Impurity production at graphite limiters in the DITE tokamak

Through a series of controlled experiments on the DITE tokamak the main impurity production mechanisms at graphite limiters are investigated. Three principal processes can release carbon impurities: deuterium chemical sputtering, physical sputtering and oxygen chemical erosion. By varying, in turn, the limiter temperature, the bombarding particle energies and the local oxygen flux the rate of each erosion process is changed independently. From these experiments it is found that oxygen erosion dominates. Hence, carbon impurities leave the graphite surface primarily in the form of CO gas. The observed radial transport of intrinsic carbon and oxygen originating from the limiter is confirmed to be similar to that resulting from the localized puffing of CO. Metal impurities produced at the graphite limiter are physically sputtered, predominantly by carbon and oxygen impurity ions. The observed transport of chromium from the limiter has been modelled with a physical sputtering transport code.