The world's largest fusion experiment explained — how a giant tokamak in southern France aims to prove that fusion energy can produce more power than it consumes.
ITER — Latin for "the way" — is a multinational fusion research project under construction in Saint-Paul-lès-Durance, France. It is designed to be the first fusion device to produce net energy: 500 megawatts of fusion power from 50 megawatts of heating input, achieving a fusion gain (Q) of 10. Understanding how ITER works requires walking through its core components, the physics it exploits, and the engineering that makes it possible.[1]
ITER is a tokamak, a doughnut-shaped (toroidal) magnetic confinement device originally conceived in the Soviet Union in the 1950s. The fundamental idea is straightforward: because fusion fuel must be heated to temperatures exceeding 150 million degrees Celsius — ten times hotter than the core of the Sun — no physical material can contain it. Instead, the fuel is held in place by powerful magnetic fields that keep the superheated plasma away from the vessel walls.[2]
The tokamak achieves confinement through a combination of two magnetic field components. A toroidal field, generated by large D-shaped superconducting magnets arranged around the vacuum vessel, wraps the plasma in the long way around the torus. A poloidal field, generated partly by an electric current driven through the plasma itself and partly by external coils, provides the twist that stabilizes the plasma against drift. Together these fields create helical magnetic field lines that particles follow, staying confined within the plasma volume.
ITER is enormous by design. The tokamak will stand roughly 30 meters tall and weigh 23,000 tonnes. Its major components include:
The vacuum vessel is a double-walled steel container that holds the plasma and provides the first confinement barrier. Weighing about 5,200 tonnes, it is lined internally with blanket modules — 440 individual units that shield the vessel from neutron bombardment and absorb heat. A tungsten divertor at the bottom of the vessel extracts helium ash and impurities from the plasma, functioning as the tokamak's exhaust system.
The cryostat, the largest stainless-steel vacuum chamber ever built (29.3 meters tall, 29.3 meters in diameter), surrounds the entire tokamak and maintains the ultracold environment needed for the superconducting magnets.
ITER will use a deuterium-tritium (D-T) fuel mix. Deuterium is extracted from seawater (about 33 grams per cubic meter), making it virtually inexhaustible. Tritium, a radioactive hydrogen isotope with a half-life of 12.3 years, is rarer and will initially be sourced from existing stockpiles. Later fusion plants are expected to breed their own tritium by surrounding the plasma with lithium blankets.[3]
When a deuterium nucleus and a tritium nucleus fuse, they produce a helium-4 nucleus (alpha particle) carrying 3.5 MeV of energy and a neutron carrying 14.1 MeV. The alpha particles remain in the plasma, providing self-heating. The neutrons, being electrically neutral, pass through the magnetic field and deposit their energy in the blanket, where it can ultimately be captured as heat.
Reaching fusion temperatures requires multiple heating systems working in concert. ITER will employ three methods:
Ohmic heating: The central solenoid acts as the primary winding of a transformer, inducing a current of up to 15 million amperes in the plasma (the secondary winding). This current heats the plasma resistively, but ohmic heating alone is insufficient — plasma resistance drops as temperature rises.
Neutral beam injection (NBI): Two neutral beam injectors will fire high-energy deuterium atoms into the plasma at energies of 1 MeV. These fast atoms transfer their energy to the plasma through collisions, providing up to 33 megawatts of heating power.
Radio-frequency heating: Ion cyclotron resonance heating (ICRH) and electron cyclotron resonance heating (ECRH) use electromagnetic waves at specific frequencies to energize ions and electrons, respectively. Together they will contribute up to 40 megawatts.[1]
ITER's primary mission is to demonstrate that a burning plasma — one sustained largely by the energy of its own alpha particles — is achievable at reactor scale. At Q = 10, the plasma would produce ten times more fusion power than the external heating power injected, with the alpha particles providing the dominant heating. This condition has never been achieved; the current record, set by the Joint European Torus (JET) in 1997, was Q ≈ 0.67.[2]
ITER will also study long-pulse operation. While JET's record-setting shots lasted seconds, ITER aims to sustain plasma for 400 to 600 seconds in its baseline scenarios, and potentially up to 3,600 seconds in advanced steady-state scenarios at lower Q.
ITER is an experimental facility, not a power plant. It will not generate electricity. The 500 MW of fusion power it produces will be dissipated as heat; there is no turbine or generator. ITER is also not designed to breed its own tritium at scale, though it will test tritium breeding blanket modules to inform the design of its successor, DEMO.[3]
ITER is a collaboration among 35 nations: the European Union (the host party), China, India, Japan, South Korea, Russia, and the United States. Construction began in 2010, with first plasma originally targeted for 2025. As of mid-2026, the project has experienced significant delays and cost overruns, with first plasma now anticipated no earlier than the early 2030s. The total project cost is estimated to exceed €20 billion. Despite the delays, ITER remains the centerpiece of the international fusion roadmap and the essential stepping stone toward commercial fusion energy.[1]