M. Kobayashi, Y. Feng, S. Morita, M.B. Chowdhuri, S. Masuzaki, M. Goto, N. Ohyabu, T. Morisaki, H. Yamada, I. Yamada, K. Narihara, A. Komori, O. Motojima and the LHD experimental group


Study on impurity screening in stochastic magnetic boundary of the Large Helical Device

Date of publication:

Oct. 2008

Key words:

21 IAEA Fusion Energy Conference, EX/9-4


The impurity transport characteristics in the scrape-off layer associated with a stochastic magnetic boundary of LHD are analyzed. The remnant islands with very small internal field line pitch in the stochastic region play a key role in reducing the impurity influx. The thermal force driven impurity influx is significantly suppressed when the perpendicular energy flux exceeds the parallel one inside the islands due to the small pitch. Application of the 3D edge transport code, EMC3-EIRENE, confirmed the impurity retention (screening) effect in the edge region. It is also found that the edge surface layers are the most effective region to retain (screen) impurities because of the flow acceleration and plasma cooling via short flux tubes. The carbon emission obtained in experiments is in good agreement with the modelling results, showing the impurity retention (screening) potential of the stochastic magnetic boundary.

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