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Why does a tokamak need both toroidal and poloidal magnetic fields?

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3D illustration of the interior reaction chamber of a tokamak fusion reactor, showing a large, cylindrical, metallic structure with curved walls and bright light panels. — fusion reactor A tokamak is a device used in nuclear-fusion research for magnetic confinement of plasma.

In a tokamak, both toroidal and poloidal magnetic fields are essential to maintain the plasma in a stable equilibrium, as the magnetic field lines must form a helical shape around the torus.1

Toroidal Field: This component points the long way around the torus. If it were absent, the plasma would be unstable, developing growing distortions or "kinks" that would cause the plasma to strike the chamber walls.1
Poloidal Field: This component is directed the short way around the machine. Without it, the plasma would not be in equilibrium; instead of following the magnetic field lines, particles would drift to the walls of the device. The poloidal field ensures that particle orbits remain contained within the reactor.1

Together, these fields create the necessary helical magnetic structure to confine the plasma effectively.1