poloidal field is the magnetic field component that runs the short way around a torus — looping through the doughnut hole and over the plasma's top and bottom — produced in a tokamak mainly by the plasma current itself.
Its near-neighbor is the 📝toroidal field, which runs the long way around and carries most of the field strength; the poloidal component is far weaker but does work the toroidal field cannot. In a purely toroidal field, charged particles drift steadily up or down and escape; adding the poloidal component twists every field line into a gentle helix, so a particle following its line samples the top and bottom of the 📝plasma and its drifts cancel. The ratio of the two components — how many toroidal circuits a field line completes per poloidal circuit — is the safety factor q, the central parameter of tokamak stability.
In a 📝Tokamak the bulk of the poloidal field comes from the megaampere-scale 📝plasma current, with external poloidal field coils — horizontal rings above and below the machine — adding the corrections that shape the plasma cross-section, hold its vertical position, and pull the exhaust field lines into a 📝divertor. This is the deep structural difference from the stellarator, which generates its entire twisted field with shaped external coils and dispenses with the plasma current altogether.