In-vessel colorimetry of Wendelstein 7-X first wall components: variation of layer deposition distribution in OP1.2a and OP1.2b

A colorimetry study of the first wall panels in the Wendelstein 7-X (W7-X) was carried out using a compact color analyzer after Operation Phases OP1.2a and OP1.2b, in which graphite test divertor units were used, to estimate the wide-range distribution of the deposition layer. The color analyzer was used to measure the intensities of the red, green and blue (RGB) channels, which correspond to reflection rates, for all first wall panels after OP1.2a and OP1.2b. A significant difference in the RGB values was found between OP1.2a and OP1.2b. The color pattern on the panels was roughly the same for all the five toroidal modules of W7-X. The deposition layer thickness was estimated from the RGB values using a single-layer model. A thin average deposition layer (10 ± 6 nm) was estimated for OP1.2a. On the other hand, a thicker average deposition layer (25 ± 8 nm) was estimated for OP1.2b. (topical issue article)