ИСТИНА

Molybdenum mirrors are planned to be used as the first mirror in the majority of optical diagnostics in ITER [1]. The tests of molybdenum, as well as other mirror materials, were made in a number of controlled nuclear fusion devices [2-4]. Nevertheless, deposition from the main wall (mainly beryllium), erosion and particle implantation can lead to losses of the mirror reflectivity. This work presents experimental results on sputtering of aluminum and molybdenum foils as well as plasma cleaning of aluminum (used as beryllium proxy) deposited on molybdenum mirrors. The experiments were made for poly- and monocrystalline (Mo(111) and Mo(110)) molybdenum mirrors. The Mo samples were treated in D2/N2 plasma with a D/N ratio varied from 0 to 100% in a dc self-bias ranging from floating potential up to -100 V. Surface and optical properties of the foils and mirrors were characterized before and after exposure by Reflectance Spectroscopy, Scanning Electron Microscopy, Electron Probe Microanalysis, X-ray Photoelectron Spectroscopy, and Atomic Force Microscopy. The sputtering yields of the Al and Mo foils increased by 5 and 8 times, respectively, with increasing of nitrogen concentration from 0 (pure deuterium) up to 20%. The observed trend of increasing sputtering yield with nitrogen concentration can be interpreted as the influence of ND3 formed in the D2/N2 plasma. On the fresh surface of the Mo foil blistering occurred at a fluence of 1023 ion/m2 and the number of blisters increased with exposure time. After polishing the foil by diamond powder the blisters did not appear up to a fluence of 1024 ion/m2 . Similarly, there were neither blisters nor significant changes in sputtering yields on diamond polished poly- and monocrystalline Mo mirrors for all Mo samples. The plasma cleaning of the Al deposit from the surface of the Mo(111) and Mo(110) mirrors in a mixture of D2/2% N2 demonstrated reflectivity recovery. The further exposure of the samples in the plasma led to improvement of the reflectivity likely due to removing carbon embedded in the surface layer at previously polishing by diamond powder. [1] V. Voitsenya, A.E. Costley,V. Bandourko, A. Bardamid,V. Bondarenko, Y. Hirooka, et al., Rev. Sci. Instrum. 72 (1) (2001) 475. [2] M. Lipa, B. Schunke, Ch. Gil, J. Bucalossi, V.S. Voitsenya, V. Konovalov, et al., Fusion Eng. Des. 81 (2006) 221. [3] K.Yu. Vukolov, M.I. Guseva, S.A. Evstigneev, A.A. Medvedev, S.N. Zvonkov, Plasma Devices Oper. 12 (3) (2004) 193. [4] G. De Temmerman, M.J. Rubel, J.P. Coad, R.A. Pitts, J.R. Drake, Proceedings of the 32nd EPS Conference on Plasma Physics, 2005 ECA, vol. 29C, Tarragona, Spain, 2005, P-1.076.