At the heart of the Tokamak Complex a deep, perfectly cylindrical well is taking shape. Here, the ITER Tokamak will be assembled from bottom up, enclosed in its stainless steel cryostat and surrounded by the thick concrete walls of the bioshield.  Newsline takes you this week on a photographic journey into one of the most complex and challenging construction projects in the world.

The US Domestic Agency achieved a major milestone in February by completing the delivery of all US-supplied toroidal field conductor to the European toroidal field coil winding line at ASG in La Spezia, Italy. All certificates of acceptance have been signed for the nine lengths of conductor, totalling over 7,000 metres. The US Domestic Agency, managed by Oak Ridge National Laboratory, has now completed its contribution of 8 percent of the toroidal field coil conductor ITER requires; the rest of the conductor has been supplied by other ITER Members.   Key industrial partners for this US procurement include: Luvata Waterbury (Connecticut) and Oxford Superconducting Technologies (New Jersey) for strand production; New England Wire Technologies (New Hampshire) for cabling; and High Performance Magnetics (Florida) and Criotec (Chivasso, Italy) for jacketing and integration. At the height of fabrication, US vendors Luvata and Oxford Superconducting Technologies were producing over five metric tons of superconducting strand per month.   The ITER facility will use approximately 80,000 km of low-temperature, helium-cooled superconducting wire to generate the immense toroidal magnetic fields needed to confine the 150-million-degree-Celsius plasma inside the ITER Tokamak. Eighteen toroidal field magnets, weighing more than 6,500 tonnes, will have a total magnetic energy of 41 gigajoules and a maximum magnetic field of 11.8 tesla.   As a partner in ITER, the US is providing hardware for multiple ITER systems. The US project will complete its next system contribution—components for the steady-state electrical network—later in 2017. Deliveries of toroidal field conductor from the US began in 2015.

Last week was marked by the first delivery of diagnostic components—Continuous External Rogowski (CER) coils—from the European Domestic Agency to the ITER Organization. The CER coils are to be located on three toroidal field coils and on the spare toroidal field coil. The CER coils are designed to measure the total electric current flowing in the ITER plasma, a key measurement for plasma control that also has relevance for safety. Rogowski coils work with just one or two extended sensors, resulting in high reliability despite the cryogenic temperatures, high vacuum and mechanical stresses at this location.   Their development in their present form started in 2002, with the decision to incorporate them in the toroidal field coil casing. It involved the ITER Organization, the European Domestic Agency, the Japanese Domestic Agency, their predecessors, laboratories and suppliers.  Each CER is a composite cylinder, measuring approximately 40 metres in length and 12 millimetres in diameter. A special groove is made in the toroidal field coil cases to house it. The ends of the CER coils, emerging from the toroidal field coils, are housed within steel protection structures that made up the bulk of the approximately half-tonne delivery from Europe.    These small rope-like coils are tasked with a difficult job—measuring the total electric current flowing in the ITER plasma from their location inside of the toroidal field coils cases. The manufacturing has been undertaken by two companies—Axon in France and Sgenia in Spain. The system has been delivered to the ITER Organization where it has undergone site acceptance tests and is now ready to travel to the next destination for the coils—Japan—for installation in the dedicated toroidal field coils.  A number of the ITER diagnostics team members ensured the process went without too many "hitches." Practical lessons learned with this first delivery will be very useful for ensuring subsequent deliveries go smoothly and efficiently.   Shakeib Arshad, project manager for the CER coils at the European Domestic Agency, also celebrated the occasion. "Delivery of the CER coils represents a big success, particularly in terms of collaboration between the ITER Organization, the European Domestic Agency, and European suppliers for these items. There are very complex technical interfaces involved coupled with a tight delivery schedule."   Read a report on the milestone from the European Domestic Agency here.

Plasma disruptions are fast events in tokamak plasmas that lead to the complete loss of the thermal and magnetic energy stored in the plasma. The plasma control system in ITER will be responsible for minimizing the number of these events—especially when running high energy deuterium-tritium plasmas—but the probability of disruption will never be zero. Therefore, it is essential that ITER be equipped with the right means to reduce the heat fluxes and electromagnetic loads resulting from these events. The central tool will be the disruption mitigation system, whose aim is to inject up to several hundred grams of deuterium and radiating gases like neon or argon. Although these quantities appear small at first sight, they have to be compared to the one gram (at most) of plasma that will be confined in ITER. Moreover, these quantities have to be delivered to the vacuum vessel within only a few 10s of milliseconds to be effective.   But designing an effective and reliable disruption mitigation system for ITER is not only a technological challenge, it also requires a good understanding of the physics mechanisms driving disruptions to allow scientists to predict that a certain design will be effective. This is why a workshop was jointly organized by ITER's Plant Engineering and Science & Operations departments to bring physics experts together with the US ITER engineers based at Oak Ridge National Laboratory (ORNL) developing the design of the disruption mitigation system. About 25 leading experts from the Members' fusion research centres and universities joined the intensive discussions during the three-day workshop from 7 to 9 March.   Michael Lehnen (left) from ITER's Stability & Control Section, and So Maruyama (centre), head of the Fuelling & Wall Conditioning Section, coordinated the meeting. The workshop participants were asked to put ITER-specific engineering constraints aside to have a fresh look at possible concepts for the disruption mitigation system. From the discussion it became clear that the most promising approach at present is to inject the material through shattered pellet injection, which is also the present baseline concept. This technique freezes deuterium, neon, or argon gas to form cryogenic pellets as large as a wine cork. These pellets are accelerated to velocities of up to 500 m/s and broken into shards by a sharp bend at the end of the flight tube. This is intended to ensure that a high fraction of the injected material is assimilated by the plasma.   During the workshop, uncertainties in the design were identified and short-term R&D is planned to address these, including dedicated experiments at the tokamaks DIII-D and JET. But longer term R&D has also been proposed to ensure that alternative injection concepts are at hand should they be required at a later stage in the project.   Due to the complex nature of plasma disruptions, significant gaps still exist in the physics understanding. Most challenging is to understand how the formation of high energy electrons—so-called runaway electrons—can be reliably suppressed during disruptions. This was the central theme and the discussion often came back to this issue. Since a definitive answer will require further research over the next few years, the design of the disruption mitigation system has to be kept as flexible as possible. The urgency for dedicated R&D in this area undeniably exists.   In the coming weeks, the outcome of the workshop will be integrated into a report summarizing the key issues which need to be addressed to improve ITER's disruption mitigation capability. This will be reported to the forthcoming meeting of the ITER Council Science and Technology Advisory Committee in May.

Located in the foothills of the French Pre-Alps, the ITER installation blends almost seamlessly into the landscape. The architects' choice of mirror-like steel cladding for the main buildings of the installation has proved efficient—contrary to the other structures in this image, the massive ITER Assembly Hall seems to fade into its surroundings. Taken with a powerful telephoto lens, the image reveals the beauty of the snow-capped mountains, which rise some 2,000 metres at a distance of 60 kilometres.   A comparison with the photograph above, taken from the very same spot one year (to the day!) earlier, highlights the progress accomplished on the ITER site. In early March 2016, the cladding on the Assembly Hall was far from complete and the vast building was still empty.   Today the main and auxiliary cranes have been installed and finishing works are underway on the building's interior, where the first large assembly tool will be erected in a six-month long operation that will begin this summer.

For almost a week, the lobby of the ITER Headquarters building was Lego construction central, as two students from Japan's Kyoto University feverishly constructed a model ITER Tokamak from piles of coloured bricks and grid sheets of sketched instructions.   Click here to relive their experience by video.