eddy current is a loop of current induced inside a conducting body by a changing magnetic field, flowing so as to oppose the change that created it.
Faraday's law makes any changing magnetic flux drive an electromotive force, and in a solid conductor there is no wire to confine the resulting current, so it circulates in closed loops within the material — eddies, from which the name. Lenz's law fixes the direction: the induced current generates a field opposing the change, so eddy currents resist relative motion, dissipate energy as heat, and delay the penetration of an external field through a conducting shell. Outside fusion the same effect drives induction heating, metal detectors and non-destructive testing.
In a 📝Tokamak every one of those consequences is a design consideration. Ramping the 📝central solenoid and shaping coils induces currents in the 📝vacuum vessel and its supports that shield the 📝plasma from the intended field on the vessel's resistive time scale. That shielding is also useful: eddy currents in the wall slow a 📝vertical displacement event (VDE) enough for feedback control to catch it, which makes vessel resistivity a stabilisation parameter rather than a nuisance. In a 📝disruption the sign reverses: the 📝plasma current collapses in milliseconds, and the currents induced in the structure cross the 📝toroidal field to produce forces of the same order as those from 📝halo currents. The two are often confused: an eddy current is induced within a conductor and closes inside it, while a halo current is conducted out of the plasma and enters the structure from outside.
Predicting those forces means modelling three-dimensional structures — ports, penetrations, joints — that no axisymmetric code represents. Battey and co-authors' 2024 paper describes ThinCurr, a thin-wall eddy current model built on the PSI-Tet finite-element code for exactly this purpose, applied to 📝SPARC and 📝DIII-D and used to study the passive 📝runaway electron mitigation coil, whose current is itself an eddy current driven by the disruption it answers. The 2020 SPARC physics basis states that eddy currents forced to flow poloidally in the vessel produce loads similar in size to the machine's projected 50-meganewton halo current forces. Both are modelled figures for a machine that has not yet run.