The horizontal ring-shaped coils positioned above and below a tokamak that shape, position, and stabilize the plasma cross-section within the vacuum vessel.
A poloidal field (PF) coil is a large superconducting solenoid wound in a horizontal ring concentric with the tokamak axis. A set of PF coils — typically six to eight, stacked above and below the midplane — produces magnetic fields in the poloidal direction (the short way around the torus). By adjusting the currents in each coil independently, operators control the shape, vertical position, and radial position of the plasma column in real time.[1]
The poloidal field coil system performs several simultaneous jobs:
Plasma shaping. The cross-sectional shape of the plasma — whether circular, elongated, or D-shaped — is determined by the PF coil current distribution. Most modern tokamaks operate with vertically elongated, D-shaped plasmas because elongation increases the achievable plasma pressure (beta) for a given toroidal field.[2]
Equilibrium control. The outward-pushing hoop force and pressure gradient of the plasma must be balanced by an inward-pointing vertical field. The PF coils provide this equilibrium field, which scales with plasma current and pressure.[2]
Divertor configuration. Specific PF coils create one or more magnetic null points (X-points) where field lines diverge away from the confined plasma and are guided into the divertor channels. The position and shape of the X-point directly control where exhaust heat and particles strike the divertor targets.[1]
Vertical stability. An elongated plasma is inherently vertically unstable. Fast-responding internal coils — sometimes called vertical stability coils or in-vessel coils — work in concert with the external PF system to prevent vertical displacement events (VDEs) that could drive the plasma into the first wall.[3]
The central solenoid (CS) — a tall stack of independently powered coil modules located on the tokamak axis — is sometimes grouped with the PF system because it also produces poloidal-direction fields. However, the CS has a distinct primary function: driving the plasma current inductively by ramping its own current and thereby inducing a loop voltage around the torus. The PF coils, by contrast, focus on shaping and positioning rather than current drive.[1]
Because PF coil currents must be varied dynamically during a plasma pulse — including rapid swings for plasma initiation and controlled ramp-down — the coils experience significant AC losses even though they use superconducting cable. Conductor design must therefore balance low-loss performance against the need for high current density and mechanical robustness under cyclic electromagnetic loading.