At the ITER construction site in Saint-Paul-lès-Durance, in the hills of southern France, engineers have finished stacking and locking together the last pieces of a superconducting magnet taller than a four-story building and heavier than a loaded Boeing 747. The central solenoid, a column of six modules weighing roughly 1,000 tons combined, is now fully assembled inside the pit of the world’s largest tokamak fusion reactor. When the machine eventually switches on, this magnet will be responsible for one critical job: generating the intense, rapidly changing magnetic field needed to drive and sustain a ring of superheated plasma at temperatures exceeding 150 million degrees Celsius.
The milestone, confirmed by the ITER Organization in a May 2025 announcement, caps more than a decade of fabrication work led by General Atomics in San Diego under contract to the U.S. Department of Energy. It also marks one of the most demanding procurement deliveries in the history of the $22 billion international project. But completing the assembly and proving the magnet works under fusion conditions are two very different things, and the hardest tests still lie ahead.
What the central solenoid actually does
A tokamak confines plasma inside a doughnut-shaped vacuum vessel using overlapping magnetic fields. The central solenoid sits in the hole of the doughnut and acts like the primary winding of a giant transformer: by ramping its current up and down, it induces an electrical current in the plasma itself, which both heats the fuel and helps keep it stable. Without that induced current, the plasma would collapse against the vessel walls in microseconds.
Each of the solenoid’s six modules contains approximately 6 kilometers of niobium-tin (Nb3Sn) superconducting cable, a material so brittle that it must be wound into coils first and then heat-treated at around 650 °C for hundreds of hours before it becomes superconducting. The fully assembled stack is designed to produce a peak magnetic field of about 13.1 tesla and store roughly 5.5 gigajoules of energy. For perspective, 5.5 GJ is enough to power about 1,500 average American homes for a full day, all locked inside a cylinder roughly four meters across.
ITER’s own communications describe the finished magnet as strong enough to lift an aircraft carrier. That comparison refers to the enormous electromagnetic forces the solenoid generates during a plasma pulse, forces that its steel preload structure must absorb without allowing the modules to shift even slightly.
How engineers know the assembly met spec
Acceptance testing during integration followed criteria documented in a peer-reviewed paper published in IEEE Transactions on Applied Superconductivity. Three numbers stand out:
- Joint resistance below 4.1 nano-ohms. The coaxial electrical joints connecting modules must carry tens of thousands of amperes. Even tiny resistance generates heat, and heat is the enemy of superconductivity. The measured values came in under the threshold.
- Insulation qualified to 30 kV (standard) and 15 kV (Paschen conditions). Paschen testing simulates the low-pressure gas environments where electrical breakdown is most likely, a realistic proxy for conditions inside the cryostat.
- Module positioning within 2 mm of target. Each module weighs more than 110 tons. Even small misalignments could distort the magnetic field geometry and destabilize the plasma, so millimeter-level precision matters.
These results confirm that the solenoid, as built, matches its design envelope at room temperature and under assembly-stage conditions. They do not yet confirm performance at operating temperature.
What has not been proven yet
The real proving ground is cold testing: cooling the entire magnet to its superconducting operating temperature near 4.5 kelvin (about minus 269 °C) and ramping it to full current. As of early June 2026, no primary-source update has confirmed final cold-test results or full stored-energy measurements taken after the solenoid was integrated into the tokamak pit. Until those results are published, the 13.1 T peak field and 5.5 GJ stored energy remain design-stage values, not measured operational figures.
Long-term joint behavior is another open question. The 4.1 nano-ohm resistance threshold was measured during assembly qualification, essentially a snapshot. Fusion-relevant operation will subject those joints to thousands of thermal and electromagnetic cycles. If resistance drifts upward over time, the joints could become localized heat sources that trigger a quench, an abrupt loss of superconductivity that dumps enormous energy into the structure and can cause physical damage.
Then there is the broader schedule. ITER has experienced repeated delays and cost overruns over the past decade. Neither Oak Ridge National Laboratory, which managed the U.S. procurement effort, nor the ITER Organization has issued a revised project timeline or budget assessment tied specifically to the solenoid milestone, based on publicly available documentation reviewed for this article. Whether this completion puts the overall schedule on firmer ground remains an open question.
Where the solenoid fits in the larger machine
The central solenoid does not operate in isolation. It must work in concert with 18 toroidal field coils, six poloidal field coils, plasma-facing components rated to absorb punishing heat loads, a cryogenic system capable of maintaining near-absolute-zero temperatures across thousands of tons of superconducting material, and high-power heating systems that will push the plasma to fusion-relevant conditions. A deviation in any one of these subsystems could change the demands placed on the central magnet, from the exact current waveforms it must deliver to the mechanical loads it must withstand during plasma disruptions.
The solenoid’s Nb3Sn technology also invites comparison with a newer generation of fusion magnets. Commonwealth Fusion Systems, a private company spun out of MIT, has built and tested a high-temperature superconducting (HTS) magnet using rare-earth barium copper oxide tape that achieved a 20 T field in a smaller, lighter package. ITER’s design predates the commercial availability of HTS magnets at scale, and the central solenoid represents the pinnacle of what low-temperature Nb3Sn technology can deliver at this size. Whether future fusion reactors follow ITER’s magnet approach or shift to HTS designs is one of the defining technical questions in the field.
Why the milestone matters beyond ITER
Strip away the superlatives and the central solenoid’s completion proves something concrete: a multinational supply chain spanning the United States, Europe, Japan, South Korea, China, India, and Russia can fabricate, ship, and assemble components that push the boundaries of superconducting magnet technology. Brittle Nb3Sn conductors were manufactured in industrial quantities, wound into coils with sub-millimeter precision, and transported halfway around the world without damage. That logistics achievement has value regardless of how ITER’s plasma experiments eventually turn out.
For funding agencies and governments that have collectively committed billions to the project, the documented progress offers both reassurance and a reminder. The reassurance is that large-scale, high-risk hardware deliveries can reach completion despite years of supply-chain disruptions and technical setbacks. The reminder is that assembly is not operation. Transparent publication of cold-test data, revised milestones, and honest budget assessments will be essential as ITER moves from construction into commissioning.
Fusion breakthroughs are built from incremental, verifiable steps, not single dramatic moments. A 1,000-ton magnet capable of generating fields that rival conditions inside stars is an extraordinary object. But its true significance will only become clear when it is cooled to near absolute zero, energized to full current, synchronized with the rest of the machine, and asked to do the job it was designed for: driving a plasma current long and stable enough to bring fusion power closer to reality.
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*This article was researched with the help of AI, with human editors creating the final content.
