Optimization of single crystal mirrors for ITER diagnostics

Abstract

Diagnostic mirrors are planned to be used in all optical diagnostics in ITER. Degradation of mirrors due to e.g. deposition of plasma impurities will hamper the entire performance of affected diagnostics. In situ mirror cleaning by plasma sputtering is presently envisaged for the recovery of contaminated mirrors.There are observations showing a signature of sputtering dependence on crystal orientation. Should such a dependence exist, the sputtering of single crystal mirrors could be minimized, thus prolonging a mirror lifetime.Four single crystal molybdenum mirrors with different orientations were produced to study the effect of crystal orientation on sputtering. Mirrors were exposed to argon plasma under identical plasma conditions relevant to those expected in the mirror cleaning systems of ITER. The energy of impinging ions was about 60 eV. The amount of sputtered material corresponded to about a hundred mirror cleaning cycles in argon.Plasma exposures did not affect the mirror reflectivity. The maximum decrease of specular reflectivity did not exceed 5% at 250 nm. The mirrors with orientations [110]/[101] demonstrated up to 42% less sputtering than the mirrors with other crystal orientations. These findings outline the advantage of a favorable crystal orientation for a cleaning of heavy contaminants from ITER mirrors.

Cleaning of two mirrors in the first mirror unit using radiofrequency capacitively coupled plasma

“…FMs, which face the plasma directly, are highly susceptible to erosion caused by charge exchange atoms as well as deposition of sputtered plasma-facing materials and wall-conditioning materials during machine operation [1,2]. Under net erosion conditions, microrelief morphology and step structures develop on the surface of FMs made of polycrystalline materials, leading to an increase in surface roughness and degradation of reflectivity [3,4]. Since the sputtering coefficients for crystals with different orientations in polycrystalline material are different, the use of single-crystal (SC) or nanocrystalline (Nc) material can effectively mitigate the formation of microrelief and stepped structures on FMs under erosion conditions [5].…”

Investigation of surface orientation dependent sputtering of Ag

“…Sputtering can be utilized beneficially in several applications, for instance in the coating industry and in surface cleaning. On the other hand, sputtering is also an unwanted effect, especially in fusion power plants, where the wall materials and the divertor will be eroded by the bombardment of energetic particles [3][4][5][6]. Therefore, both for the pros and the cons, it is important to understand the sputtering effects of materials in detail.…”