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Forschungszentrum Jülich - Research in Jülich 3_2012

3|2012 Research in Jülich 17 RESEARCH AT THE CENTRE | Fusion plasma ignites at high temperatures – which is also why the stars in the sky shine since most of them consist of plas- ma. “Light says a lot about the proper- ties of a star, and also about those of the plasma: What contaminants does it con- tain? How hot is it?” says Litnovsky. It would be useless to install cameras and measuring equipment in the wall of ITER’s chamber. The neutrons formed would immediately tarnish the instru- ments’ optical fibre. As a mirror, Litnovsky primarily uses discs that are 2cm in size and consist of the metal molybdenum. His greatest problem so far: “The mirrors quickly be- come dull because dirt accumulates on their surface. Contaminants are always produced during reactor operation.” This is why the researchers have put a chan- nel in front of the mirrors that lets light fall on them. The channel is covered with a protective flap. “Only the magnetic field produced by the plasma activates the opening mechanism of the protective flap,” says Litnovsky. “While the plasma ignites, however, when the number of contaminant particles in the air is high- est, the flap is closed.” In addition, the channel contains mechanical obstacles referred to as apertures that deflect con- taminating particles away from the mir- rors, making them more difficult for the particles to reach. This is a way of pro- tecting the mirrors. At the moment, the researcher is test- ing the system at the US research reac- tor DIII-D in San Diego. Experiments in the test reactors TEXTOR at Jülich and ASDEX Upgrade in Garching will start shortly. Later, the system will also have to prove itself in JET. :: Port plugs for plasma diagnostics Port plugs are integrated into ITER’s vacu- um vessel. Researchers from Jülich, Karls- ruhe, the Netherlands, the United Kingdom, and Hungary are involved in developing components for the CXRS port plug. This measuring system allows the plasma tem- perature and composition as well as the concentration of fusion “ash” (helium) to be analysed. For this purpose, a high-energy particle beam is directed into the plasma and the visible light produced is analysed. Jülich focuses on the CXRS mirror system that directs the light out of the plasma and makes it available for further analysis, and on the development of the mechanical components required for this purpose. Valve for preventing plasma disruption When the plasma disrupts, it quickly re- leases its entire energy to the wall of the vacuum vessel, where it causes local dam- age. However, if gas is supplied, the plas- ma energy can be converted into light en- ergy. It is then distributed homogeneously, which mitigates any beginning disruption (radiation cooling). Jülich researchers are developing and testing a valve that lets in gas quickly enough. The valve could be lo- cated at the port plugs (see 1). Divertor material The helium produced in the nuclear fusion process and some of the heat are extracted from the plasma via the divertor. This re- quires high-performance components and materials. Researchers from Jülich have de- veloped divertor modules made of solid tungsten that are being tested in the fusion experiment JET in the United Kingdom. Similar modules will also become part of ITER. In order to further optimize the diver- tor, the scientists are developing and utiliz- ing numerical models to improve the geom- etry of the divertor plates and maximize helium extraction. Measuring method for wall analysis A small amount of the fusion fuel tritium is deposited in the vacuum vessel wall and in ablated wall material. Jülich researchers are developing and testing different laser- assisted measuring techniques to deter- mine the amount of tritium deposited. They are also working on the use of re- placeable test surfaces in the ITER wall for long-term measurements of material abla- tion and redeposition. Jülich Hot Spots ITER projects at Forschungszentrum Jülich 4 3 2 1 Jülich fusion research

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