ITER Newsline / 14 February 2018


 
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IN THE MEDIA 7
 FEATURED

Gravity supports
First production unit in China

Bolted in a perfect circle to the pedestal ring of the cryostat base, 18 gravity supports will brace the curved outer edge of each toroidal field coil. These unobtrusive elements are in fact a marvel of engineering, designed to support 10,000 tonnes of dead weight and yet have the flexibility to withstand the displacement of the coils during cooldown and operation. In China, the first production unit stands 2.65 metres tall on the shop floor at the company HTXL, a sub-supplier of the Center for Fusion Science of the Southwestern Institute of Physics (SWIP). Built from 20 tonnes of ITER-grade 316LN stainless steel, the assembly resembles a Roman column, with 21 vertical plates inserted into pedestals at top and bottom and helium cooling pipes in a curled arrangement welded to each plate near the top. ITER's 18 interlinked toroidal field coils provide the superstructure that anchors the entire superconducting magnet system, including six poloidal field coils, the central solenoid and an array of correction coils. From their position at the bottom of the machine, then, the toroidal field coil gravity supports will be confronted with about 10,000 tonnes of magnet dead weight—or 580 tonnes per support. At the same time the gravity supports must withstand the electromagnetic forces of operation, seismic loads (if they occur), and thermal gradient deformation, which causes the top of the support to shrink toward the centre of the machine (~32 mm) while the bottom remains stable. This push and pull represented a severe design challenge for engineers, requiring many years of prototyping, load analyses and testing. Working from an ITER Organization design, engineers in China worked closed under the coordination of the ITER Magnet Division, the Chinese Domestic Agency and SWIP. In 2013, an engineering test platform was built at SWIP to apply the loads—and load combinations—that are expected during operation at ITER on these qualification mockups. The solution is a mix of flexibility and rigidity—a sandwich of 21 flexible plates, 30 mm thick, divided by spacers that are pre-assembled and clamped together at bottom and top with pre-stressing bars and tie rods; these bars and rods are pre-tensioned with stud tensioners. Active cooling provided at two-thirds of the height of the assembly smooths the transition between the toroidal field magnets at 4 K on top and the room temperature at the base of the structure. 'The gravity support will have to be able to resist the toroidal and vertical motion and related forces of the toroidal field coils during the Tokamak operation, but also allow their radial motion during cooldown and warmup,' explains Cornelis Beemsterboer, structural engineer for the Magnets Division and technical responsible officer for the different magnet supports. 'At the same time each vertical plate is designed to support 27 tonnes of the overall gravity load. Analysis has shown that the design meets all requirements for both room and operating temperature conditions.' The first production module in China has performed well in factory acceptance tests, including a thermal shock test, helium leak tests on the active cooling pipes, and pressure tests. Still to come is the final assessments on the final applied pre-loading of the bolts. 'The gravity supports are impossible to replace and we need to be sure that the bolts will keep the vertical plates together for the full duration of ITER operation," emphasizes Beemsterboer. The full set of gravity supports is expected on site in mid-2019 for installation in the machine on the pedestal ring ahead of the toroidal field magnets. China is also manufacturing other magnet supports for the poloidal field and correction coils—in all, more than 400 tonnes of equipment.

Conference
Fun-filled vacuum

The science of ITER is not simple. But with a bit of imagination (and a dose of humour) a way can be found to convey the most complex physics notions to a public of non-specialists. Last week, as a large number of staff members were assembled to attend the fourth presentation in a new ITER technology series, Vacuum Section leader Robert Pearce demonstrated that vacuum—the presentation's theme—not only could be made 'understandable' but could also be a lot of fun. One of the highlights of his presentation was a reenactment of the famous 1654 'Magdeburg hemispheres' demonstration, when a German scientist pumped air out of two joined copper hemispheres and had two teams of 15 horses try to separate them by pulling in opposite directions. As no horses were available in the ITER amphitheatre, the experience was conducted with two volunteers: head of ITER Project Control Office Hans-Henrich Altfeld and cooling water engineer Fabio Somboli. But like the horses of 364 years ago—and despite much pulling, puffing and panting—they were unable to overcome the atmospheric pressure that keeps the spheres attached to one another. (At this point of the presentation we learn that the atmospheric pressure comes from the gravitational force of the air molecules.) There were other very interesting facts to glean from Pearce's presentation—that an absolute vacuum cannot exist even in the deepest and darkest outer space; that vacuum can boil and freeze water simultaneously; and that extremely cold surfaces coated with finely ground coconut charcoal are perfect for imprisoning helium particles. But the most popular moment of the show was the production of a plasma, created by injecting argon gas into a small vacuum chamber equipped with a magnet, and running a current. And there we had it: ITER's First Plasma close to eight years ahead of schedule. Not inside the Tokamak vacuum vessel as expected but onstage in the Headquarters amphitheatre ...

Naive question of the week
What happens to the car keys?

We begin today a new series that aims to answer basic, even naive, questions about fusion and ITER. An image used often, when trying to convey the amount of energy stored into the ITER central solenoid, is that of a magnet lifting an aircraft carrier out of the water. Convenient images, of course, simplify reality. Here is a little more explanation on this one: 'The top and bottom halves of the central solenoid are attracted to each other with a force of 50,000 tonnes,' explains Neil Mitchell, the head of ITER Magnet Division. 'If there was a gap in the middle of the 18-metre-high component, and if a 100,000-tonne aircraft-carrier was attached to the bottom, the carrier would indeed be lifted until the gap closed.' This leads to the naive question of the week. If the massive magnet is powerful enough to lift an aircraft carrier, could it snatch the car keys from the pocket of an operator standing in the Diagnostics Building, some 30 metres away? Powerful magnets are known to do this kind of trick. Paul Libeyre, ITER Central Solenoid, Support and Performance Section Leader, remembers visiting the Philips research centre in Eindhoven (Netherlands) where some of the most powerful magnets for magnetic resonance imaging (MRI) are assembled. 'They did several demonstrations on how a powerful MRI magnet attracts anything metallic in its vicinity with considerable force—coins, trays, drip stands, and even a wheelchair! It was quite impressive.' The keyword here is 'vicinity.' Like many things in nature (light, radiowaves, gravity, sound ...) the intensity of a magnetic field follows what is called the inverse-square law. The force of a magnet decreases so rapidly that at a distance of 30 metres it has lost 99.9 percent of its original intensity. In the pocket of the diagnostics operator, therefore, car keys are perfectly safe.

Metrology
Facing the millimetre test

In the realm of the very large at ITER, some of the biggest challenges are lurking down in the millimetre range. Within the Assembly Building a massive structure is taking shape: the Sector Sub Assembly Tool (SSAT). It is the first of two assembly tools designed to equip the nine-sectors of the vacuum vessel before they are positioned in their final location in the adjacent Tokamak Pit. Metrology, the science of measurement, will play a decisive role at every step in the erection of the twin tools. Before and after each component is preassembled and put into place, metrology engineers carry out a variety of measurements and computational analysis to verify that every component is assembled and positioned in accordance with its design requirements. Utmost precision and accuracy are vital. How is this accomplished? Metrology relies on an instrument called a laser tracker that measures angles and distances to identify an object's exact position in three-dimensional space. To assist these measurements, reference points or 'nests' are identified and characterized on each component as targets for the laser tracker. Characterization is a method in metrology to capture a component's critical features to precisely position it in the next assembly phase. Due to the size of the SSAT, several metrology instrument positions are needed to achieve the best precision measurements. A network of target nests embedded in the ground of the Assembly Building helps position the instruments around the SSAT. By shooting a laser beam at the nests in the ground, the instrument obtains its exact position with respect to the SSAT, essentially measuring the distance and the horizontal and vertical angles relative to each nest. Simple trigonometry—one known distance and two known angles. It's very much like GPS, with the metrology instrument representing a cell phone and the nests representing the fixed position of global satellites that can identify the exact location of a cell phone anywhere on the Earth's surface using triangulation. The challenge lies in the size of the SSAT. With a height of 22 metres, a wing span of 20 metres and weight of 860 tonnes when temporary supports are included, small discrepancies at the bottom of the SSAT could lead to large divergences at the top. 'We have to set the SSAT as close to level with the gravity vector as possible so that the downward forces are equally distributed,' says John Villanueva, a dimensional control engineer and metrology expert. 'Gravity has to be our friend.' Assembly tolerances are set to plus or minus two millimetres and the engineers are working with a very tight tolerance budget. Always giving their best effort, they've achieved tolerances in the sub-millimetre range which provides them with extra budget allowances they could potentially apply at subsequent assembly phases. Villanueva will follow the SSAT project through all of its assembly phases. Earlier, he travelled to Taekyung Heavy Industries in Korea for the factory acceptance test of the first SSAT tool before shipment to France. Following his premise that simple physics can tell you the truth, he put a small container of water on the moving wing platform. A line on the outside of the container marked the waterline. 'I wanted to see whether the platform was level as it moved around the rail. I checked it at different stations and it was perfect; the waterline never deviated. They did a marvelous job.' He now wants to do a similar test on site when the first assembly tool is fully erected. 'I will put a marble on the rotating frame to see how it reacts ...'

Fusion research in Europe
Working it out together

In Europe, fusion research is structured around a goal-oriented roadmap that closely involves universities, research laboratories and industry. Sibylle Günter, the scientific director of the Max-Planck Institute for Plasma Physics (IPP) in Germany, can speak to how the framework works from the inside. During a recent visit to ITER Headquarters she was able to give us her views on the advantages of broad scientific cooperation on fusion and how ITER is benefitting. Europe is home to many different fusion devices, all conducting experiments in the quest for fusion electricity. How do they work together? EU countries have a long history of working together on fusion research. With ITER being the main goal of these efforts, we use many of the already existing devices to prepare for ITER operation by exploiting their specific strengths. It's like a step ladder approach—we can test new ideas with the smaller and more flexible machines. Let me give you an example. The tokamak at IPP, ASDEX Upgrade, has a very similar shape to JET (Joint European Torus) and ITER. Carbon used to be the material considered for the plasma chamber walls. But, carbon is easily eroded by the plasma and it traps the plasma fuel tritium. So, we were pioneering tungsten as the material for the plasma chamber wall instead of carbon. Tungsten is a metal that was under consideration due to its very high melting point. But as it is not fully ionized, even at fusion-relevant temperatures, it emits significant line radiation. Therefore, you can tolerate only a tiny amount of tungsten in the plasma of a fusion power plant. We ran several campaigns with tungsten as the plasma chamber wall material at ASDEX Upgrade and learned how to avoid tungsten accumulation in the plasma centre. When the results came back positive, JET used tungsten to build an ITER-like vessel wall. The ASDEX Upgrade scientists supported JET in learning to operate with metal walls. Now, JET is the machine that ITER can rely on for findings on a tungsten divertor. We have a really successful joint program in preparing ITER operational scenarios. We exploit our facilities in a joint and coordinated manner. We all focus on ITER wherever possible, but we are open-minded and are also looking ahead at DEMO, the demonstration fusion power plant that will come after ITER. The successful operation of the ITER plant is the spearhead of the European fusion research. How can work at ITER contribute to the research efforts at Europe's laboratories and fusion devices? I hope we will learn a lot. We all work for ITER because we want to see a burning plasma. We want to learn from ITER how plasma physics work in a running fusion reactor and feed the findings back into the development of our DEMO design. ITER will give us a whole catalogue of lessons learned that will help us answer many questions: How do we build a reactor? What can we learn from the ITER model on international cooperation? Which new technologies can we take on from ITER? Two different designs for a fusion power plant dominate current research—the stellarator and the tokamak. Do we need both? In 50 years of fusion research we have seen many different configurations for fusion devices, including new designs by start-up companies in the US. The first-ever fusion device was a stellarator, but it was outrivaled by the tokamak with its better confinement properties. Now, with the help of high-performance computers in optimizing the magnetic field structure, the stellarator can be made to work.

A theorist with an interest in the practical uses of physics

Sibylle Günter is a German theoretical physicist, working primarily in the field of plasma physics. Although a theorist, she is passionate about practical applications of her field. Consequently, she became a staff scientist at the Max Planck Institute for Plasma Physics, IPP, in Garching, Germany, in 1996. Four years later, she assumed the position of Head of the Tokamak Physics Division. Since February 2011 she has been IPP's Scientific Director.

There is no real competition between the two designs. Both have advantages and disadvantages. The tokamak's advantages come from its good plasma confinement properties, plus it is relatively easy to build. Its disadvantages relate mostly to the possibility of disruptions, which are the biggest fear for power plants. So, as long as we have not resolved the issue of disruptions we should continue looking at the stellarator design as an option in any case. Its advantages stem from its intrinsic steady-state operation and higher plasma densities. Does this mean that the stellarator design is considered an option for another DEMO project? In our EU roadmap, the DEMO fusion power plant is clearly based on a tokamak design. The stellarator has not yet proved that it can achieve the plasma confinement properties that the tokamak design has been demonstrating. This is the task of Wendelstein 7-X at IPP Greifswald. The EU roadmap has set a decision point regarding the future of the stellarator program at around 2025. In any case, we all work to make ITER a success, as the basis of a DEMO fusion power plant.

 OF INTEREST

Not just for smart phone batteries

The chemical element lithium may just have found itself a new application. Scientists at the Princeton Plasma Physics Laboratory (PPPL) and China's Experimental Advanced Superconducting Tokamak (EAST) have found that lithium powder can reduce periodic instabilities in plasma when used to coat tungsten surfaces in fusion devices. These instabilities are known as edge-localized modes (ELMs) and occur at the outer parts of the fusion plasma. ELMs develop regularly when the plasma enters what is known as high-confinement mode, or H-mode, which holds heat within the plasma more efficiently. ELMs can damage the divertor, a plasma-facing component that extracts heat and ash produced by the fusion reaction, and cause fusion reactions to fizzle. The researchers also found that it became easier to eliminate ELMs as the experiments progressed, possibly requiring less lithium as time went on. The results cause physicists to be confident that these techniques could also reduce ELMs in larger fusion devices that were designed to be compatible with lithium. Read the full article on the PPPL website here.

 IMAGES

Ready to lift

This large crawler crane was erected last week near the cryoplant. Beginning on 14 February, it will be used to lift vertical tanks for the storage of liquid and gaseous helium and nitrogen into position.

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Meet the scientists who want to harness the power of the Sun

Science News

50 years on, nuclear fusion still hasn't delivered clean energy (subscription only)

Science Mag.cz

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