Surface Modification of Tokamak Fixed Limiter Used in High-Field Tokamak TRIAM-1

The surface morphology and composition changes of a Mo fixed limiter used in the High-Field Tokamak TRIAM-1 after about 70,000 hydrogen plasma discharges have been investigated by means of optical microscope, SEM-EDS and AES. Significant difference in surface damage is observed between the electron drift side and the ion drift side. The damage on the electron drift side is characterized by melting, cracking, cone-like protuberance, droplet, eutectic and impurity accumulation. This is considered to be caused by the overlapped effects of the heat load from the plasma, which decreases with increasing distance from the plasma center, and the impurity accumulation which originated from the stainless steel vacuum vessel and the limiter itself. The kinds of damage on the ion drift side are melting due to arcing, sputtering, blistering and color change of the surface due to impurity accumulation. The distribution of these damages changes with the distance from the limiter edge. The damage observed on the ion drift side is considered to be caused by the synergistic effects of sputtering due to ion bombardment, impurity accumulation and heat load due to arcing.