and Z2 and radial of X and ZX. As a result, strong hysteresis was observed that the magnetic fields were not linearly proportional to the operating current even at the beginning of charging. Even at the identical operating currents of the coil during charging and discharging, a significant difference in the spatial field distribution was observed. The residual field, a primary source to the field discrepancy, was measured 2.11% of the magnet center field at its peak operating current of 50 A, which is significant for some applications including NMR (nuclear magnetic resonance) magnets. More importantly, the residual gradients in some major axial gradients, Z1 and Z2, were much more noticeable, about -9.4% and 10.2%, respectively, of those measured at 50 A. A recommended charging protocol, called "current-sweep-reversal," in terms of minimizing the hysteresis of SCF was investigated and discussed in detail.