The world’s powerful device being built in France reaches an important milestone for nuclear fusion

The world’s largest and most powerful device being built for clean energy reached an important milestone in 2025.

The International Thermo-nuclear Experimental Reactor (ITER) Project is currently under construction in a 180-hectare site in Southern France. Over 30 countries, including India, are collaborating to build the world’s largest tokamak, a magnetic fusion device to prove the feasibility of nuclear fusion as a large-scale and carbon-free source of energy.

Nuclear fusion generates energy through merging of two light nuclei to form a single heavier nucleus. Fusion reactions power the Sun and other stars.

In 2025, ITER announced that it had completed all components for the world’s largest, most powerful pulsed superconducting electromagnet system, which will function as the heart of ITER’s donut-shaped reactor, called a Tokamak. After assembling the different components of the fully assembled pulsed magnet system, it will weigh nearly 3,000 tonnes.

Finally, the experiment will allow scientists to inject a few grams of hydrogen fuel—deuterium and tritium gas—into ITER’s gigantic Tokamak chamber.  Deuterium and tritium are two hydrogen isotopes (variants), which have been identified as the most efficient way of achieving fusion.

The pulsed magnet system will then kick in to send an electrical current to ionise the hydrogen gas to create a plasma, a cloud of charged particles. Plasmas — called the fourth state of matter — provide the environment in which light elements can fuse and yield energy. Powerful magnetic fields help confine and control the plasma.

After this, external heating systems will increase the plasma temperature to 150 million degrees Celsius — about 10 times hotter than the core of the sun. At such high temperatures, the atomic nuclei fuse to produce helium nucleus, one neutron, and great amounts of energy.

The goal of ITER is to produce 500 megawatts of fusion power as output from only 50 megawatts of input heating power — a tenfold gain. In 2022, the Lawrence Livermore National Laboratory — a federal research facility in California, US — reported a major scientific breakthrough in nuclear fusion science by producing 3 megajoules (MJ) of energy from an input of 2 MJ.

The final component of ITER was constructed and delivered by the US. The country built the Central Solenoid, a component of the magnet system. Its job is to induce the majority of the magnetic flux change needed to initiate the plasma, generate the plasma current, and maintain this current during the burn time. It has also designed a support structure that will help Central Solenoid to withstand the extreme forces it will generate.

Russia has contributed the Poloidal Field magnet that will crown the top of the ITER Tokamak. It measures nine metres in diameter and weighs approximately 160 tonnes, and is the smallest of the six ring-shaped magnets that circle the ITER Tokamak. Europe has designed four ring-shaped Poloidal Field magnets onsite in France, ranging from 17 to 24 metres in diameter.  China, too, has built a 10-metre Poloidal Field magnet, which already been installed at the bottom of the partially assembled ITER Tokamak.

China is also contributing 18 superconducting Correction Coil magnets, which will help fine-tune the plasma reactions. Japan has produced 43 kilometres of Niobium-Tin (Nb₃Sn) superconductor strand that was used to create the Central Solenoid modules in the US while Korea has produced the tooling needed to pre-assemble ITER’s largest components.

India has built the Cryostat, the 30-metre high, 30-metre diameter vacuum shell made of stainless steel that encases the entire ITER Tokamak. It was designed by ITER-India — a specially empowered project organisation within the Institute for Plasma Research, an autonomous institute under the Department of Atomic Energy (DAE). L&T Heavy Engineering was roped in for the manufacturing and installation of the ITER Cryostat.

As for financing of the project, Europe, as the Host Member, contributes the lion’s share, 45 per cent of the cost of the ITER Tokamak and its support systems. China, India, Japan, Korea, Russia, and the United States each contribute 9 per cent.

“What makes ITER unique is not only its technical complexity but the framework of international cooperation that has sustained it through changing political landscapes,” Pietro Barabaschi, ITER director-general, said in a statement. ITER is expected to begin scientific operations in 2034. The Deuterium-Tritium operation could begin in 2039.