The steel structure that's being erected against the northeast side of the Assembly Hall is for a large building that will be densely packed with power supplies and energy convertors designed to deliver 40 MW of heating power into the ITER plasma. The space inside the building will be shared by two radio-wave-generating systems designed to feed energy at frequencies that match the oscillations of the different particles inside the plasma—a matching called 'resonance.' Deuterium and tritium nuclei, or ions, will be 'heated' by ion cyclotron resonance heating (ICRH); while electrons will be 'heated' by electron cyclotron resonance heating (ECRH). By way of collisions, electrons will in turn transfer the absorbed energy to the ions. In ITER, the ECRH system also generates the "spark" that, by ionizing the deuterium-tritium gaseous mix, starts up the plasma discharges. Both heating systems are based on the same principle: the conversion of electrical power into electromagnetic radiation. However, the technology they rely upon and the wave frequencies they produce are considerably different—the ECRH system can be compared to a microwave oven, with the 'food' replaced by the plasma, while the ICRH source is like a powerful radio transmitter of the kind that is still used to broadcast information from one end of the world to the other. Both systems require considerable amounts of electrical power to operate. In the Radio Frequency Building, massive transformers and rectifiers will convert the industrial 22 kV AC into DC for the wave-generating gyrotrons of the ECRH system and for the tetrodes (akin to vacuum tubes) of the ICRH system. The building is imposing—50 metres long, 43 metres wide, 25 metres high—but from the perspective of its future users it is barely large enough. Power supplies (a total input of 100 MW) for both resonance heating systems will occupy the better part of the first two floors, while the top floor will be occupied by the wave generators. Generators too need a lot of space: on the ECRH side for example, twenty-four 2.5-metre-high gyrotrons —each giving out the equivalent power of 1,000 kitchen microwave ovens—must have an interspace of five metres between them in order to avoid magnetic interference. Once generated in the Radio Frequency Building, the energetic radio beams must travel 100 metres along transmission lines all the way to the tokamak, where ECRH 'launchers' and ICRH antennas—both massive components weighing up to 45 tonnes—will deliver them deep into the plasma. Installing the sources at a distance from the machine is a choice dictated by magnetic issues: if they were any closer, the intense magnetic field of the tokamak would disrupt that of the generators' gyrotrons. Over the next few months, the building will evolve rapidly. Soon, the internal floors and side walls will be added and by early spring the building structure will be complete, allowing work to begin on added services. By mid-2018 the power supplies will be progressively installed, followed by the ECRH gyrotrons and ICRH tetrodes during the years 2019-2020. See related article in this issue: "Men of measure."

'Neither snow nor rain nor heat nor gloom of night stays these couriers from the swift completion of their appointed rounds.' [From the Greek historian Herodotus; also inscribed on the New York City Post Office.] This phrase, which has been attributed to the men and women who brave all kinds of climatic conditions to deliver daily mail, could just as well be used to describe the diligence of the ITER metrology team members who have been out monitoring the Assembly Hall since August of last year. Why are they outside in the heat of the summer and the cold of the pre-dawn winter hours? The Assembly Hall stands at over 60 metres in height and will move during windy days, expand during hot days and contract during cold days. Systems that are supported off the building structure will move with the building. This is of particular concern for the electron cyclotron transmission line, which weaves its way from the Radio Frequency Building, through the Assembly Hall and on to the Tokamak Building. As the Assembly Hall 'moves' it will distort the electron cyclotron transmission line at the junction between the buildings. These distortions will perturb the transmission of the microwave beams, which will increase the heating of the transmission line further downstream as well as the launching antenna system in the port plugs. Because the microwave beams represent up to 1MW of power with peak power densities exceeding 1GW/m², large distortions could generate large heat loads. The role of the metrology team is to characterize the maximum displacements of the Assembly Hall and to validate the predicted movements of up to +/-15 mm on a typical day. This may sound small but—relative to the alignment requirements for the electron cyclotron transmission line (waveguide supports aligned with sub-millimeter precision over a distance of 4 m)—it is enormous. In addition, the metrology team has been measuring the relative position of the three buildings to provide an 'as built' footprint, with the desired sub-millimeter precision, of the trajectory that the electron cyclotron transmission line will follow. The measurements were taken at a critical point in construction during the autumn, when the ground floor for all three buildings was sufficiently established and the opening between the Assembly Hall and the Radio Frequency Building not yet closed in. This allowed the precise positioning of the Assembly Hall relative to the Radio Frequency Building. The first round of measurements will be completed near the end of January with all information passed on to the electron cyclotron team, which will build an 'as built' CATIA model for transmission line passage. In addition, the building displacements will be used to revise the requirements on the transmission line load specifications. Another round of measurements will be performed once the Tokamak, Assembly Hall and Radio Frequency buildings are completed in the 2019 period.

In December, as toroidal field conductor unit length #133 came off the production line, the ITER community celebrated a major milestone—the end of a nine-year procurement campaign to procure 88 km of niobium-tin (Nb3Sn) superconductor for ITER's toroidal field coils. Six ITER Members have participated in ITER's longest running procurement effort: China, Europe, Japan, Korea, Russia and the United States. Considered a difficult material to work with due to the sensitivity of the Nb3Sn superconducting strands to strain, the world production capacity in 2007 at the start of the campaign did not exceed 15 tonnes per year. To meet ITER's needs, this had to be ramped up by one order of magnitude. In close association with the ITER Organization, the ITER Members developed winding and jacketing facilities, launched qualification programs for processes and tooling, and followed demanding process control and certification standards to ensure conformity with ITER's technical specifications. Some 500 tonnes of copper and niobium-tin (Nb3Sn) multifilament composite wires were produced for the toroidal field coils, then 'bundled' to form cables and contained in a structural steel jacket. Eight strand suppliers and four jacketing facilities were qualified by the ITER Organization in the course of the global procurement effort. The last toroidal field conductor unit length was jacketed in December 2016 by the European ICAS consortium in Italy from superconducting cable manufactured in the US and steel tubes sourced in Japan by the US Domestic Agency. 'This milestone represents the end of an amazing and challenging nine-year journey, which has been completed on time due to the good understanding and collaborative spirit between the partners,' said an enthusiastic Arnaud Devred, head of the Superconductor Systems & Auxiliaries Section at ITER. 'This procurement sums up much that is ITER—advanced technology, innovation, perseverance and strong international collaboration. For me, it's an excellent illustration of how ITER is bringing people together.' The many technical issues encountered along the way were overcome by facing them head-on and working out pragmatic solutions together, added Arnaud. 'This spirit is expected to continue in the next phases of the toroidal field magnet procurement, as our European and Japanese partners produce the final coils.' Eighty-eight kilometres of cable-in-conduit conductors for the toroidal field coils represents approximately 825 tonnes of material and an estimated market value of EUR 350 million.

Concrete and steel met at the end of the 19th century, never to part again. From their encounter a new material was born that revolutionized construction techniques. 'Reinforced concrete' brought the best of two worlds to building projects: concrete's resistance to compression and steel's resistance to both compression and tension. Embedding steel rebar into concrete made it possible to erect 300-metre-high skyscrapers and build viaducts tens of kilometres long. Without it, the construction of a nuclear installation, which requires exceptionally high structural resistance, would not be conceivable. At worksites throughout the world, more than 6 billion tonnes of concrete are poured every year. Depending on the nature of the construction, the density of steel reinforcement per cubic metre of concrete varies greatly: from an average of 100 kilos per cubic metre in standard civil engineering works, it can reach 600 to 700 kilos in the most strongly reinforced sections of a nuclear installation. At the heart of ITER, the Tokamak Complex will be built with 30,000 tonnes of steel (more than four times the weight of the Eiffel Tower) for a total of 100,000 cubic metres of concrete ─ an average of 300 kilos per cubic metre. The density and the geometry of the reinforcement is determined by complex computations that take into account loads, stress and—in the case of a nuclear installation—safety requirements. Construction design reinforcement drawings, which rebar installers must follow, are elaborated on this basis. As thick as 40 mm in the most heavily reinforced areas of the ITER Tokamak Complex, the steel reinforcement bars are arranged in complex patterns and layers—imagine dozens of superimposed spider webs made of steel 'thread' as thick as a maiden's wrist. Steel bars are typically 12 metres long and cannot be butt-welded to form larger continuous structures. In order to preserve structural resistance, bars must overlap by as much as 2.5 metres for the largest among them. This overlapping not only exacerbates the density challenge, but also results in a costly increase in steel consumption. Fortunately some thirty years ago, rebar installers came up with a smarter solution: they developed the 'coupler,' a small, threaded steel connector that can join two bars. Perfected about 15 years ago by a small rebar company just an hour's drive from ITER (SAMT in Saint-Chamas, France), this small piece of forged steel has become a key element in any large-size reinforced construction project. In the ITER Tokamak Complex alone, more than 250,000 couplers will be necessary. In the SAMT workshops, on the edge of the inland sea Étang de Berre, activity is buzzing. Steel bars of all calibres, manufactured in Italy, are fed into machines to be either cut to size and formed according to ITER's detailed execution drawings or threaded to accommodate couplers. Everything is carefully traced and tagged to be delivered two to three times a week at the foot of the ITER worksite cranes. Currently, 300 to 500 tonnes of steel bars and more than 4,000 couplers are integrated into the construction of the Tokamak Complex every month. Compared to the size and complexity of a project such as ITER, a small piece of steel such as a coupler could appear insignificant. But by limiting the amount of steel in the structure without altering its resistance and by reducing the costs attached to steel consumption, it has proved essential and indispensable.

At nightfall, when buildings, work areas, roads and parking lots light up, the ITER site looks like an alien spaceport. Drenched in the yellow glow of sodium lights, with its cranes reaching for the sky, the Tokamak Complex is like a launch pad minutes before a shuttle's departure; towering above, the Assembly Hall resembles a giant hangar for some mysterious spaceship bound for the confines of the galaxy... © EJF Riche/ITER Organization

The latest newsletter from the ASDEX Upgrade tokamak team in Germany reviews the improvements that were made in 2016 to the machine's in-vessel components and heating systems, discusses plans for the 2017 experimental campaign, and highlights the importance ASDEX operation to the achievement of Europe's Roadmap to the Realisation of Fusion Energy. ASDEX Upgrade, in addition to contributing to the knowledge basis required to operate ITER, is focusing increasingly on issues relevent to DEMO, the next-step fusion device. Read the December issue of the ASDEX Upgrade here.

At the EAST tokamak in China, five Italian scientists recently joined their Chinese colleagues to participate in a week-long experiment aimed at testing a voltage-driven vertical stabilization system. The successful tests, carried out in the framework of a joint ASIPP-CREATE-ENEA collaboration, were a key step on the way to MIMO (multiple input, multiple output) control of advanced tokamak configurations, capable of decoupling shape control from vertical stabilization. The new vertical stabilization system implemented and tested on EAST is identical to the one proposed for ITER, and these first tests show that it is compatible with the installation a new ITER-like multivariable shape controller for advanced configurations. Read more on the ASIPP website.