Eurofusion July 12th 2017
It is high time for Fusion in Europe to check in on the progress of ITER more regularly. Our series “The ITERsection” explains what makes this tokamak the most complex machine in the world. By reporting about the fusion puzzle, made of pieces coming from Europe, China, Russia, South Korea, Japan, India and the US, the upcoming articles won’t only introduce scientific challenges, but they will also look at the demanding engineering tasks and the involved scientists until the tokamak will be able to light up its first plasma in 2025.
No ITER, no EUROfusion
Let’s take a look at Europe which acts as one of the seven ITER members. EUROfusion has centred its research almost entirely around ITER. Dedicated experts in 28 European countries want the world’s largest tokamak to deliver what it is built for: the proof that fusion works. Until ITER finally starts operating, the Joint European Torus (JET) is the only experiment in the world able to use the real fusion fuel, Deuterium-Tritium (D-T), and the only candidate capable of executing preliminary studies for ITER with D-T. It is EUROfusion’s task to coordinate the scientific exploitation of JET. Its ITER-like wall, for instance, gives crucial information about the experiments expected within the largest tokamak on Earth.
Checking on ITER’s heart
This time we will look at some of the components that make up ITER’s heart. The article highlights one particular set of magnetic coils which create, amongst other magnets, the box to hold the super-hot plasma in place. These six poloidal field coils (PF), also called horizontal coils, are situated outside the toroidal field (TF) coil magnets, also called the vertical coils. The task of the horizontal coils is to control the shape of the plasma and to secure its stability by keeping it away from the walls. They embrace the other D-shaped coils (toroidal), from the top to bottom. The process of manufacturing PF coils is demanding. Not only are these magnets gigantic, they are made up of elements from various parts of the world that come together in order to form one section of the tokamak’s heart.
One magnet or four Blue Whales
Due to their impressive size, four of these magnets will be manufactured on the ITER site because they are simply too heavy to be shipped. The largest coil has a diameter of about 25 metres and the heaviest weighs more than 400 tons. This is about four times the weight of a Blue Whale!
The different metals and their shear size make them so unwieldy. Inside, the coils rest “cable-in-conduit” superconductors which are made of niobium-titanium, a technically challenging combination to produce. The metal is mixed with copper to form superconductor/copper strands. These are, in turn, enclosed in a stainless steel jacket.
The world’s largest magnetic field
Why does ITER need such powerful magnets at all? The world’s largest tokamak to come is thus able to create the world’s largest magnetic field. The larger size and higher field enable ITER to operate with 15 megaampere of plasma current, five times more than in today’s largest tokamaks. This naturally requires powerful electrical current which is supplied by the previously mentioned superconducting cables. Their unique feature is to transport electricity without losing precious energy to electrical resistance. Another fascinating fact is that those mega magnets need to be cooled with helium, kept at a range of 4 Kelvin (-269 °C´), in order to work properly.
Processing the coils
In May this year, a major milestone was reached for the technicians inside the coil winding facility on ITER’s soil: after gaining sufficient experience they were able to switch from using dummies to using the actual niobium-titanium superconductor which is now being used to make the first real coil (PF5). Measuring 17 metres in diameter, PF5 will be the second ring to take its place in the Tokamak assembly sequence.
Pierluigi Valente, Responsible Technical Officer for the supply of the European share of PF coils, is more than confident: “What you see today is the result of the work that commenced almost three years ago. Seeing all of these manufacturing stages in practice is extremely gratifying” he says. Nevertheless, his job is far from being completed. Three more European PF coils are set to follow between this year and 2021.
Two more to come
Thus, the four European poloidal field coils currently being built on the ITER site are a challenge on their own. But, as the article mentioned earlier, ITER’s heart needs a total of six rings. The two remaining coils are being manufactured elsewhere. Europe also contributes to the bottom coil (PF 6) which is being made in China, in accordance with an agreement made between the ITER’s European Domestic Agency Fusion For Energy and the Chinese Institute of Plasma Physics. Meanwhile, the top poloidal field coil is being produced in Russia.
Puzzling for success
And this is just the story of the six horizontal magnets around ITER’s heart. There are toroidal field coils and correctional coils all waiting to be manufactured by six different nations. The challenge will be to fit all of the pieces together at the end of the day.
From this time onwards, Fusion in Europe will report more frequently on ITER developments. The ITER spirit lives, not only from the demanding engineering tasks, but also from the people involved in the construction of the big tokamak in the South of France. As a result, in the next edition, we will be introducing ITER’s Electron Cyclotron Section Leader Mark A. Henderson who considered ITER as nothing less but a ‘fusion cathedral’.
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