In the assembly hall of the world’s largest fusion experiment in southern France, five massive superconducting magnet modules now sit stacked in a column that stretches more than four meters tall. Each one weighs about 110 metric tons and contains roughly 6 kilometers of niobium-tin superconducting cable, wound, heat-treated, and insulated over a process that took years per unit. Together, they form the nearly complete central solenoid of ITER, the 35-nation fusion project designed to prove that fusing hydrogen isotopes can produce net energy on a commercial-relevant scale.
The sixth and final module, completed at General Atomics’ facility in San Diego, is expected to arrive at the ITER site in Saint-Paul-les-Durance later in 2026. When it does, the full magnet assembly will be within reach for the first time in the project’s nearly four-decade history.
The magnetic heart of the machine
The central solenoid sits at the geometric center of ITER’s tokamak, a doughnut-shaped vacuum chamber where deuterium and tritium fuel will be heated to temperatures exceeding 150 million degrees Celsius. By rapidly ramping its magnetic field, the solenoid induces an electrical current in the plasma. That current does double duty: it heats the fuel and helps confine it in a stable ring, preventing the superheated gas from slamming into the chamber walls.
Without this component, ITER cannot attempt its primary goal of producing 500 megawatts of fusion power from 50 megawatts of heating input, a tenfold energy gain known as Q=10. The central solenoid is often called the beating heart of the tokamak because its pulsed operation drives each plasma discharge from start to finish.
All six modules were manufactured by General Atomics under contract with the U.S. ITER project office, which is managed through the Department of Energy’s Oak Ridge National Laboratory. The superconducting cable inside each module must operate at roughly 4 kelvin (minus 269 degrees Celsius), cold enough to carry enormous currents with zero electrical resistance. Achieving that performance requires exacting quality control at every stage, from cable winding to final cryogenic testing.
Steel exoskeleton delivered
Alongside the magnet modules, a less visible but equally critical piece of hardware has reached the ITER site: the central solenoid’s compression structure. Oak Ridge National Laboratory announced that the final shipment of this steel framework arrived in France in spring 2025, completing a delivery campaign that spanned multiple years.
The compression structure functions like a high-strength exoskeleton. When the solenoid cycles between high and low magnetic states, forces on the order of tens of thousands of tons try to push the stacked modules apart and bend them out of alignment. The precompression system clamps them together tightly enough to keep the assembly stable through thousands of operational pulses, distributing loads so that no individual module or joint is overstressed.
ORNL’s expertise in materials science, advanced alloys, and magnet engineering, honed through decades of work on facilities like the Spallation Neutron Source, made it a natural fit for designing steel components that must tolerate cryogenic temperatures, intense magnetic fields, and dynamic mechanical loads simultaneously. With the structural backbone now on-site, the path to final assembly depends on the arrival and integration of the last module from San Diego.
Shipping a 110-ton superconductor across the world
Getting the sixth module from General Atomics to southern France is not a simple freight job. Previous modules traveled by specialized heavy-transport ship across the Pacific, through the Panama Canal, and across the Atlantic before arriving at the Mediterranean port of Fos-sur-Mer. From there, each unit was loaded onto a custom transport frame and moved overland along a dedicated route to the ITER site, a journey that took weeks of careful planning and execution.
The ITER Organization confirmed that the sixth module was completed at General Atomics in April 2025. No official shipping date has been published as of June 2026, but the organization has indicated the module is expected on-site before the end of the year. Once it arrives, technicians will need to stack it atop the existing five, connect electrical joints between modules, and integrate the full assembly with the compression structure before cryogenic cooldown and electrical testing can begin.
Where the solenoid fits in ITER’s revised timeline
The central solenoid milestones arrive against a backdrop of significant schedule and budget pressure. In 2024, the ITER Council approved a revised project baseline that pushed the target for first plasma operations to no earlier than 2033, with full deuterium-tritium fusion experiments following several years later. The project’s total cost, shared among its 35 member nations, has grown substantially from early estimates, though precise current figures remain difficult to pin down because each partner accounts for its contributions differently.
The United States funds its share of ITER through the Department of Energy’s Office of Science. Congressional appropriations for the U.S. ITER contribution have totaled roughly $2.4 billion through recent fiscal years, covering the central solenoid modules, the compression structure, and other in-kind hardware deliveries. Neither ORNL’s announcement nor the ITER Organization’s release includes updated cost data specific to the solenoid work, so it is not possible to say precisely how spending on this subsystem compares to earlier projections.
The solenoid is also just one piece of a far larger puzzle. ITER’s vacuum vessel, cryostat, toroidal field magnets, heating systems, and diagnostic instruments must all be completed, installed, and commissioned before plasma experiments can begin. Assembly of the tokamak has been underway since 2020, and the project has described the current phase as the most complex period of integration, with components arriving from fabrication sites across Europe, Asia, and North America on overlapping schedules.
What testing still lies ahead
Stacking the modules is a visible milestone, but proving the solenoid works as designed is a separate and demanding challenge. Before ITER can run plasma, engineers must cool all six modules to their operating temperature of about 4 kelvin, verify that the superconducting joints between modules carry current without measurable resistance, and confirm that the compression structure holds its preload values through repeated magnetic cycling.
None of these commissioning results have been published as of June 2026. That is not unusual for a project of this scale; large scientific facilities typically release detailed test data only after internal review, and incremental technical milestones rarely generate the same public attention as major hardware deliveries. Still, the operational readiness of the central solenoid remains an open question until those results are in hand.
For the global fusion community, the near-completion of the central solenoid is a tangible sign that ITER’s most technically demanding magnets can be built to specification. Whether they perform to specification under real operating conditions is the next question, and answering it will require patience, precision, and a 110-ton delivery from San Diego.
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*This article was researched with the help of AI, with human editors creating the final content.
