Deuterium permeation through tungsten driven by plasma-based low-energy ion implantation

The permeation of deuterium through tungsten driven by plasma-based ion implantation with low incident energies has been investigated. For this purpose, a method was developed that enables to measure the permeation during plasma exposure in a plasma source typically used for deuterium retention experiments. The measurement results were used to investigate the influence of sub-surface damage evolution on the permeation flux. For the permeation measurements, a getter layer consisting of either zirconium, titanium or erbium was deposited on one side of the tungsten samples. Subsequently, the opposite side of the samples was exposed to deuterium plasma. Deuterium that permeated through the tungsten during the plasma exposure was accumulated in the getter layer. The deuterium amount present in the getter layer after plasma exposure was determined by ex-situ ion-beam analysis. A cover layer system on top of the getter layer prevented direct uptake of deuterium into the getter layer from the background deuterium gas present during deuterium-plasma exposure. Furthermore, it enabled a distinction of deuterium in the getter layer and at the surface of the cover layer system in the ion-beam analysis. The method was validated and applied to measure the deuterium permeation through about 25 μm thick tungsten foils at 300 K and 450 K that was caused by deuterium-plasma exposure with incident energies in the eV-range. The resulting measured steady-state deuterium permeation flux was for both exposure temperatures indistinguishable within the experimental data scatter and had a value of about 1.7 x 10¹⁴ D/(m² s). Also the measurement of even lower permeation fluxes appears in principle possible with the presented method. An advantage of the presented method is that it decouples the measurement of the permeated deuterium amount spatially and temporally from the plasma exposure. Thus, existing plasma devices typically used for deuterium-retention experiments can also be used for plasma-driven permeation experiments without the necessity to attach a complex in-situ permeation measurement setup to the devices. The results of the permeation measurements were combined with the results of microstructural analyses and measurements of the deuterium retention. Thereby, the influence of sub-surface damage evolution on the permeation flux could be investigated. An increased deuterium retention below the plasma-exposed surface was correlated with the observation of sub-surface damage evolution. It was ascribed to traps evolving in the vicinity of the observed sub-surface damage. Sub-surface damage evolution as well as increased deuterium retention were only observed for 300 K exposure temperature. Still, the permeation flux at 300 K and 450 K was indistinguishable within the experimental data scatter. This was the case although the estimated upper limit of deuterium loss from the solute phase to evolving traps was of the same order of magnitude as the steady-state permeation flux. This phenomenon was modeled, reproduced and investigated with one-dimensional diffusion-trapping simulations. Diffusion-limited boundary conditions at plasma-exposed and permeation side and an implantation distribution and reflection yield chosen based on implantation simulations resulted in a good agreement of the simulated steady-state permeation flux with the experimentally determined value. To fully match the simulated to the experimental steady-state permeation flux, only a minor correction of the reflection yield was necessary for the best set of implantation-simulation input parameters. The evolving sub-surface trap profile was implemented based on deuterium retention measurements. The diffusion-trapping simulations also showed that a stronger decrease of the steady-state permeation flux would have to be expected for trap evolution deeper below the deuterium-plasma-exposed tungsten surface. The maximum ratios of solute-deuterium to tungsten atoms present during deuterium-plasma exposure at 300 K and at 450 K were estimated using the measured steady-state permeation flux. These values may be useful in future attempts to develop and test models that describe damage and associated trap evolution below the surface of tungsten caused by deuterium-plasma exposure. A detailed comparison of the experimentally determined and simulated deuterium retention after plasma exposure indicated limitations of the current diffusion-trapping model, which can give indications for future model improvements. The results presented in this thesis contribute to an improved understanding of the influence of sub-surface damage evolution in tungsten due to deuterium-plasma exposure on deuterium permeation and form a solid basis for further investigations of the plasma-driven permeation of deuterium through tungsten.