The magnetic field component that wraps the long way around a torus, confining plasma against radial escape and setting the stage for stable fusion conditions.
In any toroidal magnetic confinement device—tokamak, stellarator, or spherical torus—the toroidal field (TF) is the component of the magnetic field directed the long way around the doughnut-shaped vacuum vessel. It is the dominant contributor to plasma confinement in most designs and is generated by a set of discrete coils arrayed poloidally around the torus.
Each toroidal-field coil carries a large current that produces a magnetic field looping through the bore of the torus. Because the coils are discrete rather than continuous, the field is slightly stronger on the inboard (high-field) side and weaker on the outboard (low-field) side. This 1/R dependence—where R is the major-radial distance from the machine axis—is a fundamental property of toroidal geometry and drives several important drift effects on charged particles.
The toroidal-field magnets are typically the largest, heaviest, and most expensive components of a fusion device. They must withstand enormous electromagnetic forces—the centering force that pulls each coil inward toward the machine axis, and the overturning torques generated during plasma disruptions. Modern designs use niobium-tin (Nb3Sn) or rare-earth barium copper oxide (REBCO) high-temperature superconductors to reach fields above 12 T on the conductor, enabling more compact machines.2
The toroidal field alone cannot confine a plasma in equilibrium; a poloidal field component is also required to create nested, helical field lines that average out particle drifts. However, the strength of the toroidal field directly sets the achievable plasma pressure through the beta limit: β = 2μ0⟨p⟩ / B2. Higher toroidal fields allow higher absolute pressures at the same normalized beta, which is why high-field approaches—pioneered by MIT's SPARC and Commonwealth Fusion Systems' ARC—have attracted intense interest.3
Because real TF coil sets have a finite number of coils, the field exhibits periodic variation known as toroidal-field ripple. Ripple causes enhanced fast-ion losses and can degrade confinement. Designers minimize ripple by increasing the number of coils or adding ferromagnetic inserts between coils, as done in JT-60SA.