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Data Needs from the Present JAERI Tokamak Experiments

Impurity control experiments have been summarized in DIVA and JFT-2 at JAERI. Plasma purity improved by a divertor, by a titanium (medium-Z) wall or by a carbon (low-Z) wall has remarkably favorable effects on confinement, stability and high-density operation. Highly-ionized impurity ions accelerated by a sheath potential in front of the bombarded wall play a predominate role in generation of the wall impurity. Detailed study of a scrape-off layer proposes the scaling law on the scrape-off plasma. According to the proposed scaling law, the impurity control is essential to realize successfully fusion-state tokamaks in future. Ion (including highly-charged impurity ion) sputtering, chemical sputtering (potential disadvantage of low-Z carbon materials) and impurity behavior near the wall are of great importance for the future impurity control.