AC Loss Study in REBCO HTS Tape and Four-Tape Stack Carrying AC Current With DC Offset

In rapid-cycling synchrotrons (RCSs), superferric magnets wound with high-temperature superconductors are energized by ac current with a dc offset. However, the presence of the dc offset may increase the ac loss in the magnets. To investigate the influence of the dc offset on ac loss of superconductors, simulations of a single high-temperature superconducting (HTS) tape and a four-tape stack carrying various combinations of ac currents with dc offsets were carried out using the T-A formulation. The 4 mm-wide REBCO HTS tape considered in this work was SuperPower wires with a self-field critical current of 104.98 A. For both the single tape and the four-tape stack, the amplitude of ac current ranges from 0.1 to 0.9 times their respective critical currents, while the dc offset varies from 0 to 0.8 times their respective critical currents. Regarding the four-tape stack, the simulated critical current of 88.9 A was employed. The normalized current density and perpendicular magnetic field distributions of the single tape and the four-tape stack were presented for different ac and dc values. Compared to the cases without the dc offset, small dc offsets almost have no influence on ac loss of both the single tape and the four-tape stack, while a significant loss increase is observed with high dc offsets. For a given combination of ac current and dc offset, the influence of the dc offset on ac loss is more pronounced in the four-tape stack than in the single tape.