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

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 (Bth). Understanding and predicting Bth and dynamic loss in the HTS tapes is crucial for designing the field coils with high stability and efficiency. In this study, Bth, 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 (IDC) is varied from 11.6 A to 254 A, corresponding to i0 (= IDC / Ic0) ranging from 0.03 to 0.7, where Ic0 is self-field critical current of 362.9 A at 65 K. The analytical equation for a single tape: Bth = Bp(1-i¬0) is extended to the average and individual Bth for multi-tape stacks by modifying i¬0 and Bp. Where Bp is the effective penetration field of the HTS tapes which can be found from the maxima of Γm0 (normalized magnetization loss without DC current by the squre of the external magnetic field). Comparison of the simulation results shows that not only the average Bth of the stacks, but also the Bth of individual tapes can be predicted by the extended analytical equation for stacks. The edge tapes have the lowest Bth and highest AC loss values in the stacks due to the large shielding current that expells the external magnetic fields.