The doughnut-shaped machine that dominates fusion research — how magnetic fields confine a star-hot plasma, and why most of the world's biggest fusion projects chose this design.
If you have heard of fusion energy, you have almost certainly heard of the tokamak. The word — a Russian acronym coined in the 1950s — describes a machine that traps super-hot plasma inside a doughnut-shaped (toroidal) vacuum chamber using powerful magnetic fields. More than 200 tokamaks have been built worldwide, and the concept underpins ITER, the 35-nation megaproject under construction in southern France.1
Fusion fuel — typically a mix of deuterium and tritium, two heavy forms of hydrogen — must be heated to roughly 150 million degrees Celsius before the atomic nuclei move fast enough to overcome their mutual electrical repulsion and fuse. At those temperatures the fuel becomes a plasma: a roiling cloud of charged particles. No solid wall can touch it without melting, so the tokamak uses magnetic fields as an invisible cage.
A tokamak combines two magnetic fields to confine the plasma. The first, called the toroidal field, is generated by large D-shaped coils wrapped around the vacuum vessel. This field runs the long way around the doughnut. The second, called the poloidal field, is generated partly by external coils stacked above and below the machine and partly by a strong electrical current driven through the plasma itself.2
Neither field alone can hold the plasma stable. Together, they create helical (corkscrew-shaped) magnetic field lines that wind around the inside of the doughnut. Charged particles follow these spiraling lines, staying away from the walls.
The plasma current that contributes to the poloidal field is usually driven by a large solenoid (a coil of wire) running through the center of the doughnut, called the central solenoid. It works like the primary winding of a transformer: ramping the current in the solenoid induces a current in the plasma, which acts as the transformer's secondary winding.3
This transformer action is inherently pulsed — eventually the solenoid runs out of room to ramp further. Sustaining a plasma current continuously (so-called "steady-state" operation) is one of the major engineering challenges for future tokamak power plants and requires supplementary current-drive techniques such as neutral-beam injection or radiofrequency waves.
The plasma current itself provides some heating through electrical resistance (ohmic heating), but this becomes less effective as the plasma gets hotter. Additional heating is supplied by neutral-beam injectors, which fire high-energy atoms into the plasma, and by radiofrequency antennas that pump electromagnetic energy into the plasma at frequencies chosen to resonate with ion or electron motion.4
The tokamak concept proved its worth in 1968 when the Soviet T-3 device demonstrated plasma temperatures far beyond what other confinement schemes could achieve at the time. Since then, Europe's JET set the record for fusion power at 69 megajoules in a single pulse in 2021, and ITER aims to produce 500 MW of fusion power from 50 MW of heating input — a gain factor (Q) of 10.5
Tokamaks face several persistent challenges: plasma disruptions (sudden losses of confinement that can damage the vessel), the need for a massive central solenoid that limits how compact the machine can be, and the complexity of handling tritium fuel. Newer designs — compact tokamaks with high-temperature superconducting magnets, spherical tokamaks with a slimmer core — aim to shrink the device and reduce cost. Meanwhile, some researchers pursue entirely different confinement geometries like the stellarator, which dispenses with the plasma current altogether.
Despite these hurdles, the tokamak remains the most extensively studied and best-understood path to fusion energy, and the one closest to demonstrating net power gain at reactor scale.