halo current is current that leaves a disrupting tokamak plasma through its cool outer edge, flows through the surrounding vessel structure and returns, closing a circuit through the machine and driving large mechanical forces where it crosses the toroidal field.
The halo is the layer of open field lines surrounding the confined 📝plasma, and in normal operation almost nothing flows there. A 📝vertical displacement event (VDE) changes that: the plasma column loses position control, drifts into contact with the wall, and part of the 📝plasma current transfers onto field lines that intersect the 📝vacuum vessel. The vessel becomes a conductor in series with the plasma. Hundreds of kiloamperes cross the 📝toroidal field, and the resulting J × B force lands on the structure rather than on the plasma — which is why halo currents, more than heat, set the mechanical design of a 📝Tokamak's vessel and its supports.
Two quantities set the load, and both were established by measurement on operating machines. The halo current fraction is the share of pre-📝disruption plasma current diverted through the halo; the toroidal peaking factor is the maximum halo current density divided by its toroidal average. 📝Alcator C-Mod measurements published by Granetz and colleagues in 1996 found a typical peaking factor near 2, values as high as 5, and an asymmetry that rotates toroidally at a few kilohertz — ruling out fixed wall features as its cause — with the vessel motion predicted from the measured currents matching the displacement actually observed. 📝ITER's halo current specification rests on a multi-machine database of peaking factor against halo fraction built from measurements of exactly this kind. A halo current is distinct from an eddy current, though a disruption produces both: halo current is conducted out of the plasma into the structure, while an eddy current is induced within a conductor and closes inside it.
For 📝SPARC the number is a projection. The 2020 physics basis led by 📝Ryan Sweeney states that the machine is being designed to withstand predicted unmitigated axisymmetric halo current forces up to 50 meganewtons — a modelled design load for a tokamak that has not yet produced a plasma, not a measured result. The 2026 📝ARC disruption paper finds every load, halo currents included, within a factor of two of SPARC's.