Impact of plasma-wall interaction on the nitrogen transport in fusion experiments

Controlling the plasma-wall interaction is one of the greatest challenges on the way to a fusion reactor. Experiments in the tokamak ASDEX Upgrade posed an important step to overcome this challenge: It could be shown that the admission of nitrogen to the plasma reduces the power flux to the wall and at the same time improves the plasma performance. However, it was also realized that nitrogen from the plasma is implanted into the tungsten wall surfaces of ASDEX Upgrade and becomes chemically bound in the tungsten. The figure illustrates the manifold interactions of nitrogen with a tungsten surface.

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Through the combination of laboratory experiments, computer simulations and dedicated ASDEX Upgrade experiments it was possible for the first time to describe this interaction quantitatively. The underlying physical picture is shown in the figure: The implantation of nitrogen into tungsten induces the formation of tungsten nitride. As nitrogen does not diffuse into the bulk material, the surface can only store a limited amount of nitrogen and excess nitrogen escapes back into the plasma. The combination of this newly established surface model with plasma transport calculations in the code WallDYN correctly describes the nitrogen balance in ASDEX Upgrade. The simulations reveal that the nitrogen balance is dominated by the storage of nitrogen in the surface.

With this work Gerd Meisl graduated at the TU München in January 2015.

G. Meisl et al., New J. Phys. 16, 093018 (2014),
http://dx.doi.org/10.1088/1367-2630/16/9/093018

G. Meisl et al., J. Nucl. Mater. 463 (2015) 668,
http://dx.doi.org/ 10.1016/j.jnucmat.2014.10.031

 
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