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@PHDTHESIS{Matveeva:190219,
      author       = {Matveeva, Maria},
      title        = {{I}nfluence of the surface composition and morphology on
                      the reflectivity of diagnostic mirrors in a fusion reactor},
      volume       = {261},
      school       = {Universität Düsseldorf},
      type         = {Dr.},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothe, Verlag},
      reportid     = {FZJ-2015-03143},
      isbn         = {978-3-95806-051-7},
      series       = {Schriften des Forschungszentrums Jülich Reihe Energie $\&$
                      Umwelt / Energy $\&$ Environment},
      pages        = {158 S.},
      year         = {2015},
      note         = {Universität Düsseldorf, Diss., 2014},
      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.},
      keywords     = {Dissertation (GND)},
      cin          = {IEK-4},
      cid          = {I:(DE-Juel1)IEK-4-20101013},
      pnm          = {172 - Tokamak Physics (POF3-172)},
      pid          = {G:(DE-HGF)POF3-172},
      typ          = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
      url          = {https://juser.fz-juelich.de/record/190219},
}