toroidal field coil is one of the D-shaped magnets arrayed around a tokamak's vacuum vessel that together produce the machine's main confining magnetic field, running the long way around the torus.
The near-neighbor distinction is with the 📝poloidal field coil: toroidal field coils encircle the 📝plasma vertically, like ribs, and generate the 📝toroidal field — the strong field that confines the plasma — while poloidal field coils are horizontal rings that shape and position it. The toroidal field coils are the strongest magnets in a 📝Tokamak and largely set its performance, because 📝fusion power density rises steeply with field strength: doubling the field yields roughly sixteen times the power density from the same plasma volume. Their characteristic D shape is structural, following the curve along which the enormous magnetic forces on the conductor become pure tension.
Because copper coils at this scale would consume more power than a plant could sell, practical toroidal field coils are superconducting. 📝ITER's eighteen coils use low-temperature 📝superconductor at fields around 12 tesla on the conductor; 📝High-Temperature Superconducting (HTS) Magnets push toward 20 tesla, which is what allows a compact machine like 📝SPARC to pursue the performance of a far larger one.