Temporal Evolution of Erosion/Deposition on Tungsten Surfaces Exposed to SOL Plasmas and its Impact on Plasma-Surface Interactions

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Temporal evolution of erosion/deposition on W surfaces exposed to SOL plasma composed of D+ and C4+ impurities, and its impact on plasma-surface interactions such as ion reflection and physical sputtering have been calculated. In particular, changes in the thickness of the W targets due to (i) C impurity concentration in the SOL plasma, and due to the additional contribution of (ii) chemical sputtering (CD4 emission) and (iii) thermal diffusion of C impurity deposited on the target are focused on. Changes in the depth profile of the deposited C are also considered. The calculated results have been compared qualitatively with experimental data which were obtained in TEXTOR-94. (i) Erosion of Wnear exposed close to the LCFS and deposition of C impurity on Wfar exposed far from the LCFS are reproduced by assuming different C impurity concentrations at the two targets. The deposition is due to the high C impurity concentration and the low plasma ion temperature, whereas the erosion is due to the converse reasons. However, the depth profiles are in disagreement with the measured growth of the W–C contamination layers. (ii) By the weak contribution of the chemical sputtering, the erosion of Wnear and the deposition on Wfar are also reproduced. Since the amount of the deposited C is decreased by the chemical sputtering, the depth profiles are also in disagreement with the experimental data. (iii) By the significant contribution of the thermal diffusion, the erosion/deposition at the two targets is also reproduced. In addition, the depth profiles for Wnear and Wfar, which show there is a deep penetration of the deposited C due to the thermal diffusion, are in good agreement with the experimental ones.

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