29th Symposium on Fusion Technology (SOFT 2016)

O4A.4 First direct comparative test of single crystal rhodium and molybdenum mirrors for ITER diagnostics

8 Sep 2016, 12:00

20m

Forum Hall 2nd floor (Prague Congress Centre)

Board: 4

Oral D. Diagnostics, Data Acquisition and Remote Participation O4A

Speaker

Andrey Litnovsky (Forschungszentrum Jülich)

Description

All optical and laser diagnostics in ITER will use mirrors to observe the plasma radiation. In the severe ITER environment mirrors may become contaminated with plasma impurities hampering the performance of corresponding diagnostics. To counteract the mirror contamination, an in-situ mirror cleaning is proposed, which relies on ion sputtering the contaminants together with affected mirror material. However, such a cleaning introduces high demands on reflectivity and on sputter resistance of mirror materials. Previous research demonstrated the decisive advantages of single crystal (SC) molybdenum(Mo) under erosion conditions over polycrystalline concepts. Until now the production of rhodium (Rh) mirrors with an excellent reflectivity was limited to polycrystalline thin coatings due to high cost of rhodium and design challenges. Recently, the first single crystal rhodium mirrors became available and tests have been started at the Forschungszentrum Jülich. In a direct comparative test two SC Rh mirrors and two SC Mo mirrors were exposed under identical conditions in steady-state helium plasmas in the linear plasma device PSI 2. The energy of impinging He-ions was ~100 eV well matching conditions expected in the in-situ cleaning system in ITER. The temperature of the mirrors was ~300^ooC, the total fluence was 1.9×10²¹21 ion/cm²2. During exposure molybdenum mirrors lost 450-600 nm of their material due to sputtering. Rhodium mirrors lost more than 1 µm. Exposure impact on mirrors corresponded to 50-100 cleaning cycles, thus addressing the entire mirror lifetime in ITER. Nevertheless, rhodium mirrors have preserved their specular reflectivity, showing the maximum degradation of less than 7% at 250 nm. The diffuse reflectivity was preserved. Molybdenum mirrors demonstrated moderate decrease of specular reflectivity of 12%-25%. Since the moderate degradation corresponds to the entire service life of the mirror, the obtained results open new perspectives for the use of single crystals in ITER diagnostics.