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(EX/P2-17) Behavior of Plasma Facing Surface in the Large Helical Device

T. Hino¹⁾, Y. Hirohata¹⁾, N. Inoue²⁾, A. Komori²⁾, LHD Experimetal Group²⁾, S. Masuzaki²⁾, O. Motojima²⁾, Y. Nobuta¹⁾, N. Noda²⁾, N. Ohyabu²⁾, A. Sagara²⁾, H. Suzuki²⁾, Y. Yamauchi^1)
 
1) Department of Nuclear Engineering, Hokkaido University, Sapporo, Japan
²⁾ National Institute for Fusion Science, Toki, Japan

Abstract.  From the initial experiment of LHD, material probes have been installed along the poloidal direction. After each experimental campaign, the degree of wall conditioning and the plasma wall interactions have been systematically investigated using surface analysis. It is shown that both glow discharge cleanings and main discharge shots have largely contributed to the wall cleaning.The plasma stored energy has increased as the progress of the wall cleaning. Since helium gas has been employed for both main discharges and discharge cleaning, helium retention has been observed to be dominant. From 3rd experimental campaign, graphite tiles have been newly placed in the divertor leg region, and reduction of Fe impurity level has been observed. This reduction was clarified by the surface analysis, i.e. carbon deposition on entire region of first walls. In addition, it is seen that retention of impurity gas has increased by installation graphite tiles. The plasma wall interactions due to main discharge shots and glow discharges have been also investigated.

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