The paper presents results from the impurity control limiter (ICL) experiment, which was installed in the DITE tokamak. The ICL was designed so that the majority of ions leaving the plasma strike the undersides of its graphite tiles, which are angled towards the wall. These experiments confirm that the impurities sputtered from the ICL are ionized predominantly in the scrape-off layer and that there is significant deposition of neutral carbon on the walls. Both the spatial distribution of ionization and the toroidal distribution of carbon on the walls are in good agreement with code predictions. In helium discharges, the insertion of the ICL resulted in a 50% reduction of the total radiation and a 37–66% reduction in the impurity contribution to Zeff compared with the values obtained with rail limiters of conventional geometry. In deuterium discharges, the reduction in the total radiation was only 20%; this is attributed to the greater importance of the wall sources of carbon and oxygen in deuterium plasma relative to those in helium.
Impurity production at graphite limiters in the DITE tokamak