Influence of Impurities on the Fuel Retention in Fusion Reactors

Doctoral thesis English OPEN
Reinhart, Michael (2015)
  • Publisher: Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag

The topic of this thesis is the influence of plasma impurities on the hydrogen retentionin metals, in the scope of plasma-wall-interaction research for fusion reactors.This is addressed experimentally and by modelling. The mechanisms of the hydrogenretention are influenced by various parameters like the wall temperature, ionenergy, flux and fluence as well as the plasma composition. The plasma compositionis a relevant factor for hydrogen retention in fusion reactors, as their plasma willalso contain impurities like helium or seeded impurities like argon.The experiments treated in this thesis were performed in the linear plasma generatorPSI-2 at Forschungszentrum Jülich, and are divided in 3 parts: The firstexperiments cover the plasma diagnostics, most importantly the measurement ofthe impurity ion concentration in the plasma by optical emission spectroscopy. Thisis a requirement for the later experiments with mixed plasmas. Diagnostics likeLangmuir probe measurements are not applicable for this task because they do notdistinguish different ionic species. The results also show that the impurity ion concentrationscannot be simply concluded from the neutral gas input to the plasmasource, because the relation between the neutral gas concentration and impurity ionconcentration is not linear.The second and main part of the experiments covers the exposure of tungsten samplesto deuterium plasmas. In the experiments, the impurity ion type and concentrationis variated, to verify the general influence of helium and argon on the deuteriumretention in tungsten samples exposed at low temperatures. It shows that heliumimpurities reduce the amount of retained deuterium by a factor of 3, while argonimpurities slightly increase the total retention, compared to exposures to a pure deuteriumplasma. Cross-sections of the exposed tungsten surfaces via TEM-imagingreveal a 12-15 nm deep helium nanobubble layer at the surface of the sample, whilefor the cases of pure deuterium or deuterium + argon exposures, a damaged layer ofless than 5 nm thickness is observed. Connections between the helium nanobubblesprovide a path for the molecular deuterium to the surface, which leads to the reductionof the total deuterium retention. The second part of the tungsten exposuresinvestigates the influence of helium impurities under the variation of the ion fluence.It is found that the reduction factor for the deuterium retention stays constant inthe fluence range investigated
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