A plain-language guide to the most successful fusion device ever built — how magnetic fields trap plasma in a doughnut shape and bring it to the temperatures needed for fusion.
A tokamak is a machine that uses magnetic fields to confine a hot plasma (ionised gas) in the shape of a doughnut (torus). The plasma must reach temperatures of 100–200 million degrees Celsius — ten times hotter than the centre of the Sun — for the hydrogen nuclei inside it to fuse together and release energy.[1]
A tokamak uses two magnetic field components that combine into a helical (twisted) pattern:
Toroidal field: Large D-shaped coils wrapped around the torus create a field that goes the long way around the doughnut. This is the primary confining field (5–13 tesla in current devices, up to 20+ T in next-generation machines).
Poloidal field: A large current (millions of amperes) driven through the plasma itself creates a field that goes the short way around. This current is induced by a central solenoid — essentially a giant transformer with the plasma as the secondary winding.
The combination of these two fields creates helical field lines that spiral around the torus. Charged particles follow these lines, staying confined in the middle rather than drifting to the walls.[2]
The plasma current provides some heating (ohmic heating), but this becomes less effective at high temperatures. Additional heating systems push the plasma to fusion conditions:
Neutral beam injection: High-energy atoms are fired into the plasma, where they ionise and transfer their energy through collisions. Radio-frequency heating: Electromagnetic waves at specific frequencies (ion cyclotron, electron cyclotron) resonate with the plasma and transfer energy.
Plasma particles that escape the core hit a specially designed component called the divertor — a heavily armoured target at the bottom of the vessel that handles extreme heat loads (10–20 MW/m²) and pumps away helium ash and impurities.[3]