The ITER superconducting magnets will operate at 4 K (minus 269 °C, a temperature barely above absolute zero) and will carry extremely strong electrical current (up to 68 kA). How will these first-of-kind components behave in such extreme conditions? Part of the question can be answered by testing the magnets at 80 K (minus 194 °C), which is not overly complex or costly and reproduces 90 % of the thermal and mechanical constraints a coil will be exposed to during its lifetime. However, as superconductivity is only established when a magnet is brought to 4 K, a test at 80 K says nothing about the electrical behaviour of a coil under operating conditions. Although not planned initially, a window of opportunity has just opened for testing some of the ITER coils at 4 K—a big project within the project. 'Testing at 4 K and at 80 K are completely different operations,' explains David Grillot, ITER deputy program manager for plant systems and former leader of the Cryogenics Section. 'Considering that a toroidal field coil's mass is in excess of 300 tonnes, the colder test requires considerable infrastructure—a large cryostat, a dedicated power supply, an electrical feeder and associated instrumentation, and an interface with a large refrigerator located inside the cryoplant. It's not comparable at all to testing at 80 K, which is relatively simple and has been done routinely.' All six ITER ring-shaped poloidal field coils and 14 out of 19 D-shaped toroidal field coils will have been tested at the more clement temperature of 80 K before their installation in the ITER machine. Testing at 4 K, however, was only done for the US-procured central solenoid modules. 'US ITER demanded that a full-current 4 K factory acceptance test be performed on each central solenoid module before shipment to ITER,' says Neil Mitchell, ITER's historical magnet expert. 'It definitely made sense as a 4 K test can achieve conditions pretty close to a module's operating point in ITER.' For reasons that have to do with size, shape, the nature of the conductor and with how individual magnets (central solenoid modules or single toroidal field coil) contribute to the total field generated by the coil arrangement (6 stacked modules in one case, 18 coils circling the vacuum vessel in the other), a 4 K test on a toroidal field coil does not completely reproduce the actual conditions the coil will face during the machine's operational phase. It has, however, an important virtue: it enables the testing of the strategic joints that connect the seven double pancakes that constitute a toroidal field coil. 'Making these joints is a hand operation and although we take lots of precautions, deviations are always possible.' Because of the equipment, processes and procedures involved, a 4 K test amounts to a 'partial but real-scale commissioning of the tokamak's magnetic system,' says David. 'A 4 K test is about much more than just testing a coil ... it's about validating a whole infrastructure (the power supply system, the cryogenics, the feeders, the control-command system ...). Along with minimizing risks, this partial commissioning will be like a general rehearsal that will save us a considerable amount of time.' Proceeding to 4 K tests has long been discussed inside ITER. For years, experts and management have confronted their analyses and debated the pros and cons. Confidence in the manufacturing process and other considerations eventually led to the decision not to proceed. But the mood has changed and 'a window of opportunity' has opened. The issues encountered with the vacuum vessel sectors and thermal shield piping and the necessary repairs have interrupted the assembly sequence. Consequently, toroidal field coils will remain accessible for two or three years to come—time enough to build a cold test facility and proceed to the testing of 'as many coils as possible.' 'Although we are not starting from scratch—coils at JT60-SA and W7-X have been tested at 4 K—the size of the ITER coils and hence the dimensioning of the installation present considerable challenges. Also, we need to finalize the facility in two years when it would normally take twice that time,' says David. But things are moving quickly. The project is presently in its preliminary design phase and the main contracts are already being signed. After a final design review, planned next April, the installation's assembly phase could begin in the first months of 2025 and operation by the end of that year. "Considering that by that time one or two sector modules will already be installed in the assembly pit, and that a complete cold test at 4 K requires 4 to 6 months, the plan is to test at least one coil from each manufacturer (Mitsubishi and Toshiba in Japan; ASG-SIMIC in Europe),' says David. The installation and its 350-tonne cryostat (11 m x 22 m—the widest and longest load to travel the ITER Itinerary) will be located in the vacated eastern part of the winding facility used by the European Domestic Agency to fabricate four largest poloidal field coils, and which is still equipped with powerful lifting systems. As a side benefit to building the cold test facility for the toroidal field coils, the size of installation's cryostat will allow for the testing at 4 K of PF1 from Russia, the smallest of the tokamak's six poloidal field coils.

Ahead of repairs, sector module #6, which was extracted from the Tokamak pit in early July, is being dismantled in one of the twin sub-assembly tools. One toroidal field coil has already been removed and placed into storage, the other is still attached to the tool's open wing. Removal of the thermal shield panels has not started yet. Newsline caught this image as light and reflections play tricks on the component's silver-plated surface—making the huge sector look fleetingly as if it had been equipped with the winged shoes of the Roman god Mercury.

A lucky set of circumstances brought together different experts on neutral beam injection for a few days at ITER in October. Achieving the requirements for the ITER neutral beam system in terms of power, energy, and pulse length simultaneously is a major challenge that benefits from open exchange and collaboration. To access, study and sustain burning plasmas on ITER with a high-fusion-power amplification factor, the use of external heating and current drive system plays an essential role. In particular, ITER will be equipped with two heating neutral beam injectors (with a provision of a third injector) and a neutral beam line for diagnostic purposes. To support research and development on the ITER neutral beam injection system prior to its procurement, the ITER Neutral Beam Test Facility was launched in 2012. The test facility includes two test beds—SPIDER, for the installation, optimization and operation of a full-size ITER negative ion source, and MITICA, a full-size ITER 1MV neutral beam injection system. The test facility is hosted at the Italian research laboratory Consorzio RFX in Padova, Italy, and is supported by the contributions of the ITER Organization, the European, Japanese and Indian Domestic Agencies, the EUROfusion consortium, and Consorzio RFX. Since 2019, a new agreement between Consorzio RFX and the ITER Organization has clarified roles and responsibilities as construction at the Neutral Beam Test Facility ends and operation in support of ITER intensifies.  Europe has been actively involved in the development of radiofrequency-driven ion source physics and technology from the start; in fact, ITER's negative ion source is based on a technology that has evolved over several generations of prototypes at the Max Planck Institute for Plasma Physics in Garching, Germany. A cooperation agreement signed in 2020 between the ITER Organization, Consorzio RFX and EUROfusion now formalizes the involvement of European experts in the ITER Neutral Beam Test Facility by providing for the participation of up to 14 professionals per year. Additionally, EUROfusion is contributing up to 6 professionals per year to ongoing work at the reduced-size prototype ion sources as part of stepwise R&D leading to the achievement of ITER's challenging neutral beam injection requirements. ITER is of key importance in the European research roadmap for the realization of fusion energy, as it aims to prove the scientific and technological feasibility of fusion as a future energy source. EUROfusion is contributing to the preparation of ITER scientific exploitation and is leveraging the ITER outcomes and lessons learned to inform the design of Europe's next-stage device, DEMO. In this context, a specific EUROfusion Work Package—'Preparation of ITER Operation' (PrIO)—was created recently that includes R&D on the ITER radiofrequency neutral beam ion sources, with both long-pulse discharge targets on the ELISE prototype in Garching and participation in the ITER Neutral Beam Test Facility. On 26 and 27 October 2023, EUROfusion experts working to support the R&D for the ITER neutral beam injection system at Consorzio RFX and IPP Garching had the occasion to convene at ITER Headquarters at the same time as world experts who are part of the ITER Neutral Beam Advisory Committee (formed as part of the 2019 agreement). This was a unique opportunity to develop synergies and bridges between experts, as well as to present the work performed by the EUROfusion experts to the ITER team and the committee. During a 'teaser' session in the ITER Council room, experts gave two-minute summaries of their work. This was followed by a poster exposition in the lobby, a presentation of the ITER Project, a worksite tour, a tour of France's WEST tokamak (next door to ITER), and an informal dinner to continue the scientific exchange in a collaborative atmosphere. Because achieving the requirements for the ITER neutral beam system in terms of power, energy, and pulse length is a major challenge, this type of 'coming together of the minds' was appreciated by all. The following is a list of the posters that were presented: Riccardo Agnello: Investigation of SPIDER beam and plasma by multiple diagnosticsRiccardo Casagrande: Support to the design and testing of RF solid-state generators for NBTF and ITER NBICaterina Cavallini: Support for operations, management, monitoring and verifications of NBTF cooling systemNuno Cruz: Instrumentation & Control activities in support of the ITER NBSylvestre Denizeau: Heating of SPIDER drivers during nominal operations: numerical estimation and comparison with calorimetryDaniel López-Bruna: 3D calculations of RF inductive coupling in the drivers of SPIDERCarlo Poggi: Diagnostics operation and development for SPIDER and MITICABasile Pouradier Duteil: Study and optimization of the use of caesium in large negative ion sources for nuclear fusionAlastair Shepherd: Support of NBTF operations, diagnostics and beam current measurementLuca Trevisan: Automation engineer for the NBTFNiek den Harder: Beam studies in view of ITER NBI systemsAdrian Heiler: Understanding Cs and work function dynamics at NNBI ion sources for ITER Guillermo Orozco: Mechanical engineer for neutral beam ion sourceDimitri Yordanov: Plasma diagnostics, ELISE operation and modellingRoman Zagorski: Experimental validation of the fluid solver for SPIDER

On Friday 3 November, ITER Director-General Pietro Barabaschi took part in a panel discussion on nuclear fusion organized as part of the Berlin Science Week.  Moderated by Sakura Pascarelli (Scientific Director, European XFEL), "Nuclear fusion for a decarbonized future" featured: — Pietro Barabaschi, Director-General, ITER Organization (keynote)— Masaya Hanada, Director General, Naka Fusion Institute Japan— Steven Cowley, Director, Princeton Plasma Physics Laboratory, USA— Piergiorgio Sonato, President, RFX Consortium, Italy— Hartmut Zohm, Head of Tokamak Division, MPI-IPP, Germany After describing project status, Director-General Barabaschi commented on how ITER is contributing to the world fusion research program. "Fusion is an innovation program ... it's not just one project. [...] We see now a renaissance of fusion around the world. A lot of private investment interest, a lot of companies, a lot of opportunities for young people. [...] A lot of new ideas will come up and I believe that ITER can provide some support. [...] Others may learn from what we are doing; it's part of our job I believe." You can watch a replay of the public event at this address.