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Creating an ultra-high vacuum inside the vast toroidal chamber of the °ÄÃÅÁùºÏ²Ê¸ßÊÖ Tokamak—the aptly named "vacuum vessel"—is imperative to initiating plasma operations. Mechanical pumps will do the first part of the job, evacuating the air and most of the molecules from the 1,400 m³ vessel and reducing pressure to 1/10,000th that of the atmosphere (pressure is how vacuum is measured).   This, however, will not be sufficient. The quality of the vacuum needed on °ÄÃÅÁùºÏ²Ê¸ßÊÖ is in the range of 1/10,000,000,000th that of the atmosphere, close to the deep-space void and impossible to achieve with a mechanical pumping system.   By chance, there is a simple law of physics that can take over when pumping machines reach their limit.   When a molecule or an atom encounters an extremely cold surface, it loses the best part of its energy and slows down to near immobility. This phenomenon is called "adsorption" and its intensity is proportional to surface temperature: the colder the surface, the more irresistible its holding power ...   A cryogenic pump—or cryopump for short—is based on this very principle. In °ÄÃÅÁùºÏ²Ê¸ßÊÖ, there will be six torus cryopumps positioned around the vacuum vessel and entrusted with a double mission: perfecting the high vacuum inside the vacuum vessel prior to operation and evacuating helium ash, unburnt fuel and all exhaust gases during plasma shots. Another two cryopumps will be installed on the cryostat to provide the vacuum that thermally insulates the magnet system from the environment.   Every °ÄÃÅÁùºÏ²Ê¸ßÊÖ cryopump is equipped with 28 "cryopanels" that will be cooled down to 4.5 K (minus 268.5 °C) by a flow of supercritical helium. These extremely cold surfaces will make an extremely effective particle trap.   The °ÄÃÅÁùºÏ²Ê¸ßÊÖ cryopumps are based on a simple law of physics: when a molecule or an atom encounters an extremely cold surface, it loses the best part of its energy and slows down to near immobility—a phenomenon know as "adsorption." The cryopanels (one metre long, 20 centimetres wide) are coated with a very fine, porous carbon matrix obtained from ground coconut-shell charcoal. Despite their relatively small size, they provide an immense surface for particles to stick to: if developed (flattened out), each carbon matrix would cover 5.5 square kilometres—an area close to 13 °ÄÃÅÁùºÏ²Ê¸ßÊÖ platforms.   In August last year, a pre-production cryopump for the torus pumping system, built in collaboration by the °ÄÃÅÁùºÏ²Ê¸ßÊÖ Organization and the European Domestic Agency, was delivered to the °ÄÃÅÁùºÏ²Ê¸ßÊÖ site. The massive and highly sophisticated component is presently being tested in a laboratory that the °ÄÃÅÁùºÏ²Ê¸ßÊÖ vacuum team has set up in the neighbouring CEA-Cadarache, close to the hall that hosts the WEST tokamak.   "No one has ever built a cryopump comparable to this one. It's absolutely unique and we have to familiarize ourselves with it," says Roberto Salemme, the °ÄÃÅÁùºÏ²Ê¸ßÊÖ vacuum engineer who oversees the small team from the Air Liquide-40/30 consortium implementing the test program.   The °ÄÃÅÁùºÏ²Ê¸ßÊÖ cryopumps will connect directly to either the vacuum vessel (1,400 cubic metres) or the cryostat (8,500 cubic metres). In order to mimic these conditions, a "dome" (0.4 cubic metres) has been installed to seal the open end (left) of the pre-production cryopump. The valve inside the cryopump—the world's largest all-metal high vacuum valve—is one of the main focuses of the tests. Its head is 80 centimetres in diametre, weighs 80 kilos and travels along a 40-centimetre shaft stroke. When closing, it must lock with a precision of 0.1 millimetre to tighten its all-metal seal.   "We need to characterize the valve's mechanical properties and behaviour and precisely measure the forces that need to be exerted to move it along the shaft and obtain the required sealing at both atmospheric pressure and under vacuum," explains Roberto.   Once installed in the °ÄÃÅÁùºÏ²Ê¸ßÊÖ machine, the cryopump will connect directly to the vacuum vessel. In order to mimic this configuration in the lab, the pump has been equipped with a dome that seals its open end and allows the creation of a vacuum inside—not °ÄÃÅÁùºÏ²Ê¸ßÊÖ-grade, but sufficient to characterize the mechanical operation of the valve in "real" conditions.   There is a lot that still needs to be explored, measured and characterized before the cryopumps can enter series fabrication, and tests under cryogenic conditions will be essential to establishing the detailed succession of °ÄÃÅÁùºÏ²Ê¸ßÊÖ operational sequences.   All that can be done in advance—like the ongoing tests at the °ÄÃÅÁùºÏ²Ê¸ßÊÖ lab at CEA— will simplify the commissioning to be performed by the vacuum team for First Plasma and beyond.    

It is close to midnight in the brightly lit basement of the °ÄÃÅÁùºÏ²Ê¸ßÊÖ bioshield and, tonight, the first plot of the Tokamak "crown" is to be poured. The operation is of strategic importance: the crown will support the combined mass of the Tokamak and its encasing cryostat (23,000 tonnes) while transferring the forces and stresses generated during plasma operation to the ground. Thousands of tonnes of concrete have already been poured into the different areas of the Tokamak Complex. But every operation is unique, due to differences in pouring techniques, concrete formulations, and even prevailing weather and temperature conditions. Like in most challenging situations—and the pouring of the supporting crown is certainly one—the correct pouring of the different geometrical shapes of the structure was rehearsed and validated on a real-size mockup before being implemented. It took four hours to pour approximately one hundred cubic metres of high-performance concrete into the first plot, which represents one-fourth of the total crown volume. Another plot is scheduled to be poured in about three weeks.

The heat flux sustained by the targets of the °ÄÃÅÁùºÏ²Ê¸ßÊÖ divertor will be higher still—by ten times—than that of a space vessel re-entering Earth's atmosphere. Meticulous prototyping and test campaigns are underway to prepare for the manufacturing of these highly technical components; last month, a milestone was reached in Europe in the development program for the inner vertical target. In °ÄÃÅÁùºÏ²Ê¸ßÊÖ, the hot plasma can only be maintained if waste gases and impurities are continually exhausted from the machine. The component in charge of the exhaust process is situated at the bottom of machine—composed of a supporting structure in stainless steel and three plasma-facing components, the divertor will face heat fluxes of 10-20 MW per square metre. The bombardment will be particularly intense for the vertical targets, which are positioned at the intersection of magnetic field lines. Approximately 300,000 individual monoblocks in tungsten will armour the divertor plasma-facing components. Each one must be shaped and positioned precisely; in this photo, metrologists from the European Domestic Agency are performing dimensional checks. Tungsten, a shiny, silvery-white refractory metal that has a high melting temperature (3400 °C), has been chosen as the armour material for the plasma-facing components and prototyping and testing phases are underway for the dome (Russia), the outer vertical target (Japan), and the inner vertical target (Europe) to ensure manufacturability and performance in line with challenging °ÄÃÅÁùºÏ²Ê¸ßÊÖ specifications. The maximum expected temperature on the tungsten surface of the inner vertical target will be about 1000 °C in normal operating conditions and 2000 °C in off-normal conditions. During a pre-qualification phase in Europe for the inner vertical target, multiple suppliers had fabricated and tested small-scale (~1/20th) mockups of the tungsten monoblocks mounted around cooling channels. As a follow-up step, pre-qualified manufacturers were invited to produce full-scale prototypes—half-tonne, 1.5-metre-long components that are made of a massive curved steel support structure armoured with 1,104 tungsten monoblocks actively cooled by pressurized water. "This scale-one prototype demonstrates industry's ability to manufacture such a demanding piece—fulfilling the °ÄÃÅÁùºÏ²Ê¸ßÊÖ requirements on line with nuclear standards, in particular in terms of welding techniques," says Frédéric Escourbiac, leader of the Divertor Section. "On top of this, the prototype respects the stringent tolerances imposed by the need to align the components in the machine perfectly. It's a great first step; we need now together with our Domestic Agency partners to deploy the effort to ensure a similar level of performance during series production. Ansaldo Nucleare (Italy) is the first supplier to complete its prototype, in collaboration with its main subcontractors Ansaldo Energia, ENEA and Walter Tosto. The prototype will now be shipped to the Efremov Institute in Saint Petersburg for a series of thermal tests. Read the full story on the European Domestic Agency website.

In mid-May, factory acceptance tests were successfully carried out on the second gyrotron of the Russian procurement program by specialists at the Institute of Applied Physics and GYCOM Ltd. Twenty-four energy-producing devices called gyrotrons will operate on °ÄÃÅÁùºÏ²Ê¸ßÊÖ as part of the machine's electron cyclotron resonance heating system. These powerful sources of microwave radiation are tasked with a number of important missions: pre-ionization ("starting" the plasma), plasma heating and current drive, and the stabilization of local instabilities.   The first gyrotron was developed at the Institute of Applied Physics (Russian Academy of Sciences) back in 1964, generating 6W at 10GHz for continuous operation. Since then, scientists around the world have steadily increased gyrotron output power and, today, °ÄÃÅÁùºÏ²Ê¸ßÊÖ needs are driving the program.   The tests conducted on the second gyrotron manufactured in Russia demonstrated full compliance with °ÄÃÅÁùºÏ²Ê¸ßÊÖ Organization technical requirements (1 MW power at the required 170 GHz in continuous mode).

A second batch of tanks for °ÄÃÅÁùºÏ²Ê¸ßÊÖ's water detritiation system has been delivered by Europe for installation in the Tritium Building. The new arrivals—two small holding tanks and two feed tanks—were delivered on 30 May and are temporarily stored in a large warehouse on site.   Tritium is a highly valuable resource that will be recovered at °ÄÃÅÁùºÏ²Ê¸ßÊÖ in a complex recycling process for re-injection into the plasma. Gases containing low levels of tritium from various locations in the Tokamak Complex—such as the neutral beams cells and the port cells—will undergo a water-based cleansing process in the air detritiation system. The resulting tritiated water will be stored in large holding tanks that arrived at °ÄÃÅÁùºÏ²Ê¸ßÊÖ in spring 2015 and became the first processing components to be installed in the Tokamak Complex a year later.   The two newly arrived small holding tanks with a capacity of approximately 7 cubic metres complement the four larger holding tanks. They are designed to hold tritiated water resulting from the maintenance of the water detritiation system.   Two larger feed tanks, 12 cubic metres each, ensure the continuous feeding of tritiated water from all six holding tanks into the water detritiation system. In several steps, electrolyzer units and a catalytic exchange column will treat the tritiated water to recover the valuable tritium fuel for the fusion reaction.   Procured by the European Domestic Agency, the four stainless steel tanks were manufactured in Spain by ENSA with subcontractors ENWESA and Empresarios Agrupados.

Are you a writer or an artist interested in fusion? Then the latest call of EUROfusion, the European consortium of national fusion research institutes, may be something for you. For the third time, EUROfusion invites creative minds to be inspired by all things fusion. Choose any of the eight provided topics and let your creativity on the loose. Winners will have their work published in the autumn edition of the magazine Fusion in Europe. The eight topics include fusion as a must-have in the future energy mix; fusion as a benefit for you, us and society; fusion as a melting pot for different scientific fields; fusion as a driver of innovation, and a few others questions. Consult EUROfusion to see all topics and more information on how to participate in this call. The deadline for the submission of proposals is 25 June.