The world’s largest tokamak, under construction in southern France, designed to demonstrate sustained deuterium–tritium burning plasma at a plasma energy gain factor Q ≥ 10.
ITER (originally an acronym for International Thermonuclear Experimental Reactor, now styled simply as a word meaning “the way” in Latin) is a tokamak under construction at the Cadarache research centre in Saint-Paul-lès-Durance, southern France. The project unites 35 nations—the European Union (through Fusion for Energy), China, India, Japan, South Korea, Russia, and the United States—making it the largest multilateral science collaboration since the International Space Station.[1]
ITER’s tokamak confines a hydrogen plasma inside a D-shaped vacuum vessel surrounded by 18 superconducting niobium-tin (Nb3Sn) toroidal field coils and 6 poloidal field coils fabricated from niobium-titanium (NbTi). The magnets operate at 4.5 K. The device is designed to produce 500 MW of fusion power from 50 MW of external heating, yielding a plasma energy gain Q ≥ 10—a verified design target established in the ITER Physics Basis.[1] This would be the first demonstration of a magnetically confined burning plasma, in which alpha-particle self-heating dominates the plasma energy balance. It is essential to note that Q ≥ 10 refers to plasma gain, not net electrical output; ITER will not generate electricity.
The core mission is to demonstrate the feasibility of sustained D–T fusion at reactor-relevant conditions. Key objectives include: maintaining a burning plasma for hundreds of seconds; demonstrating integrated plasma-control scenarios at Q ≥ 5 in steady-state-like operation; and testing tritium breeding blanket module technologies in a fusion neutron environment.[2] ITER will also serve as an essential testbed for plasma–material interactions at reactor-scale heat and particle fluxes, particularly on its tungsten divertor.
Site preparation began in 2007, with the first concrete for the tokamak complex poured in 2013. Assembly of major components—including the cryostat base, vacuum vessel sectors, and toroidal field coils—began in 2020.[3] The project has experienced significant cost growth and schedule delays. First plasma was originally targeted for 2025 but has been revised multiple times. In June 2024, the ITER Council approved a New Baseline that pushed first plasma to approximately 2034, with full D–T operations following several years later. Total project costs, initially estimated at roughly €5 billion (2001 value), now exceed €20 billion.[4] These delays have prompted debate about the relative pace of private-sector fusion ventures.
Despite schedule challenges, ITER remains the flagship of the international magnetic fusion programme. Its results on burning-plasma physics, alpha-particle transport, disruption mitigation, and steady-state scenario development are expected to inform the design of successor demonstration power plants (DEMO) in Europe, China, Japan, and elsewhere. No other facility currently under construction can access the reactor-scale burning-plasma regime that ITER targets.
ITER’s auxiliary heating systems—33 MW of neutral beam injection, 20 MW of ion cyclotron resonance heating, and 20 MW of electron cyclotron resonance heating—will provide flexible plasma control and current-drive capability. The device will also host six test blanket modules from partner nations, providing the first integrated tests of tritium breeding, heat extraction, and neutron shielding in a fusion neutron environment. These engineering tests are as important to the path toward a power plant as the plasma physics programme itself.[2]