Fig. 2-9 Divertor magnetic configuration and C redeposition on the divertor plate

Fig. 2-10 Electron micrograph of C deposition (~20 micrometer-thick deposition on the substrate) on the divertor plate of JT-60 These nearly 20 layers (6 micrometer from the surface) correspond to ca. 20 discharges. The inclined columnar structures correspond to the lower power divertor discharges.

Fig. 2-11 Schematic for columnar structure formation Inclined columns are formed on the lower temperature substrate under irradiation of inclined incident adsorbate atoms.

In present-day tokamaks, graphite armor tiles are used for plasma-facing components to protect the vacuum vessel against plasma heat flux from the core. For the divertor configuration adopted, carbon (C) deposition is dominant on the inboard divertor plate (graphite), while erosion is dominant on the outboard divertor plate (Fig. 2-9). To clarify the deposition mechanism of these C layers, a transmission electron microscopic study was conducted on the C deposition layer on the JT-60 divertor plate. A photograph shows a cross-sectional view of the top surface layer of C deposition on the inboard side of the divertor of JT-60 (Fig. 2-10). The analysis specimens were fabricated using a focused ion beam technique. Inclined columnar structures were more obvious for the relatively low power discharges of this divertor configuration. Lamellar structures were formed for the higher power discharges. Columnar structure formation was attributed to the lower temperature of the divertor plate surface for those discharges and to the impinging of C ions to the divertor plate surfaces at an incline. Low atom migration and a self-shadow effect on the substrate lead to formation of columnar structures (Fig. 2-11). Rapid diffusion of hydrogen (H) takes place through the deposition layers of a columnar structure due to the existence of open pores between columns. H adsorption and desorption properties of those columnar structures, which are relevant to the tritium inventory issue in the thermo-nuclear fusion experimental reactor will be adressed in future studies.

Reference Y. Gotoh et al., Transmission Electron Microscopy of Redeposition Layers on Graphite Tiles Used for Open Divertor of JT-60, J. Nucl. Mater., 329-333, 840 (2004).

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