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One of the most deleterious impurities is oxygen, which originates from the inevitable oxidation process in the material inside the tokamak. Boron (B) is an effective oxygen getter by easily forming chemical compounds such as boron oxide. Boronization is a commonly used method of wall conditioning that has been employed in various devices[1-3].
In KSTAR, boronization was performed using carborane (C2B10H12)[4]. B layers are replenished during operations using an impurity powder dropper (IPD), which can inject precise, reproducible amounts of powdered materials into plasma discharges[5].
In KSTAR, wall conditioning effects, such as reduced impurity emission and radiated power, following B injection have been reported[6]. However, the duration of these beneficial effects is limited by the erosion of the boron layer during plasma operation. For this reason, two samples were collected: a dust sample from the lower tungsten cassette divertor (TCD), and a coupon from the outer mid-plane region, collected using a specially designed structure with a slit opening.
Post-mortem analysis using ellipsomtery revealed that thickness of the TCD dust and the coupon sample were 3 um and 400 nm, respectively. The XPS spectra showed that all samples exhibited boron (B1s), carbon (C1s), tungsten (W4f), and oxygen (O1s) peaks. However, their chemical compositions differed. For the B1s spectra, the TCD dust exhibited two peaks at 187.5 eV and 190.6 eV, corresponding to elemental boron and boron nitride (BN) [7], respectively. In contrast, the coupon sample showed only a single peak at 191.2 eV, which is attributed to a B–O bond. The O1s peak for both samples appeared at approximately 532 eV, which can be assigned to either C–O or B–O bonding[8]. In case of C1s spectra, both samples displayed a large peak for C-C (284 eV). The coupons sample had an additional peak at ~287 eV, corresponding to a C-O bond. The W4f spectrum also differed between the samples. The dust samples showed peaks at 34.8 and 36.8 eV corresponding to the species of W2O5 and WO3[7], whereas the main W4f peaks for coupon sample is at ~ 30 eV. The observed differences in chemical composition indicate that the boron layers experienced different plasma exposure conditions depending on their retrieval locations. In this work, we report a comprehensive characterization of the boron-containing coated layers, including their elemental compositions and chemical states.