Impact of Divertor Magnetic Balance and Particle Drifts on Radiating Divertor Behavior in DIII-D

Recent DIII-D experiments show that both magnetic balance between upper and lower divertors and the particle drifts in the SOL and divertors are important to understanding how argon impurities accumulate in double-null plasmas in a ``puff and pump" radiating divertor scenario. The best results to-date in terms of coupling a radiating divertor approach to an H-mode plasma with minimal degradation in $\tau_E$ have been produced in a double-null shape biased opposite to the ion ${\bf B}\times\nabla B$ drift direction. Edge transport modeling with UEDGE [1] shows that the maintenance of low core impurity levels in this configuration can be largely ascribed to the central role of particle drifts. Additionally, it is found that ion ${\bf B}\times\nabla B$ drift direction plays a much more important role than divertor closure in controlling plasma density.\par \vskip6pt \noindent [1]~T.D.\ Rognlien, et al., Phys.\ Plasmas {\bf 34}, 362 (1994).