Characterization of the in situ leading-edge-induced melting on the ITER-like tungsten divertor in EAST

Understanding tungsten melting behavior and its influence on plasma operation is one of the main concerns for ITER, which will be operated with a full tungsten divertor. Similar to ITER, actively cooled tungsten cassette modules have been successfully installed for the upper divertor in EAST for a stronger heat exhaust ability. However, an unexpected in situ tungsten melting phenomenon has been observed around the strike point area on both the inner and outer targets during recent EAST plasma campaigns. It was identified that such tungsten melting was caused by the leading edges, which leads to much high heat load on the protruded edges of the cassette modules. All tungsten melting occurred only at the edges of the cassette modules where larger misalignment up to millimeter scale was formed. The melted layer, which was mainly driven by the electromagnetic force, was moved either up or down under different conditions. Only a few bridge connections along the vertical direction could be formed. Such tungsten melting ejected a large number of droplets into the core plasma, and resulted in a sharp increase of tungsten impurity and power radiation and could eventually lead to disruptions. With droplet ejection and melted layer removal, the melted corner seems to form a moderate chamfer structure and thus may mitigate the temperature rise. Under current operation conditions, EAST can tolerate such melting to some extent. Several solutions to help avoid this kind of melting have been proposed according to the thermal analysis for future EAST operations.