Superconducting magnets wind helical field lines around a donut-shaped (toroidal) vacuum vessel, confining a 100-million-degree plasma of hydrogen isotopes away from the walls. Deuterium and tritium nuclei fuse when confinement, temperature, and density are all high enough — the Lawson criterion.
nτE·T ≥ ~3×10²¹ keV·s/m³ (Lawson triple product for ignition)
P_fusion ∝ n² · <σv>(T) (reaction rate rises steeply with T)
- Mode — switches between magnetic (tokamak, steady toroidal plasma) and inertial confinement (a compressed pulsed fuel pellet).
- Toroidal field — stronger fields squeeze the plasma tighter, raising confinement quality and reducing turbulent transport.
- Plasma current — drives the poloidal field component that twists field lines into confining flux surfaces.
- Ion temperature — sets the fusion reaction rate; power rises sharply above ~10 keV.
- Fuel density — fusion power scales roughly with density squared, but too much raises disruption risk.
Real-world: ITER (France) and private ventures like Commonwealth Fusion Systems' SPARC aim to cross the Lawson threshold for net energy gain, a step toward commercial fusion power plants.