Comparative analysis of impurity transport in the peripheral plasma in the Large Helical Device for carbon and tungsten divertor configurations with EMC3-EIRENE

Impurity transport in the peripheral plasma in the Large Helical Device for carbon and tungsten divertor configurations is analyzed using a three-dimensional edge plasma simulation code (EMC3-EIRENE). Long pulse plasma discharges in LHD have often been interrupted by the emission of iron dust from the surface of the vacuum vessel. The iron ions in the peripheral plasma originating from the iron dust can enhance the sputtering of impurities on the divertor plates. The influence on the peripheral plasma by the sputtered impurities is investigated in the iron dust emission case for both the carbon and the tungsten divertor configurations. The simulation reveals that the dependence of the radiation power by the sputtered impurities on the plasma density and the iron emission rate is quite different between the two divertor configurations. It is found that, for the tungsten divertor, a high plasma density operation is advantageous for controlling the radiation power and the impurity ion content. In the high plasma density operation, the radiation power by tungsten ions is suppressed to less than that by carbon ions for the carbon divertor even in the case of high iron dust emission rates due to the reduced sputtering rate by lowered divertor electron temperatures. (author)