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Wall conditioning improves plasma performance in fusion devices by reducing the amount of impurities, especially carbon- and oxygen-based impurities, in the plasma [1,2]. Standard wall-conditioning procedures include baking and glow-discharge cleaning (GDC) of the inner walls using hydrogen or helium plasmas for removing impurities contained at the plasma facing components (PFCs). The amount of impurities can be further mitigated by depositing thin boron layers on PFCs surfaces (boronization). These boron layers act as oxygen getters [3, 4].
Wendelstein 7-X (W7-X) [5,6] is an advanced stellarator fusion reactor with major parts of its first wall comprised of carbon components. Since 2022 (operational period OP 2.1) the device has operated with a water-cooled carbon-fibre-composite (CFC) divertor. Boronizations have been incorporated into the wall conditioning procedures since 2018 (operational period OP 1.2b).
In the present work, a series of samples exposed to boronizations or plasmas in W7-X were analyzed. All samples were exposed using the multi-purpose manipulator (MPM) equipped with the MAT1 probe head during operational phases OP 2.2 and 2.3. The characteristics of the samples (material composition, surface roughness) varied. The boronizations were implemented with a B2H6 (90%) – He (10%) gas mixture. Samples were exposed to hydrogen plasmas for multiple discharges of varying duration, with electron densities ranging from 2 to 16 * 1019 m-2. In addition, a number of samples were exposed to He-GDC plasmas. The sample analysis was carried out at the Tandem Accelerator facility of the Max-Planck Institute for Plasma Physics in Garching, Germany using Nuclear Reaction Analysis (NRA) with a 3 MeV 3He beam.
The measured amounts of boron in the deposited layers varied significantly between boronizations, up to a factor of 4. No correlations were observed between the deposited amount of boron and the substrate composition or its surface roughness. Carbon and oxygen concentration profiles did not scale proportionally with boron. For samples exposed to hydrogen plasma, the net erosion rate of boron was substantially higher than that of carbon or oxygen. In some cases, a net oxygen uptake rather than erosion was recorded.
[1] J. Winter, Plasma Phys. Control. Fusion 38 (1996) 1503
[2] A Goriaev et al., Phys. Scr. T171 (2020) 014063
[3] M. Mayer et al., Nuclear Materials and Energy 41 (2024) 101778
[4] S. Sereda et al., Nucl. Fusion 60 (2020) 086007
[5] Beidler et al., Fusion Technology, 17(1) (1990) 148–168.
[6] T.S. Pedersen et al., Phys. Plasmas 24 (5) (2017) 055503.