Highlights 2014

Research news from the division Plasma Edge and Wall

First experiments with the new divertor manipulator system in ASDEX Upgrade

Parallel to the installation of massive tungsten target tiles in the outer divertor of ASDEX Upgrade a new manipulator system was designed, assembled and integrated in the tokamak, which allows exposing two adjacent test target tiles at the outer divertor plate (image above). The test tiles can be exchanged by means of an airlock system without breaking the vacuum of the ASDEX Upgrade vessel. This allows detailed studies of the influence of plasma-material interactions on the retrieved test tiles using a multitude of ex-situ diagnostics available in the surface analysis laboratories of IPP. The new manipulator system is the first of its kind, which allows exposure of full-size target plate samples and sample exchange in between experiment days. The manipulator also includes an active tile cooling system, which for the first time allows dedicated tests of target component concepts under reactor relevant thermal load conditions and realistic magnetic field geometry. The system was successfully commissioned in the current experimental campaign 2014. In a first experiment, erosion properties of tungsten were investigated. This included on the one hand quantification of the influence of surface roughness on the tungsten erosion yield and on the other hand deriving direct evidence for so-called prompt re-deposition of eroded tungsten atoms within their first gyro-orbit after ionisation. To facilitate sample analysis, two custom bulk tungsten tiles with grooves for insertion of smaller samples were prepared. Erosion rates were quantified by measuring the thickness of respective marker layers before and after plasma exposure using ion beam analysis. This method provides in the same measurement also quantitative information on the deposition rate of other plasma impurities. Both quantities are essential for estimating the life-time of divertor target plates, which contribute significantly to the operating costs of a fusion reactor.

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