Speaker
Viacheslav Petrovich Budaev
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1012.pdf
Stochastic clustering of material surface under high-heat plasma load in
fusion devices
V. P. Budaev 1,2
1
National Research University “MPEI”, Moscow, Russia
2
National Research Center “Kurchatov Institute”, Moscow, Russia
Materials of various chemical composition and initial crystalline virgin structure (tungsten,
carbon materials and stainless steel) have been studied after the irradiation by high heat
plasma fluxes in nuclear fusion facilities [1]. High-temperature plasma load on the plasma
facing material in fusion devices during transients (disruption, ELMs, VDE etc.) produces
several multiscale effects including surface erosion, redeposition of eroded materials,
melting and melt motion over the surface, inhomogeneous solidification leading to specific
surface clustering conditions which are strictly different from any other conditions of
solidification and clustering of materials previously analysed. This study has demonstrated
evidences of inhomogeneous stochastic clustering of the surface with properties of the
self-similarity of granularity from nano- to macroscale. In particular, the hierarchical
granularity and self-similarity with cauliflower-like shape of tungsten surface have been
revealed for the first time. The clustering of materials irradiated by high-temperature plasma
qualitatively differs from the ordinary Brownian surface roughness and from clustering
under other conditions. This difference is shown by comparing the results with those for the
molybdenum sample after exposure in the magnetron plasma discharge and for the industrial
steel casting with the ordinary roughness formed typically at solidification after melting. The
specific property of material solidification and clustering under plasma influence in fusion
devices is due to a material’s (ions, clusters, melt on the surface etc.) motion under the
influence of stochastic electromagnetic field formed by the near-wall turbulent plasma. This
field ensures memory effects, long-term correlation and conditions for the growth of
agglomerates with a self-similar structure [2, 3]. In addition to such a process, effects of
irregular motion and relaxation of the material (melt) on the surface contribute to the process
of clustering at the extreme heat load on the material surface. These multiple effects are
responsible for the fractal growth mechanism at scales from several tens of nanometers to
hundreds of microns [2, 3]. Collective (synergistic) effects, rather than the specific physical
and chemical properties of the virgin materials, dominate in such stochastic clustering. The
reported experimental results possibly indicate universal mechanisms of stochastic clustering
of materials under the high-heat plasma load in a fusion device. The quantitative
characteristics of the statistical inhomogeneity of such surface structure, in particular, the
broadening of the multifractal spectrum by 0.5–1.2, are in the range observed for typical
multifractal objects in nature. The work was supported by the Grant RSF № 16-19-10531.
[1] V.P. Budaev, Physics Letters A 381, 43, (2017) 3706
[2] V.P. Budaev, JETP Letters 105, 5 (2017) 307
[3] V.P. Budaev, et. al., JETP Letters 95, 2 (2012) 78
