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| Hauptseite > Publikationsdatenbank > Influence of the surface composition and morphology on the reflectivity of diagnostic mirrors in a fusion reactor |
| Hauptseite > Publikationsdatenbank > Influence of the surface composition and morphology on the reflectivity of diagnostic mirrors in a fusion reactor |
| Dissertation / PhD Thesis/Book | FZJ-2015-03143 |
2015
Forschungszentrum Jülich GmbH Zentralbibliothe, Verlag
Jülich
ISBN: 978-3-95806-051-7
Please use a persistent id in citations: http://hdl.handle.net/2128/9201
Abstract: Plasma-surface interactions in fusion devices represent a critical issue for the design and operation of diagnostic systems based on transmission of light signals from plasma where metallic mirrors will be used as first plasma-viewing elements. In this work, the behavior of metallic mirrors is investigated with respect to the influence of plasma-induced changes of surface morphology and composition on the surface reflectivity. The work is based on the series of dedicated experiments performed in tokamaks TEXTOR and DIII-D, in which pre-characterized metallic mirrors were exposed during the plasma operation (TEXTOR) and during the thermo-oxidative wall conditioning (DIII-D). The morphology, composition and optical properties of the mirror surfaces are analyzed post-mortem. To understand the observed surface modifications, ion-surface interactions are modelled with the SDTrimSP code. The processes responsible for modifications of the surface morphology and composition under energetic particles bombardment, the surface erosion and formation of deposited layers, are investigated. Measurements show that the specular reflectivity of a mirror strongly depends on the surface roughness. The surface roughness after a plasma exposure depends on the crystalline structure of the material due to the fact that grains with different crystalline orientations have different sputtering rates. An increase of the surface roughness resulting from non-homogeneous sputtering of a polycrystalline material leads to a significant drop of the specular reflectivity due to diffuse scattering of the incident light. It is shown that under net erosion conditions coatings with nano-sized crystallites demonstrate a similar behaviour compared to polycrystalline materials. On the contrary, single crystalline mirrors are sputtered uniformly, thus show significantly less roughening and preserve the reflectivity better. The mirror reflectivity depends also on the surface composition, which can be changed due to plasma-surface interactions. The thickness of the affected surface layer depends on the balance between the processes of erosion, deposition, particle implantation, diffusion, and chemical reactions. For instance, carbide formation is observed on mirrors exposed in TEXTOR, thus contributing to the decrease of the surface reflectivity. The independence of the resulting depth distributions of carbon atoms from the mirror temperature suggests that the volume diffusion of carbon is very slow and can be neglected when comparing the diffusion depth with the ion implantation depth or the thickness of the eroded layer within the time scale of the experiment. It is shown that formation of carbides and oxides slows down the volume diffusion and prevents deeper penetration of impurity atoms into the surface. Measurements and modelling give a strong indication of a dynamic equilibrium established between the different physical and chemical processes involved. This equilibrium results in similar thicknesses of carbide layers formed on all molybdenum mirrors independent on the incident particle fluence and sample temperature. The overall balance between erosion and deposition processes on the mirror surface depends strongly on plasma parameters. Net erosion conditions are beneficial for metallic mirrors since such conditions do not lead to an unpredictable layer growth. It is demonstrated in this work that the balance between erosion and deposition can be shifted towards net erosion by means of intentional injection of gaseous species in the vicinity of the mirror surface during the plasma exposure.
Keyword(s): Dissertation
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