Highlights 2016

Research news from the division Plasma Edge and Wall

Interaction of Deuterium Plasma with pre-nitrided Tungsten Surfaces

Experiments in the tokamak ASDEX Upgrade have shown that the addition of nitrogen to the plasma reduces the power flux to the wall and at the same time improves the plasma performance. However, nitrogen from the plasma is implanted into the tungsten surfaces and becomes chemically bound. This bound nitrogen influences plasma-surface-interaction processes and hydrogen retention in the wall material.

The interaction of nitrogen-containing tungsten surfaces (WN_x) with hydrogen plasmas and their influence on hydrogen retention was investigated in laboratory experiments by Gao Liang in the frame of his PhD thesis at IPP.

Confocal laser scanning microscope image of a tungsten surface after deuterium implantation. The right-hand side of the sample was covered with a 100 nm thick WNx layer prior to D implantation. The pure tungsten surface (left) shows strong blistering (formation of gas-filled cavities underneath the surface), while no blisters occur on the WNx-covered half (right). — Confocal laser scanning microscope image of a tungsten surface after deuterium implantation. The right-hand side of the sample was covered with a 100 nm thick WN_x layer prior to D implantation. The pure tungsten surface (left) shows strong blistering (formation of gas-filled cavities underneath the surface), while no blisters occur on the WNx-covered half (right).

Image: IPP

Confocal laser scanning microscope image of a tungsten surface after deuterium implantation. The right-hand side of the sample was covered with a 100 nm thick WN_x layer prior to D implantation. The pure tungsten surface (left) shows strong blistering (formation of gas-filled cavities underneath the surface), while no blisters occur on the WNx-covered half (right).

Image: IPP

One important result is the finding that nitrogen implantation into tungsten strongly suppresses hydrogen diffusion.

At 300 K the diffusion of H in WN_x is negligible and at 600 K it is still much lower than in pure W.

Additionally he developed a refined method to determine the deuterium depth profile at surfaces with significantly improved depth resolution.

With this work L. Gao graduated at the Ruhr University Bochum in January 2016.

L. Gao et al., Nuclear Fusion 56, 016004 (2016),
http://dx.doi.org/10.1088/0029-5515/56/1/016004

L. Gao et al., Physica Scripta T159, 014023 (2014),
http://dx.doi.org/10.1088/0031-8949/2014/T159/014023