Temporal evolution of erosion/deposition on tungsten surfaces exposed to SOL plasmas and its impact on plasma-surface interactions

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. (author)