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New mirror system for ITER tested in DIII-D

Ralph Schorn, Forschungszentrum Juelich

Front side of the test set-up of Jülich's mirror station with the passive magnetic protective shutters facing the fusion plasma. Copyright: Forschungszentrum Jülich. (Click to view larger version...)
Front side of the test set-up of Jülich's mirror station with the passive magnetic protective shutters facing the fusion plasma. Copyright: Forschungszentrum Jülich.
An international working group coordinated by Forschungszentrum Jülich, Germany, has completed a new mirror system for ITER ... and for its successors. The system—referred to as a "mirror station"—has shutters that open and close automatically to protect optical components from being contaminated by particle flows in the vacuum vessel. The researchers have been testing the practical applicability of the module at the US research reactor DIII-D in San Diego since mid-March.

Optical diagnostics are indispensable for nuclear fusion experiments. The light produced in a plasma speaks volumes about its properties, such as its composition and the concentration of various isotopes and elements. Due to the intense neutron radiation, it will only be possible to observe the light indirectly, using mirror systems positioned at the plasma edge. In this zone, however, the mirrors are exposed to contamination from beryllium and tungsten particles removed from the wall materials during contact with the hot plasma.

Rear side of Jülich's mirror station: The light collected from the nuclear fusion plasma is redirected to suitable measuring systems for analysis. (Click to view larger version...)
Rear side of Jülich's mirror station: The light collected from the nuclear fusion plasma is redirected to suitable measuring systems for analysis.
The new mirror system for ITER has fast shutters made of monocrystalline molybdenum, which only uncover the mirror during the main phase of the plasma pulse. The shutters thus protect the sensitive optical components when the plasma is ignited, as the risk of contamination is at its highest during this phase. Since the very strong magnetic fields in the vacuum vessel interfere with electrical circuits, Jülich's mirror station relies entirely on passive control. An additional magnetic field component is utilized for this purpose. It emerges as soon as the tokamak plasma ignites and it acts on a magnetic ferrite core in the "mirror station" which passively opens the protective shutters.

Jülich's mirror station, ready to be tested in the DIII-D Tokamak. (Click to view larger version...)
Jülich's mirror station, ready to be tested in the DIII-D Tokamak.
"We have already tested electromagnetic loading of the system in a tokamak environment and used software codes developed at Jülich to minimize the release of contaminating atoms and their redeposition on the mirror surfaces. We believe that our development will make a very substantial contribution to making optical measurements possible at ITER ," says project head Dr. Andrey Litnovsky at Jülich's Institute of Energy and Climate Research. After DIII-D, the practical applicability of Jülich's "mirror station" will be put to the test at the Chinese fusion experiment EAST in Hefei, at the ASDEX Upgrade operated by the Max Planck Institute for Plasma Physics in Garching near Munich, Germany, and at Jülich's TEXTOR Tokamak.

Further information on fusion research at Forschungszentrum Jülich can be found here.


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