The final segment of the central solenoid structure has been delivered to ITER

The last shipment of components of the support structure for the central solenoid delivery was prepared in Pennsylvania during January 2025. Credit: US ITER

US ITER has successfully delivered all components for the central solenoid's support structure—an essential part of the 60-foot-tall superconducting magnet at the core of the ITER fusion reactor. This intricate assembly, made up of over 9,000 individual parts, was produced by eight U.S. manufacturers spanning six states.

ITER, currently being assembled in France, is a global initiative aimed at proving the feasibility of generating and maintaining fusion energy at power-plant scale for extended durations.

The final shipment, which crossed the Atlantic in January, marked the completion of more than ten years of collaborative work by US ITER personnel and American suppliers. Companies involved in the effort included Kamatics (Connecticut), Major Tool & Machine (Indiana), Keller Technology (New York), Hamill Manufacturing, Precision Custom Components, and Superbolt (Pennsylvania), Petersen (Utah), and Robatel Technologies (Virginia).

"Creating and building the unique superconducting central solenoid presents a significant engineering challenge," said David Vandergriff, a senior project engineer at ORNL who has been part of the central solenoid team since he joined US ITER in 2016. "However, this magnet won't function effectively for ITER without its support structure."

Eight companies across the United States fabricated components for the support structure, including Superbolt in Pennsylvania, whose technology is key to holding the structure together against extreme forces. Credit: Superbolt

The support structure acts like an exoskeleton or cage, enclosing the central solenoid located at the heart of the ITER machine. The solenoid itself consists of six stacked magnet modules, each weighing 121 tons. It is primarily responsible for producing the magnetic flux changes required to start the plasma, drive the plasma current, and sustain that current throughout the burn phase.

The primary function of the structure is to keep the six central solenoid modules precisely aligned within tight tolerances, measured in millimeters,” explained Kevin Freudenberg, US ITER’s engineering technical director. “But the true complexity arises during operation. At critical moments, the vertical force acting on the module stack can reach up to 60 meganewtons — more than twice the thrust of a launching space rocket.”

To build a structure capable of withstanding such immense forces generated by the central solenoid, the team had to address several engineering challenges. One such challenge involved creating 27 specialized connectors, known as tie plates. These vertical bars link the lower key blocks on the solenoid’s base platform to the upper key blocks at the top, forming a cage-like enclosure around the central solenoid.

“When we started the initial design, there were major concerns about whether it was even possible to manufacture each tie plate as a single piece,” Freudenberg explained. “These components are 50 feet long and need to be perfectly straight, adhering to very tight tolerances. We collaborated with forging facilities and specialized engineering firms to make it happen, ultimately shifting from our original two-piece design to a single-piece solution.”

One engineering challenge was designing and fabricating the 50-foot-long components called tie plates, which had to meet strict tolerances. Credit: US ITER

Another significant hurdle was securing the structure’s components strongly enough to endure the extreme forces produced by the central solenoid. To tackle this, the team partnered with a U.S. company that developed a fastening solution known as Superbolt® technology.

With four central solenoid modules already installed and the final two slated to be stacked later this year, the team is now preparing for the next stage of building the support cage at the ITER site in southern France.

The true reward for the team will be witnessing the structure being assembled around the central solenoid — seeing more than ten years of hard work come together and knowing it brings us one step closer to ITER becoming operational,” said Freudenberg.