An approach to controlled fusion that uses strong magnetic fields to hold a hot plasma in a stable configuration long enough for fuel ions to fuse and release net energy.
Charged particles in a magnetic field experience a Lorentz force that causes them to gyrate around field lines in tight helical orbits. The radius of this gyration (the Larmor radius) is typically a few millimeters for ions in a fusion-grade plasma at 5 T, far smaller than the dimensions of the confinement vessel. By arranging magnetic field lines into closed or quasi-closed configurations, plasma can be confined away from material walls for the seconds-to-minutes duration required for sustained fusion reactions.[1]
The fundamental challenge is that a straight magnetic field confines particles only perpendicular to the field; they are free to stream along it. All practical MCF configurations must therefore close or twist the field lines so that particles remain within a bounded volume. The two dominant solutions are the tokamak and the stellarator, both of which use toroidal (doughnut-shaped) geometries.[2]
The quality of magnetic confinement is measured by the energy confinement time τE: the characteristic time over which the plasma would cool if all heating were turned off. Achieving the Lawson criterion for D–T fusion requires τE on the order of several seconds at densities around 1020 m−3.[3]
In practice, energy and particles leak across field lines much faster than classical collision theory predicts. This anomalous transport is driven primarily by plasma microturbulence. Empirical scaling laws, validated across dozens of tokamak experiments worldwide, relate τE to machine size, magnetic field strength, plasma current, and other parameters. The most widely used, the IPB98(y,2) scaling, underpins the ITER design basis.[3]
Three methods are standard: neutral beam injection (NBI), radiofrequency heating (ICRH and ECRH), and ohmic heating. Ohmic heating becomes ineffective above ~3 keV as resistivity drops with rising temperature.[2]
The JET tokamak held the record for fusion energy production (59 MJ in a sustained D–T pulse in 2021 — a verified result) before its retirement in 2024. ITER, under construction at Cadarache, France, is designed to achieve Q ≥ 10 with 500 MW of fusion power. The Wendelstein 7-X stellarator in Germany has demonstrated record confinement times for its configuration class.[3]