Wide-range evaluation of the deposition layer thickness distribution on the first wall by reflection coefficient measurements

A simple method to evaluate the thickness distribution of the deposition layer formed on the first wall is proposed using an innovative measurement concept of the optical reflection coefficient, which is measured as the RGB (red, green, blue) value using a compact color analyzer. Analysis of the samples exposed to plasmas during an experimental campaign shows the relationship between the thickness of the deposition layer and the reflection coefficient, which is followed by the single layer model. The reflection coefficient clearly indicates the thickness of the deposition layer between 10 and 100nm. The reflection coefficients of stainless steel plates on the helically twisted coil in one of the 10 toroidal sections of the vacuum vessel in the Large Helical Device (LHD) are measured. There is almost no deposition layer on the inner side of the torus, however, the deposition layer reaches a thickness of over 100nm on the first wall near the divertor region. On the outer side of the torus, almost the entire area is covered by the deposition layer. Reflection coefficient measurements indicate that approximately 60% of the area on the measured coil can is coated with a deposition layer over 10nm thick, which suggests that this area plays a role in the wall retention. Keywords: Reflection coefficient, Color analyzer, Deposition layer, Wall retention, Helium