A cutting-edge X-ray process confirms microscopic cracking and keeps ITER on track to succeed. When a handful of tiny leaks were discovered in one section of ITER's immense thermal shield system, project leaders faced an existential question: was this an isolated problem that could be easily fixed by replacing a bit of piping, or was it a more serious issue affecting all 20 kilometres of thermal shield cooling pipework? And while the question seemed straightforward, finding the answer required a remarkable collaboration with the CERN particle physics laboratory using innovative X-ray tomography and advanced microscopy technologies that allowed scientists to measure cracks so small they had never been measured before. "It was a complete first to use this kind of X-ray tomography to characterize a leak path of this magnitude," says Robert Pearce, the Vacuum System Project Leader. "We were able to identify micron level cracks that could have completely disabled the machine if they were allowed to grow over time." By now, the saga of the cracks in ITER's thermal shield system has been well reported. In July 2020, routine helium tests detected a pinhole leak in a thermal shield cooling pipe on vacuum vessel sector #6 that was likely caused by a welding issue. This discovery prompted an examination of more than 300 thermal shield panels and in 2021 additional helium testing revealed three more, much smaller, leaks. The source of the leaks was hypothesized to be stress corrosion cracking from chlorides that had been trapped in the piping during the silver-plating process. Corrosion of stainless steel is rare, but examples can be seen when stainless-steel jewelry is tarnished in chlorine swimming pools and there are even extreme cases of stress corrosion cracking resulting in pool roofs collapsing when stainless-steel beams suffer prolonged exposure to chlorine. The ITER team removes a piece of thermal shield cooling pipe for testing. A few leaks that could be easily identified, accessed, and fixed would not have been an especially big problem for ITER. Minor vacuum leaks occasionally occur in fusion devices, and they have been found and successfully repaired at other tokamaks such as the European JET and Korean KSTAR. However, the fact that the leaks were thought to be caused by stress corrosion cracking was deeply troubling. If this hypothesis was true, it meant corrosion was likely occurring and progressing elsewhere in the thermal shield piping and could lead to hundreds or thousands of additional leaks in years to come. It was thus crucial to find out if there was widespread corrosion-induced cracking that would lead to other leaks in the thermal shields cooling pipes. But there was just one problem: no existing techniques could assess such minuscule cracks. To solve this problem, the ITER vacuum group turned to the Mechanical and Materials Engineering team at CERN, the European Organisation for Nuclear Research. ITER has had a Cooperation Agreement in place with CERN since 2008 to draw on the expertise in material assessment and magnets that was developed during the construction of the Large Hadron Collider. The first step was to analyze the material in the spots where ITER's helium testing expertise had accurately pinpointed leak positions. Because there was no in situ technique for this, sections of the pipe were removed and sent to CERN. Pieces were placed in an X-ray tomography machine, where they were rotated and scanned for approximately six hours. The resulting scans offered initial 3D images of the route of the leak, which provided a precise location for accurate local scans and a fuller metallurgical inspection. Thin slices of pipe were taken from the location where the leak path was seen and then polished. This approach provided a tenfold increase in resolution compared to existing procedures and suddenly, the leak and the chloride-caused cracking were fully visible. It was the first known characterization of the path of a corrosion-induced vacuum leak in the 10-6 to 10-7 Pascal cubic-metre-per-second range, which corresponds approximately to a leak that would fill a birthday-party helium balloon in 30 years. A groundbreaking view of leak paths is achieved by new X-ray techniques developed by ITER and CERN. "The fact that we could see such a small leak gave us some confidence that this technique might be useful for seeing cracks that were not yet leaks," says Pearce. "Without this new 3D process, you would never know where to cut and analyze further, so it would have been like looking for a needle in a haystack." Having established the technology to visualize the cracking, the second stage began. In the summer of 2022, sections of the 14-millimetre-in-diameter, 2-millimetre-thick pipe were removed from what were called "regions of interest" on the thermal shield that were selected because of visible corrosion spots. Again, pipe sections were sent to CERN where they were sliced, polished, and X-ray tomographed. The images clearly revealed sub-surface granular and transgranular cracking that had started penetrating a quarter of the way through the pipes. "It was quite unprecedented. We were looking at the cross-section and we could see typical lightning bolt structure that was like what you would see in a summer storm and evidence of chemicals that caused it," says Stefano Sgobba, the engineer in charge of the Materials team of CERN who oversaw the testing. "These advanced tools gave us a much deeper understanding of the process and the chronology of events." With stress corrosion cracking confirmed on the very first piece of pipe analyzed by this combination of techniques, it was safe to conclude it was happening throughout the thermal shield cooling system. And although there wasn't immediate leaking everywhere, stress corrosion cracking can grow by between 0.1 mm to 10 mm per year depending on the conditions. By the time fusion would be ignited at ITER, the cracking could have led to thousands of leaks and the tokamak could have stopped functioning. The existing 20 kilometres of thermal shield piping at ITER have now been declared unusable and a repair/replacement strategy has been designed for the vacuum vessel thermal shield, with a more resistant grade of stainless steel and the forgoing of the silver-plating process in future welded constructions to avoid any chance of residual chlorides.The joint ITER-CERN research into the stress corrosion cracking was presented at the 2023 IEEE Symposium on Fusion Engineering (SOFE) and the 28th International Conference on Magnet Technology (MT-28), while the new tomography process will be the subject of an academic paper, all of which will help shape guidelines for the construction of future fusion devices. Meanwhile at ITER, with a potential mission failure averted, the project forges ahead. "We've got repairs to make, but it's still positive," says Robert Pearce. "ITER isn't unrepairable, the body of knowledge has been expanded, and we're in the solution phase."

Recruitment opens today for the next Monaco-ITER Postdoctoral Fellowship campaign. If your PhD was awarded after 1 January 2021—or if you are about to obtain one—and you are interested in participating in one of the great scientific and technical challenges of the 21st century, this may be an opportunity for you!

On Friday 15 December 2023, as staff posed for a group photo in front of newly arrived toroidal field coil #18, Alessandro Bonito-Oliva, Head of the Tokamak Program  at ITER since 1 November, was still wearing his Fusion for Energy hard hat. Whether an oversight or deliberate, it hinted at the immense collaborative effort under European responsibility to manufacture and deliver ten first-of-a-kind superconducting toroidal field coils, each as tall as a five-storey building and as heavy as a fully loaded Boeing 747. The delivery of the massive component marked the completion not only of the European procurement program, but that of the project-wide effort to procure 19 D-shaped coils (including one spare) for the ITER machine. For Alessandro, who had managed the magnets program of the European Domestic Agency for more than 15 years, it was "mission accomplished." As part of the ITER Organization now, "Sandro" will oversee the integration of the coils into the machine—a potent illustration of the push to ensure that competencies developed in the ITER Domestic Agencies are not lost during the machine assembly phase. Sandro and ITER met in 1993, long before the design of the machine was finalized. A theoretical physicist by training and an expert in the quantic sorceries of superconductivity, he had been working for industry since 1986, participating in the early developments of magnetic resonance imaging (MRI), venturing into superconducting motor research (a technological impasse), and contributing to CERN's Large Electron-Positron's (LEP's) magnets before entering the world of fusion at the Next European Torus (NET) project. In the meantime, the ITER project was gradually acquiring substance, with the manufacturing and full testing of a "toroidal field model coil" in the planning. Ansaldo, the Italian company for which Sandro was working at the time, was among the contractors involved. That was his first encounter with ITER and its monumental magnets. "The shape of the model was more like a racetrack, and its size was about one-third that of the present D-shaped coils, but it was totally relevant from a technological point of view," he remembers. In the mid 1990s, Sandro participated in the manufacturing of a model toroidal field coil for the projected ITER machine. Although one-third smaller and of a different shape than the final components, the model "was totally relevant from a technological point of view." The model coil is on display outside the Institute of Technical Physics (Karlsruhe Institute of Technology) in Germany. Following that experience Sandro moved to Oxford Instruments in 1997, a major actor in the field of superconducting magnets where he was to spend close to 10 years, followed by a stint at the National High Magnetic Field Laboratory in Florida. The early ITER experience however had struck a chord. "I had this dream of going back to the ITER project and completing the job started with the model coil and being responsible for the development and production of the actual toroidal field coils." In the early days of 2008, the dream became reality. In Saint-Paul-lez-Durance, France, the 42-hectare scientific platform was being levelled and equipped, while each Member was establishing a Domestic Agency to procure machine components and industrial systems. Ten toroidal field coils out of a total 19 fell under Europe's scope; another 9 fell under Japan's. For Fusion for Energy, the European Domestic Agency, Sandro was the right man to lead the production of the components: a physicist who knew "what's behind the equations" and someone with enough industrial experience to manage the delicate relationship between supplier and customer, to understand the difficulties, needs and points of view of both sides and to appreciate the technological complexity. "Toroidal field coils are by far the most technologically complex superconducting magnets ever manufactured and one of most complex components in the whole ITER machine," says Sandro. "Think of the precision (0.001% over a kilometre of conductor length) required when matching the conductor to the 750 metres of grooves in the radial plates on a double spiral trajectory. Think of the 1.5 kilometres of laser welding for the radial plate covers, or of the heat treatment at 650 °C required by the brittle niobium-tin compound!" However daunting, the challenges were met. By September 2013, the first double pancakes had entered production in Italy and less than seven years later the first European coil was ready for shipping. Between April 2020 and December 2023, Europe delivered the full scope of 10 toroidal field coils to the ITER site. All in all, over the years, 700 people from nearly 50 companies were involved in the manufacturing process. Ten thousand miles away in Japan, a similar process resulted in the supply of 9 toroidal field coils to the ITER Organization. At the same time, Sandro's responsibilities had expanded: appointed Magnets Head of Unit by Fusion for Energy in 2013, the production of the ring-shaped poloidal field coils entrusted to Europe now fell under his responsibility. "This new project was the most complex in terms of project management," Sandro recalls. "First, we needed to put together a large onsite factory, where 6 different suppliers and about 100 workers and engineers would have to work side by side. But even more important was the necessity of creating a 'team spirit' between all the entities, contractors and individuals—a challenge we achieved over time with effort ... and a good number of celebrations!" In 2023, with only a few tasks to complete on the final poloidal field coil, it was again "mission accomplished" for Sandro. What to do now that the "dream of his life" had been realized? Inside the "large onsite factory" put together to accommodate the manufacturing of the four largest ITER poloidal field coils, representatives of the European Domestic Agency Fusion for Energy and its six contractors celebrate the finalization, in July 2023, of poloidal field coil #4. A new idea began to grow as Sandro spent six months in Naka, Japan, working on the JT-60SA tokamak to repair the electrical insulation of the different superconducting coils after an electrical event had caused a pause in commissioning. "It was the first time I found myself in front of a completely assembled tokamak and it was an inspiring experience—after all, superconducting magnets are not just works of art, they are an integral part of the tokamak machine..." Joining the ITER assembly project was a natural next step and part of an ultimate mission—producing a working ITER machine.   To his new job, Sandro brings the important perspective of having worked on the supplier side. "Relations between the ITER Organization and the Domestic Agencies are complex because within a shared objective the approach can differ depending on whether you are reasoning as a supplier or as the machine owner." Under the re-organization currently underway at the ITER Organization, there is a definite will to integrate these perspectives, along with talent from the Domestic Agencies, in order to optimize the continuity from design to manufacturing to assembly and eventual operation. It is important, for instance, that a Domestic Agency's involvement does not stop with the delivery of components. "When you've produced a technically complex component and accumulated all the technical knowhow, you can make a real contribution onsite during integration and commissioning. My first months here have been an incredible source of motivation, which at this advanced stage of my career I thought would be impossible to find. I would strongly encourage others to follow in my steps!"   

Fusion researchers are constantly challenging the scientific state of the art and improving the technology. This drive lay the basis the conditions for innovation, much of which can be exploited in other sciences and industrial sectors for the benefit of society. The SOFT Innovation Prize rewards outstanding achievements in fusion energy research showcasing opportunities of valorization in the sector. It is intended for researchers and/or industrial actors who find new solutions, possibly with wider applications, to the challenges of fusion. SOFT stands for Symposium on Fusion Technology—the name of  conference where the prize is awarded. Following the success of the previous editions, the European Commission is holding the contest again in conjunction with SOFT 2024 (23-27 September 2024, Dublin, Ireland). There are no specific categories for this prize. Participants are free to submit an application concerning any physics or technology innovation that has been developed in magnetic confinement fusion research and that has market potential or has been taken up (or recognized) by industry to be further developed for the market. Entries will be assessed on originality and replicability, technical excellence, economic impact, exploitation, and plans for further development. Proposals are studied by an independent jury composed of experts in technology transfer, from business and academia. The deadline to submit applications has been extended to 15 February 2024. See further details on the SOFT Prize website.