Tokamak

A doughnut-shaped machine that confines plasma by combining the magnetic field of external coils with the field produced by a current driven through the plasma itself. The dominant approach to magnetic confinement.

The name is a Russian acronym for a toroidal chamber with magnetic coils. The concept was developed in the early 1950s by Igor Tamm and Andrei Sakharov in the Soviet Union.

With external coils alone, the field lines simply run once round the doughnut, and the plasma drifts outward and escapes. A tokamak therefore drives a large current through the plasma; the field of that current twists the lines into a helix, which holds the plasma steady^[1].

The turning point came in 1968, when the Soviet T-3 reported temperatures far beyond any other design. Few believed it, so in 1969 a British team travelled to Moscow and measured the plasma by laser scattering, confirming about 10 million °C^[2]. Research worldwide converged on the tokamak after that.

The weakness is that the plasma current must be sustained. That makes continuous operation difficult, and if the current collapses suddenly — a disruption — the machine takes large forces. ITER, JT-60SA and SPARC are all tokamaks.