toroidal field is the magnetic field component that runs the long way around a torus, supplied in a tokamak by external coils and providing the bulk of the field that confines the plasma.
Its near-neighbor and complement is the poloidal field, which runs the short way around the torus; a 📝Tokamak needs both, because a purely toroidal field lets particles drift out of the machine. The toroidal component is by far the stronger of the two, produced by a ring of D-shaped coils spaced around the vessel, and it falls off with distance from the machine's central axis — the field on the inboard side of the 📝plasma is always stronger than on the outboard side, which is why instabilities favor the outer edge.
Toroidal field strength is the most consequential number in tokamak design: fusion power density scales roughly with the fourth power of the magnetic field, so doubling the field yields a machine that can be dramatically smaller for the same output. ITER's conventional superconductors reach 5.3 tesla on axis; 📝SPARC is designed for 12.2 tesla on axis using 📝High-Temperature Superconducting (HTS) Magnets, which is what allows a machine roughly one-fortieth ITER's volume to target comparable plasma performance.