screening current is the persistent, self-induced current that circulates within the width of a superconducting tape as the magnetic field around it changes, adding a field of its own that distorts the field the magnet was built to produce.
The mechanism is ordinary induction with an extraordinary consequence. Any conductor meeting a changing magnetic field develops currents opposing the change; in a normal metal they decay in milliseconds, and in a finely filamented 📝low-temperature superconductor the loops are confined to filaments micrometers across. 📝REBCO offers neither escape. Because the 📝coated conductor architecture lays the 📝superconductor down as a single unbroken film several millimeters wide, induced loops span the whole tape width, and because the film has no resistance they do not decay. What remains is a magnetization frozen into every turn of the winding, and its field superimposes on the intended one.
The practical damage is threefold. Field quality suffers: the central field of a solenoid drifts from its design value, homogeneity degrades, and the error depends on the coil's entire charging history rather than on its present current, so the same operating point can give different fields depending on how it was reached. The screening currents also flow in a strong field and therefore feel a 📝Lorentz force, loading the tape in ways the nominal transport current alone does not predict. Mitigations trade against one another — striating the film into narrow filaments, deliberately overshooting and backing down the current, or correcting the field with shim coils. The near-neighbor to keep distinct is 📝AC loss: both originate in the same magnetization, but AC loss is energy converted to heat while a screening current persists after the field stops changing and costs accuracy rather than watts.
Peng Xu of Brookhaven National Laboratory, Mark D. Bird and Ernesto Bosque of the National High Magnetic Field Laboratory at Florida State University, and Dylan Kolb-Bond of 📝Commonwealth Fusion Systems examined the effect in the geometry high-field magnets actually use in "Some Consequences of Screening Currents in Solenoids Constructed from REBCO Double-Pancakes," IEEE Transactions on Applied Superconductivity 35(6), September 2025 (doi:10.1109/TASC.2025.3570868) — a coil built by 📝pancake winding, where each flat spiral presents its tape broadside to the axial field.