In a cavernous assembly hall at a facility in southern France, engineers are stacking the final modules of a magnet taller than a five-story building. The ITER central solenoid, 59 feet of superconducting coil designed to carry 40,000 amps of current at temperatures just a few degrees above absolute zero, is approaching completion after a 15-year manufacturing and assembly campaign. When finished, it will be the most powerful pulsed superconducting magnet ever constructed for fusion energy research.
The milestone, confirmed in peer-reviewed papers published in IEEE Transactions on Applied Superconductivity, marks the removal of one of the single largest hardware risks from ITER’s path toward producing a sustained fusion reaction. But the road to this point was anything but smooth, and the magnet’s completion arrives against a backdrop of persistent schedule delays and ballooning costs for the international fusion project as a whole.
A magnet built to ignite a star
ITER is a tokamak, a doughnut-shaped reactor designed to confine superheated plasma with powerful magnetic fields and, for the first time in history, produce more fusion energy than it consumes. The central solenoid sits at the heart of that machine. Its job is to induce and drive the electrical current that flows through the plasma, shaping it into the precise configuration needed for fusion reactions to occur. Without it, ITER cannot operate.
The solenoid is built from six stacked coil modules, each wound from niobium-tin superconducting cable and cooled to roughly 4.5 Kelvin, about minus 450 degrees Fahrenheit. At that temperature, the cable loses all electrical resistance, allowing enormous currents to circulate without generating heat. Each module was individually tested to its full operating current of 40,000 amps (40 kA) before being cleared for assembly, a manufacturing campaign that alone consumed roughly a decade.
The scale is hard to overstate. The finished magnet stands 59 feet tall, and the assembly process required a purpose-built 43-ton lifting fixture just to handle the modules. Engineers at Oak Ridge National Laboratory, which led the U.S. contribution to the central solenoid, have pursued sub-millimeter alignment goals while stacking components that weigh as much as a loaded freight car.
Tolerances measured in millimeters and nano-ohms
Superconducting magnets are unforgiving. Even tiny imperfections in electrical connections or physical alignment can trigger a “quench,” an event where part of the magnet abruptly loses superconductivity and dumps its stored energy as heat. At the currents and energies involved in the central solenoid, a quench is not just a nuisance; it can damage hardware and set a project back by months.
That is why the assembly tolerances documented in a second IEEE paper matter so much. The coaxial joints connecting the solenoid’s modules measured below 4.1 nano-ohms of resistance, a figure so small it is essentially the electrical equivalent of a perfect connection. Electrical insulation between modules was qualified at 30 kilovolts under Paschen discharge conditions, and module positioning came within 2 millimeters of target across the full 59-foot stack.
For context, 2 millimeters is roughly the thickness of a nickel. Holding that tolerance across a tower of superconducting modules operating near absolute zero is what separates a working fusion magnet from a very expensive piece of scrap metal.
The joint resistance crisis that cost years
The project did not reach this point on schedule. The first central solenoid module ran into serious trouble during early validation when joint resistance measurements came back higher than acceptable. The problem forced engineers to redesign the joints and repeat acceptance testing from scratch, a process that consumed years and pushed the overall timeline well beyond original projections.
Oak Ridge National Laboratory reported that the reworked module eventually received a “clean bill of health,” but the episode illustrates a recurring theme in large-scale fusion engineering: problems that look manageable on paper can become multi-year detours when they surface in hardware operating at the extremes of temperature, current, and precision.
The setback also fed into broader criticism of ITER’s management and cost trajectory. The project, funded by a consortium of 35 nations, has seen its timeline for achieving first plasma slip repeatedly. The central solenoid’s troubles were one piece of that larger picture, though the magnet team’s eventual success in meeting its performance targets stands as a counterpoint to the narrative of perpetual delay.
What still has to happen
As of mid-2026, the central solenoid’s assembly is ongoing but not yet finished. No verified public source specifies a firm date for when the fully assembled magnet will be installed in the ITER tokamak pit. ORNL program updates confirm that the work continues, but completion and installation timelines should be treated as projections rather than locked dates.
The total cost of the central solenoid program has not been disclosed in the primary engineering literature or in ORNL’s institutional releases. Estimates from secondary sources vary, and none have been verified against official U.S. Department of Energy or ITER Organization financial disclosures.
Perhaps the most consequential unknown is long-term performance. The peer-reviewed papers document manufacturing quality and assembly tolerances in impressive detail, but they do not model how the solenoid will behave over thousands of plasma pulses during ITER’s operational life. Superconducting magnets can degrade under repeated thermal cycling and electromagnetic stress, and whether the joint resistance and insulation values achieved during acceptance testing will hold up across years of plasma campaigns remains an open question.
Where the central solenoid fits in the fusion race
ITER is not the only fusion effort making headlines. Private companies like Commonwealth Fusion Systems and TAE Technologies are pursuing smaller, faster reactor designs, some using high-temperature superconducting magnets that were not available when ITER’s central solenoid was designed. The competitive landscape has shifted significantly since the solenoid’s manufacturing began in the early 2010s.
But ITER remains the only project designed to demonstrate net energy gain from fusion at reactor scale, and the central solenoid is the component that makes its plasma physics possible. Completing the most powerful pulsed superconducting magnet ever built for fusion does not guarantee that ITER will achieve its goals, but it removes one of the tallest engineering hurdles standing in the way. For a project that has weathered 15 years of setbacks, redesigns, and budget battles, getting the magnet right counts as something worth noting.
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*This article was researched with the help of AI, with human editors creating the final content.
