The impacts of lithium and silicon coating on the W source in EAST

Application of lithium (Li) or silicon (Si) wall coating in the Experimental Advanced Superconducting Tokamak (EAST) has proven to be an effective method to reduce fuel recycling and control impurity level, and also to improve the plasma performance. In 2014, the upper graphite divertor in EAST was upgraded into a full tungsten (W) one with ITER-like actively water-cooled monoblock structure. Note that there is still large surface area of first wall covered by graphite tiles, including the lower divertor, NBI shine through armor, the outboard guard limiters, etc. In 2016 spring campaign, both Li and Si were used to coat the first wall in experimental sequences that lasted more than one month each. The spectroscopic observation reveals that compared to Si coating, Li coating more effectively suppresses in-vessel impurities, thus mitigating the W source in upper divertor. This is further quantified by a reduction of the effective W sputtering yield calculation. Carbon (C) impurity is suggested as the main impurity governing W sputtering, and correlates inversely with the wall coating evolution during both one-day experiments and the whole campaign. The impurity concentration increases measurably after every vacuum vessel exposure to air during the campaign; substantial time is required for impurities to return to baseline levels. Real-time Li aerosol injection into the upper divertor effectively reduces the W sputtering by cooling the edge plasma and dissipating the power flux to divertor target, consequently providing an active tool for radiation divertor control.