Surface Modification of ITER-like Mirrors after One Hundred Cleaning Cycles Using Radio-Frequency Plasma

Sanchez, Fabien and Marot, L. and Steiner, R. and Mathys, D. and Hiret, P. and Soni, K. and Antunes, R. and Kisiel, M. and Romero-Muñiz, C. and Moser, L. and Le Guern, F. and Piqueras Meseguer, J. J. and Meyer, E.. (2023) Surface Modification of ITER-like Mirrors after One Hundred Cleaning Cycles Using Radio-Frequency Plasma. Journal of Nuclear Materials, 581. p. 154382.

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In ITER, the metallic first mirrors (FMs) will undergo erosion due to their proximity to the fusion plasma and deposition of materials originated from the first walls (mainly beryllium). In-situ plasma cleaning is a promising technique to conserve the FMs optical properties by means of ion sputtering. In this work, the evolution of the optical properties of single-crystal (Sc) and nanocrystalline (Nc) molybdenum (Mo) and rhodium (Rh) mirrors were investigated up to 100 cycles of consecutive contamination and cleaning. Aluminum oxide (AlO) was used as contaminant to replace the toxic beryllium. The plasma cleaning was carried out using a capacitively coupled argon (Ar) plasma excited by a 60 MHz radio-frequency generator resulting in the formation of a self-bias applied on the mirrors of -280 V. The plasma potential being around 30 V, the Ar ion energy was about 310 eV. The optical properties of the mirrors were assessed using ex-situ reflectivity measurements. Moreover, the surface topography was characterized by means of scanning electron microscopy (SEM), focused ion beam (FIB) and roughness measurements using atomic force microscopy (AFM). ScMo and ScRh mirrors formerly exposed to 80 successful cleaning cycles using aluminum/tungsten (Al/W) deposits and air storage exhibit drastic changes in their optical properties after being subject to cleaning cycles using AlO as contaminant. Additionally, freshly polished ScRh were exposed to identical cleaning cycles. All Sc mirrors exhibited pits induced by the polishing procedure using diamond paste in addition of mounds/wavy patterns. The carbon incorporated during the polishing process was demonstrated to be responsible for the pitting of the surface. The Nc mirrors preserved their initial reflectivities after up to 100 cycles. The surface topography was systematically characterized and an average erosion rate for NcRh mirrors of about 59 nm per cycle has been estimated from FIB cross-sections. The optical properties of the Nc mirrors showed a superiority in the present study in comparison to the Sc materials due to the influence of their polishing
Faculties and Departments:05 Faculty of Science > Departement Physik > Physik > Nanomechanik (Meyer)
UniBasel Contributors:Marot, Laurent and Meyer, Ernst
Item Type:Article, refereed
Article Subtype:Research Article
Note:Publication type according to Uni Basel Research Database: Journal article
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Last Modified:13 Jun 2023 13:39
Deposited On:13 Jun 2023 13:39

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