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Description
Re-baselining ITER to have a full tungsten wall eliminates the strong impurity gettering capabilities of beryllium. Consequently, the intrinsic oxygen level makes it challenging to start-up the plasma in limiter configuration and achieve high-performance operating conditions in divertor configuration. The selected procedure in ITER to getter oxygen is boronization [1] by depositing a thin amorphous BD film (a-B:D, thickness ~100 nm) via glow discharge on the wall [2]. This film rapidly erodes during nominal plasma operation in high-flux areas. The oxygen gettering lasts longer due to coverage of the first wall and recessed areas. Thus, the layer lifetime determines how often boronization is required. Codes simulating the plasma-wall interaction, like ERO 2.0, provide predictions for this lifetime. In this case, physical and chemical erosion primarily determine the lifetime, and the absolute yields are not yet known precisely. This necessitates conducting experiments to measure the absolute erosion rates and benchmark the ERO 2.0 code for predicting the lifetime of thin boron films.
This contribution presents an analysis of thin boron film erosion experiments conducted at the linear plasma device PSI-2. Magnetron sputtering created 100 nm thick boron and a-B:D films covering polished tungsten substrates, simulating a boronized wall in a fusion reactor. Deuterium discharges at PSI-2 provide the flux of low-energy deuterons onto the films for studying their near-threshold erosion behavior. The biasing of the sample enables an impact energy scan from 40 eV to 100 eV. Additionally, limiting the fluence of the deuterons to 3∙10^23 m^−2 avoided the complete removal of the thin films, allowing for post-mortem layer thickness measurements. Comparing the thickness before and after plasma exposure gives the net erosion rates of the boron layers [3].
Spatially resolved emission spectroscopy provides information on gross erosion by measuring the boron 2p-3s transition (249.8 nm). The spatial distribution of the emission, i.e., the decay length, is compared to the emission profile predicted by ERO 2.0 and used to benchmark the code. Finally, varying the surface temperature of a bulk boron sample enables an investigation into the temperature dependency of chemical erosion by measuring the BD A-X transition around 433 nm. The PSI-2 results, in the impact energy range covered, suggest that the chemical erosion is small compared to physical sputtering.
[1] J Winter et al J. Nucl. Mater. 162-164 713 (1989)
[2] R Pitts et al NME 42 101854 (2025)
[3] M Sackers et al NME 45 102003 (2025)