5-9 September 2016
Prague Congress Centre
Europe/Prague timezone

P2.191 Investigation of repetitive cleaning of first mirror using radio frequency plasma

6 Sep 2016, 14:20
1h 40m
Foyer 2A (2nd floor), 3A (3rd floor) (Prague Congress Centre)

Foyer 2A (2nd floor), 3A (3rd floor)

Prague Congress Centre

5. května 65, Prague, Czech Republic
Board: 191
Poster I. Materials Technology P2 Poster session

Speaker

Jiao Peng (Institute of Plasma Physics)

Description

First mirror (FM) lifetime is one of critical issues for the optical diagnostic system in ITER since it greatly influences the performance of relative diagnostic. In ITER, repetitive cleaning is expected to give a positive solution to the frequent replacement of FM, thus prolonging its lifetime. Three cleaning cycles using radio frequency argon plasma were applied to the stainless steel mirror with the dimensions of 35mm×30mm×2mm to understand the evolution of mirror optical and morphology performance during each cycle. The aluminum film with the thickness of about 100nm was deposited on the mirror surface under the identical conditions in each cycle. When three cycles were implemented with same cleaning parameters, it was found that with the complete removal of film, the total reflectivity was restored up to 98% of its initial value. Nevertheless, the roughness grew as more second phases and the deformation of grain boundaries arose, leading to increase of the diffuse reflectivity of 16% and 27% in the second and third cycles, respectively. It was explained by excessive sputtering due to the formation of thinner film and lower adhesion strength between substrate and film which were measured by Ellipsometer and Nanoindenter, respectively, on the rougher surface. After lowering the selfbias during the last two cycles and with shorter cleaning time in the third cycle, the diffuse reflectivity was improved to about 10% and 15% on the basis of the good recovery of total reflectivity, respectively. In conclusion, at given conditions every repetitive cleaning cycle will increase the surface roughness, accordingly degrading the specular reflectivity, and inevitably have a negative effect on FM lifetime. It is highly essential to improve the mirror specular reflectivity by optimizing the cleaning parameters in each cycle in order to maintain its specular reflectivity as high as possible.

Co-author

Jiao Peng (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui, China)

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