(EX/P4-14) Hydrogen Retention and Carbon Deposition in Plasma Facing Wall and Shadowed Area of JT-60U

K. Masaki1), T. Tanabe2), Y. Hirohata3), Y. Oya4), T. Shibahara5), T. Hayashi1), K. Sugiyama2), T. Arai1), K. Okuno4), N. Miya1)
1) Japan Atomic Energy Agency, Naka, Ibaraki-ken, 311-0193 Japan
2) Kyushu University, Fukuoka, 812-8581 Japan
3) Hokkaido University, Sapporo, 060-8628 Japan
4) Shizuoka University, Shizuoka, 422-8529 Japan
5) Nagoya University, Nagoya, 464-8603 Japan

Abstract.  Evaluation of fuel inventory and its retention process are critical issues for a next-step fusion device, especially with carbon-based wall. In order to resolve the issues, the hydrogen retention and carbon deposition for the plasma facing surfaces and plasma shadowed areas of JT-60U have been performed. In JT-60U, erosion/deposition analyses for the plasma facing wall and carbon thirteen methane gas puffing experiment have shown that carbon impurity produced by erosion in the outer divertor can be transported to the inner divertor through the private region as well as the SOL plasma, and deposited in the inner divertor. For the plasma shadowed area, local carbon transport to the inboard direction was appreciable in addition to long-path transports. The amount of hydrogen isotopes in the inner divertor (deposition dominant) was larger than that of the outer dvertor (erosion dominant). The highest hydrogen isotope retention was observed in the redeposition layers at the bottom of the outer dome wing tile. This is probably because the bottom of the outer dome wing tile was facing to the outer divertor and its surface temperature was kept rather low, and neutral pressure at the pumping slot was high. This indicates that carbon eroded at the outer divertor was directly transported to the outer dome wing and deposited with large amount of hydrogen retention. This local carbon transport is also very important in carbon deposition and hydrogen retention. Nevertheless, the amount of the hydrogen isotope retention in such area (0.13 in a ratio of hydrogen isotopes over carbon) is still smaller than those observed in JET, because of high baking temperature (600K) and high surface temperature during NB heated discharges. The local carbon redeposition and the hydrogen retention seem strongly dependent on the divertor geometry and the position of the pumping slots as well as the surface temperature. In other words, a divertor structure with pumping slots in the private flux region and high base temperature could be effective in suppressing carbon deposition in the plasma shadowed area of the inner pumping slot and in reducing the hydrogen retention.

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