poloidal field coil is one of the external, horizontal ring-shaped magnets encircling a tokamak that shape the plasma, hold it in position, and contribute to its stability.
The near-neighbor distinction is with the 📝toroidal field coil: toroidal field coils are the vertical D-shaped magnets that produce the machine's main confining field, while poloidal field coils are the horizontal hoops stacked above, below, and around them that sculpt the field's cross-section. Their contribution to the 📝poloidal field is secondary but decisive — the bulk of that field comes from the current flowing in the 📝plasma itself, and the coils' job is to bend and trim it: setting the plasma's elongated shape, steering the 📝X-point that directs exhaust into the 📝divertor, and countering the plasma's constant tendency to drift.
Because a shaped, elongated plasma is vertically unstable, the poloidal field system is also a control system: its currents are adjusted in real time, on millisecond timescales, using magnetic measurements of where the plasma actually is. 📝ITER carries six such ring coils around its eighteen toroidal field magnets, alongside the 📝central solenoid — the separate magnet stack in the machine's bore that drives the 📝plasma current in the first place — and every 📝Tokamak uses some version of the same arrangement.