Influence of the surface composition and morphology on the reflectivity of diagnostic mirrors in a fusion reactor
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Beschreibung
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 postmortem.
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.
Autor*in
Maria Matveeva