The current densities flowing in high-temperature superconducting wires are mostly non-uniform during operations (caused by the screening current effect). This is a major challenge to high-field superconducting magnets based on (RE)Ba2Cu3O7-x (REBCO; RE, rare earth) tapes, as to cause (a) uncertainty of the field distribution and (b) concentrated uneven Lorentz force that damages the magnet. For no-insulation (NI) REBCO coils, the current distribution would be even complicated when external time-varying fields induce extra transport current circulating in NI coils. For this study, a numerical model is established, and the results are compared with hall sensor measurements for an NI coil sample. Different with conventional insulated coils, for NI coils, it was measured that the variation of axial fields (parallel to REBCO surface) could introduce remnant current densities persisting on REBCO tapes, which generate remnant fields. These remnant fields would cause the distortion of the spatial field distribution of a magnet, and were also measured to exhibit constant field decay with logarithmic time. These remnant fields pose extra concerns for NI coils for applications in NMR/MRI magnets in terms of threatening the spatial field homogeneity and temporal field stability of these magnets. Practical cases are simulated, i.e., NI coils are under time-varying fields generated by its adjacent NI coils or background magnets during quench/fault. The induced current (and the resulted Lorentz forces) concentrates on one-side edge of REBCO tapes that is adjacent to the quenched/fault coils; this observation helps understand and protect the mechanical damage observed in recent operations of NI REBCO magnets.
