World’s largest tokamak

330-ton superconducting magnet for fusion plasma cooled by world’s largest tokamak       

ITER is an international collaboration of more than 30 countries to demonstrate the viability of fusion, the power of the sun and stars, as an abundant, safe, carbon-free energy source for the planet. The fully assembled pulsed magnet system will weigh nearly 3,000 tons. It will function as the electromagnetic heart of ITER’s donut-shaped reactor, called a Tokamak. ITER’s geopolitical achievement is also remarkable: the sustained collaboration of ITER’s seven members, China, Europe, India, Japan, Korea, Russia and US. Thousands of scientists and engineers have contributed components from hundreds of factories on three continents to build a single machine. Pietro Barabaschi, ITER Director-General, says, “What makes ITER unique is not only its technical complexity but the framework of international cooperation that has sustained it through changing political landscapes. This achievement proves that when humanity faces existential challenges like climate change and energy security, we can overcome national differences to advance solutions. The ITER Project is the embodiment of hope. With ITER, we show that a sustainable energy future and a peaceful path forward are possible.”  Following are the steps of how this pulsed superconducting electromagnet system work:-

Step 1. A few grams of hydrogen fuel—deuterium and tritium gas—are injected into ITER’s gigantic Tokamak chamber.

Step 2. The pulsed magnet system sends an electrical current to ionize the hydrogen gas, creating a plasma, a cloud of charged particles.

Step 3. The magnets create an “invisible cage” that confines and shapes the ionized plasma.

Step 4. External heating systems raise the plasma temperature to 150 million degrees Celsius, ten times hotter than the core of the sun. 

Step 5. At this temperature, the atomic nuclei of plasma particles combine and fuse, releasing massive heat energy.

Now, engineers are preparing for high-current trials at ITER’s newly activated Magnet Cold Test Facility, following a 12-day cooling process which lowered a 330-ton toroidal field coil to its functioning temperature of 4 Kelvin (minus 269 degrees Celsius). This operational milestone was celebrated during a site visit by the ITER Council’s Management Advisory Committee. This program verifies magnet performance before the parts are locked into the main reactor. Over the course of four to six months/coil, the team will subject components to full operational electrical currents, which reach 68 kiloamperes (kA) for the toroidal field units and 48 kA for the poloidal field units. “Eighteen D-shaped toroidal field coils, six ring-shaped poloidal field coils, and the six independent modules of the central solenoid-with a combined stored magnetic energy of 51 Gigajoules (GJ)-will produce the magnetic fields that initiate, confine, shape and control the ITER plasma,” said ITER.

At full operation, ITER is expected to produce 500 megawatts of fusion power from only 50 megawatts of input heating power, a tenfold gain. At this level of efficiency, the fusion reaction largely self-heats, becoming a “burning plasma.” By integrating all the systems needed for fusion at industrial scale, ITER is serving as a massive, complex research laboratory for its 30-plus member countries, providing the knowledge and data needed to optimize commercial fusion power. Constructed from niobium-tin (Nb3Sn) and niobium-titanium (Nb-Ti), these alloys require liquid helium immersion to lose their electrical resistance. Superconducting systems are essential for industrial-scale fusion because they generate intense magnetic fields with minimal electricity compared to copper variants. However, this state relies on keeping temperatures, current levels and magnetic forces below strict physical limits. If these thresholds fail, the material undergoes a “quench,” reverting to a standard resistive state that releases sudden heat. Consequently, a major goal of these cold runs is verifying that the automatic safety sensors detect these thermal changes instantly.

Running these benches alongside existing plant networks provides early data on how the central control systems, power feeds, vacuums and cooling setups interact. This parallel testing reveals vulnerabilities prior to final plant commissioning. Though the setup cannot mimic the exact environment of an active fusion reaction, it measures how the magnets handle stress, monitors insulation behavior and checks the integrity of internal superconducting joints. Managing components which weigh hundreds of tonnes demands significant logistics, including a 20-meter-long cryostat chamber, heavy electrical links and direct lines to the facility’s primary helium cryoplant. ITER built this testing area inside an assembly hall formerly used by Fusion for Energy for shaping large exterior coils, capitalizing on the building’s existing heavy cranes and its layout. ITER Director-General Pietro Barabaschi noted that adapting this existing footprint allowed the organization to lower project risks logically before starting full system integration. He added that the facility will eventually serve the broader commercial fusion market by sharing technical insights. 

The past five years have witnessed a surge in private sector investment in fusion energy R&D. In November 2023, the ITER Council recognized the value and opportunity represented by this trend. They encouraged the ITER Organization and its Domestic Agencies to actively engage with the private sector, to transfer ITER’s accumulated knowledge to accelerate progress toward making fusion a reality. In 2024, ITER launched a private sector fusion engagement project, with multiple channels for sharing knowledge, documentation, data and expertise, as well as collaboration on R&D. In April 2025, ITER hosted a public-private workshop to collaborate on the best technological innovation to solve fusion’s remaining challenges. Once ITER finishes its own scheduled runs on the initial niobium-tin (Nb3Sn) coil and subsequent manufacturer deliveries, private fusion ventures will gain access to the testing area. “This is important for ITER as well as an example of how ITER can support the wider fusion ecosystem by creating knowledge, infrastructure and operational experience that others can use,” concluded Barabaschi. 

Under the ITER Agreement, Members contribute most of the cost of building ITER in the form of building and supplying components. This arrangement means that financing from each Member goes primarily to their own companies, to manufacture ITER’s challenging technology. In doing so, these companies also drive innovation and gain expertise, creating a global fusion supply chain. Europe, as the Host Member, contributes 45% of the cost of the ITER Tokamak and its support systems. China, India, Japan, Korea, Russia, and US each contribute 9%, but all Members get access to 100% percent of the intellectual property.