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ePaper created 2013-02-01, 09:35:07 | version 1.24.0
16 Research in Jülich 3|2012 Mirror, Mirror – and the Wall T he metal tungsten is really really heavy. That becomes clear when Dr. Philippe Mertens puts a piece into your hand. “Wolfram has emerged as the material of choice for the wall of the fusion chamber,” says the project leader at the Institute of Energy and Cli- mate Research. “It only melts at temper- atures above 3400°C, has a high mass and does not atomize.” It therefore has ideal properties for a material that comes into contact with a plasma at a temperature of 100 million °C. DRESS REHEARSAL Tungsten is tried and tested as a ma- terial for the filaments in conventional light bulbs and will form a critical part of ITER’s reactor: the divertor. “That’s the ‘plug hole’ of the fusion reactor and the only zone where the wall of the reactor interacts directly with the plasma,” says Mertens, “which is why it has to with- stand the greatest loads.” The many tungsten lamellas, which are only about 5 cm in size but very heavy, are strung together like the links of a bicycle chain. Thousands of these lamellas put together in rows of four form the ring-shaped divertor. Only a few of them can still be found in Mertens’ laboratory at Jülich. “The lion’s share has already made the jour- ney to the United Kingdom,” he says. At the fusion reactor JET in Culham, the components from Jülich will have to demonstrate in make-or-break tests whether they fully meet the require- ments. A total of two tonnes of tungsten have been installed there by Mertens and his team. The less complicated part of JET’s reactor wall consists of the much lighter metal beryllium. Graphite is still in the running, but is lagging far behind. For a long time, the material consisting of pure carbon — which everyone knows as pencil leads – was considered the most promising can- didate for the chamber wall. “However, it has turned out that carbon reacts with the fuel and can form radioactive depos- its in the reactor,” explains Mertens. The official and final decision on a material for ITER’s wall will be taken in no more than one year’s time – based primarily on the results of the tests in the United Kingdom. So far, everything seems to favour the version installed there made of beryllium and tungsten. There is, however, still one uncertainty: “Tungsten must never ever melt during operation,” says Mertens. “We will now have to definitely rule this out in practi- cal tests.” If the heat is evenly distributed across the entire divertor, it is extremely unlikely that the tungsten will melt. Al- though the plasma is 100 million °C hot at its core, the temperature drops con- siderably towards the plasma edge. In addition, the heat is not transferred so quickly to the solid material. It must withstand temperatures of a maximum of 2000 °C – which should not be a problem for tungsten. “But if one of the lamellas protrudes from the wall just a tiny bit more than the others, then it will be in contact with the plasma in that place,” says the researcher. “The tem- perature would then become too high there, even for tungsten.” However, Mertens is optimistic. “JET is already in operation, and so far, our tungsten wall has been a huge success.” ALL DONE WITH MIRRORS Dr. Andrey Litnovsky still has some time before the dress rehearsal. He con- ducts research on mirrors for ITER. They will direct light out of the fusion cham- ber so that it can be analysed outside the reactor. Light is created when the Preparations for ITER, the international demonstration reactor for fusion power, which is currently being built in the south of France, are now seriously under way. A reactor wall developed by the Jülich plasma researchers is currently being exposed to final tests. A mirror system, also from Jülich, still has a little more time. Construction of ITER, the international demonstration reactor for fusion power, has been under way in Cadarache in the south of France since July 2010. It will be put into operation in November 2020.