Variation in particle control with changes in divertor geometry

Full text: Recent experiments on DIII-D show the importance of magnetic balance [i.e., the degree to which the divertor topology is biased toward single-null (SN) or double-null (DN)] in determining how efficiently deuterium particles can be removed with divertor pumps and, ultimately, in determining how well core density can be controlled. In-vessel pumping at the inner and outer divertor targets was done from a single divertor at two poloidal locations. We find that the pumping rate at the outer divertor target is much less sensitive to changes in the magnetic balance than that at the inner target. This sensitivity results from interplay between the scrape-off layer (SOL) geometry and the poloidal distribution of particles entering the SOL. These pumping rates also depend strongly on edge density. UEDGE modeling suggests that ∇B- and E x B-induced particle flows in the SOL are qualitatively consistent with observation. Even for a passive secondary divertor, as possible for ITER in an advanced tokamak scenario, density control may be only weakly dependent on magnetic balance.(author)