Influence of Impurities on the Fuel Retention in Fusion Reactors

The topic of this thesis is the inuence of plasma impurities on the hydrogen retention
in metals, in the scope of plasma-wall-interaction research for fusion reactors.
This is addressed experimentally and by modelling. The mechanisms of the hydrogen
retention are inuenced by various parameters like the wall temperature, ion
energy, ux and uence as well as the plasma composition. The plasma composition
is a relevant factor for hydrogen retention in fusion reactors, as their plasma will
also contain impurities like helium or seeded impurities like argon.
The experiments treated in this thesis were performed in the linear plasma generator
PSI-2 at Forschungszentrum Jülich, and are divided in 3 parts: The rst
experiments cover the plasma diagnostics, most importantly the measurement of
the impurity ion concentration in the plasma by optical emission spectroscopy. This
is a requirement for the later experiments with mixed plasmas. Diagnostics like
Langmuir probe measurements are not applicable for this task because they do not
distinguish dierent ionic species. The results also show that the impurity ion concentrations
cannot be simply concluded from the neutral gas input to the plasma
source, because the relation between the neutral gas concentration and impurity ion
concentration is not linear.
The second and main part of the experiments covers the exposure of tungsten samples
to deuterium plasmas. In the experiments, the impurity ion type and concentration
is variated, to verify the general inuence of helium and argon on the deuterium
retention in tungsten samples exposed at low temperatures. It shows that helium
impurities reduce the amount of retained deuterium by a factor of 3, while argon
impurities slightly increase the total retention, compared to exposures to a pure deuterium
plasma. Cross-sections of the exposed tungsten surfaces via TEM-imaging
reveal a 12-15 nm deep helium nanobubble layer at the surface of the sample, while
for the cases of pure deuterium or deuterium + argon exposures, a damaged layer of
less than 5 nm thickness is observed. Connections between the helium nanobubbles
provide a path for the molecular deuterium to the surface, which leads to the reduction
of the total deuterium retention. The second part of the tungsten exposures
investigates the inuence of helium impurities under the variation of the ion uence.
It is found that the reduction factor for the deuterium retention stays constant in
the uence range investigated.