AC loss is the energy a superconductor dissipates as heat when the current it carries or the magnetic field around it changes — the reason a zero-resistance conductor is not a zero-loss one.
Zero resistance is a property of the direct-current steady state. Change the conditions and a 📝superconductor dissipates through three distinguishable mechanisms. Hysteresis loss comes from the superconductor itself: magnetic flux enters as quantized vortices pinned to defects in the crystal, and driving those vortices across the material against their pinning costs work, released as heat, in proportion to the area swept out on the magnetization loop. Coupling loss comes from currents that circulate between superconducting filaments or tapes through the resistive metal joining them, which is why cables are twisted or transposed — shortening the loop shortens the loss. The third is ordinary induction heating by 📝eddy current in the non-superconducting metal of the conductor: copper stabilizer, substrate, solder, and steel jacket.
Two distinctions matter. The first is against 📝screening current, which arises from the same magnetization but is largely persistent and lossless once the field stops changing; its cost is field error, while AC loss is energy gone to heat. The second is between transport-current loss, produced by the current a conductor carries, and magnetization loss, produced by an applied field sweeping across it — a split that matters because a conductor's geometry affects each differently. In a 📝Tokamak neither can be ignored: the 📝central solenoid and 📝poloidal field coil set ramp on every pulse, and even nominally steady 📝toroidal field coil windings see field changes during charging, discharge, and a 📝disruption. Every watt deposited at 20 kelvin has to be lifted out by 📝cryogenics that spend far more power at room temperature than the heat they remove, and the same watt erodes the temperature margin standing between the winding and a 📝quench, so AC loss sizes the refrigeration plant and constrains how fast a magnet may be driven.
For the twisted stacked-tape geometry underlying cables of the 📝VIPER cable family, Philip C. Michael, Theodore Golfinopoulos, Alexey Kaplan and Amy Watterson of the 📝MIT Plasma Science and Fusion Center, with Christopher Craighill, Dylan Kolb-Bond, Colin McCormack, 📝Charlie Sanabria and Erica Salazar of 📝Commonwealth Fusion Systems, published "Transport Current Loss Modeling for a Twisted-Stacked-Tape Cable" in IEEE Transactions on Applied Superconductivity 36(3) (doi:10.1109/TASC.2025.3630611).