Speaker
Description
Erosion, re- and co-deposition processes in Wendelstein 7-X (W7-X) lead to the formation of deposits on plasma-facing components (PFCs). Owing to the carbon-based wall-tiles, these deposits are carbon-dominated including hydrocarbons, oxides,and boron co-deposits. Characterizing these deposits is essential for understanding material migration and fuel retention.
A handheld spectroscopic ellipsometer has been developed for the first combined in-vessel thickness and refractive index measurements of the deposits in W7-X. Previous material migration investigations at W7-X relied on spectroscopic observations, on ex-situ analyses [1], including marker-layer experiments [2]. These methods provide valuable detail but have drawbacks, e.g., they require the removal of PFCs. In-vessel colorimetry [3] enables wider-range studies; however, it is limited by the use of three measurement wavelengths (red, green, blue), the lack of relative phase information from both polarization components, and the assumption of uniform optical properties of the deposits across the vessel.
The newly developed diagnostic is a compact white-light ellipsometer (frame 8.7 × 44 cm) with a stepwise rotational analyzer, operating at an incidence angle of 70°. Light from a tungsten-halogen source is coupled via optical fibers, and reflected spectra (350 - 950 nm) are recorded for analysis. Deposited layer properties are derived by fitting the measured spectra; Bayesian data evaluation provides full error propagation, increasing robustness to geometric and instrumental uncertainties [4].
Laboratory tests on SiO₂ standards (nominal thickness 492 nm) demonstrated an overall measurement accuracy of around 1 nm [5]. The system was validated on amorphous C:H layers (7 nm and 30 nm) and ex-situ W7-X wall components. Subsequently, the in-vessel measurements on W7-X PFCs are conducted on stainless steel wall panels, poloidal closure and pumping gap panels, covering an area of ~71 m². The approach enables wide-range, resource-efficient mapping of deposits around the torus. Future developments include optimization for rough layers and substrates, and multi-layer modeling.
References:
[1] C. P. Dhard et al., Phys. Scr. 96 (2021) 124059.
[2] M. Mayer et al., Nucl. Fusion 62 (2022) 126049.
[3] G. Motojima et al., Nucl. Mater. Energy 43 (2025) 101934.
[4] U. v. Toussaint et al., AIP Conf. Proc. 872 (2006) 272-9.
[5] M. Krychowiak, Rev. Sci. Instrum. 95 (2024) 113511.