Experimental results on boundary plasmas, resulting surface interactions and extrapolation to large fusion devices

Abstract

Experimental results on boundary plasmas, impurity origins, and impurity transport in the boundary plasmas are summarized in relation to impurity control and wall erosion. Important parameters in the boundary plasma are surveyed in present-day tokamaks and the relation between the boundary plasma and the main plasma is induced. It is shown that the important parameters of the boundary plasma in a large device can be calculated from the assumed main plasma parameters. Impurity release from the first wall is governed mainly by the boundary plasma. Ion sputtering is shown to be the dominant process in the stable phase of a discharge. In addition to these processes, arcing, sputtering by charge-exchange neutrals etc., which may become serious in a large device, are considered. All released impurities do not flow into the main plasma, even without a divertor. Impurity contamination in the main plasma is governed by the impurity transport and atomic processes in the boundary plasma as well as by the transport in the main plasma and by the impurity production. Therefore, the study of impurity transport in the boundary layer is important and is summarized. It is shown that the impurity transport in the boundary layer is well simulated by Monte-Carlo calculation. Combining these results, impurity problems in a large device are discussed.