A tokamak confines hot hydrogen-isotope plasma in a torus using powerful magnetic-field coils, keeping it away from any material wall while it is heated to fusion-relevant temperatures. Strong toroidal and poloidal fields resist plasma pressure and reduce turbulent energy loss, extending the energy confinement time τ. Fusion becomes self-sustaining once the triple product of density, temperature and confinement time crosses the Lawson criterion.
Lawson criterion: n · T · τ > ~3×10²¹ keV·s/m³
Confinement time (simplified): τ ∝ B² / (n · loss_rate)
Fusion power density ∝ n² · ⟨σv⟩(T)
- Magnetic field — stronger coil fields squeeze and better confine the plasma, raising τ and stabilizing the torus.
- Plasma density — more fuel particles per m³ raises the fusion reaction rate, but too much can trigger instabilities.
- Plasma temperature — must reach ~10 keV (~100 million °C) for deuterium-tritium fusion cross-sections to become significant.
- Field lines toggle — reveals the helical magnetic field lines that guide and confine the charged plasma particles.
Devices such as ITER and JET use this exact triple-product logic to plan the field strength, density and heating power needed to reach net energy gain.