Experiences and lessons from the mitigation of leading edge induced melting on actively cooled ITER-like W/Cu monoblocks for divertor target in EAST

EAST is the first tokamak to feature fully actively water-cooled ITER-like W/Cu monoblocks on the divertor target. In 2014 and 2021, the graphite tiles in the upper and lower divertor were upgraded to W/Cu monoblocks, respectively. With the increase in plasma parameters, severe melting phenomena were inspected at the leading edges of upper divertor from 2017 to 2020. Through theoretical analysis and numerical simulation, two main factors, radial misalignment and chamfer structure have been identified to mainly impact the leading-edge induced melting of MBs. It was found that assembly misalignments increased gradually during the plasma discharges from 2015 to 2017 and the maximum misalignment can reach up to 3 mm, which play an important role that is responsible for the melting during 2018 to 2020. In 2020, engineering assembly improvements were implemented, resulting in a significant improvement compared to the previous situation, with all misalignments brought below 1 mm, and thus effectively prevent melting at the leading edge on upper divertor. In addition, the modification of chamfer structure to 1.5 × 17 mm at inter-CMs of the lower divertor further mitigated the issue of leading-edge-induced melting compared to upper divertor. Such experiences and lessons from the mitigation of leading edge induced melting provide important references for future fusion devices.