One divertor or two?
The familiar image of the ITER Tokamak cutaway, with a large divertor siting at the bottom of the vacuum vessel, can be misleading. All tokamaks, including ITER, can operate in a magnetic configuration in which two divertors would be required, one in upper part of the vacuum vessel, one in the lower part — what specialists call "double null." EAST is a device which has decided to go for double null operation from the start. The machine has upper and lower divertor capability, even though operators can choose to run with one divertor only ("single null") any time they wish.
By installing the "test" tungsten divertor at the top of the vacuum vessel while maintaining the existing, partially carbon divertor at the bottom, EAST can commission and learn to run either solely in single null upper on the highly complex new divertor or in double null on both upper and lower components at the same time. Or, in case of problems, only at the bottom in single null lower. Although the new tungsten divertor is designed to operate at high power, running on the lower carbon targets is easier given the previous experience the EAST team has developed with its first divertor and the fact that carbon is a more forgiving material from the point of view of plasma impurity contamination.