Numerical simulation on the threshold field and AC loss in vertical stacks of REBCO tapes carrying DC transport currents under perpendicular AC magnetic fields

Abstract In high-temperature superconducting (HTS) coil applications, such as field coils for superconducting motors, HTS tapes in vertical stacks carry a DC transport current under an external AC magnetic field. In such a complex electromagnetic situation, HTS tapes generate AC losses consisting of magnetization loss and dynamic loss, when the amplitude of the ripple fields exceeds the threshold field ( B th ). Understanding and predicting the B th and dynamic loss in HTS tapes is crucial for the design of field coils with high stability and efficiency. In this study, B th , dynamic loss and magnetization loss for a single tape and 2-, 6-, 10- and 20-tape vertical stacks are numerically studied in detail under external perpendicular magnetic fields ranging from 10 mT to 1 T. The transport DC current level ( I DC ) is varied from 11.6 A to 254 A, corresponding to i 0 ( =I DC / I c0 ) ranging from 0.03 to 0.7, where I c0 is a self-field critical current of 362.9 A at 65 K. The analytical equation for a single tape, B th = B p (1 − i 0 ), is extended to the average and individual B th for multi-tape stacks by modifying i 0 and B p . Here, B p is the effective penetration field of the HTS tapes, which can be found from the maxima of Γ m0 (the normalized magnetization loss without DC current by the square of the external magnetic field). Comparison of the simulation results shows that not only the average B th of the stacks but also the B th of individual tapes can be predicted by the extended analytical equation for stacks. The edge tapes have the lowest B th and highest AC loss values in the stacks due to the large shielding current that expels the external magnetic fields.