central solenoid is the tall stack of magnet coils running down the center of a tokamak's doughnut hole, whose changing magnetic field drives the plasma current by induction.
The device is a transformer with one turn missing: the solenoid is the primary winding, and the 📝plasma ring itself is the secondary. Ramping the solenoid's current changes the magnetic flux through the center of the torus, and by Faraday's law that changing flux drives the megaampere-scale 📝plasma current around the ring. The same induction provides the machine's first heating — the plasma's resistance converts driven current to heat — though this ohmic heating fades as the plasma gets hotter and its resistance falls.
Induction has a built-in limit: a transformer only drives current while its flux is changing, and the solenoid can ramp only so far. This makes the inductively driven 📝Tokamak inherently a pulsed machine, and it is why non-inductive current drive — waves, particle beams, and the plasma's own bootstrap current — is the route to steady-state operation. The solenoid also claims the most contested real estate in the machine: it must fit inside the doughnut hole along with the inboard legs of the 📝toroidal field coils, which is a defining constraint for low-aspect-ratio designs. ITER's central solenoid, 18 meters tall and built from six modules, is among the largest superconducting magnets ever constructed.