Behavior of impurities in the scrape-off layer under long duration discharges in TRIAM-1M

In order to understand the behavior of impurities under long pulse operation in the TRIAM-1M and to extract issues for the plasma facing materials for steady state tokamaks, deposition of impurities on a collector probe set in the SOL and the damage of the limiters were examined. Mo limiters were the major sources of metallic impurities. Due to interaction with the edge plasma during a long pulse discharge, the surfaces of the limiters were slightly melted and evaporated Mo intruded continuously into the main plasma. In the SOL there was Fe, Cr and Ni sputtered from the wall, that could hardly penetrate into the main plasma due to the trapping effect of the poloidal limiters. Large cavities and cracks were formed at the interface between the unmelted matrix and the melted layer of the limiters. Such defects parallel to the surface reduce the effective thermal conductivity and thus enhance the damage and evaporation of impurities.