Studies of high-δ (baffled) and low-δ (open) pumped divertor operation on DIII-D

We report new experimental results with the Radiative Divertor Project-outer baffle (RDP-OB) and cryopump in both upper single-null (USN) and double-null(DN) ELMing H-mode discharges. The baffled divertor reduced the core ionization (similar to 2-2.5x), in reasonable agreement with predictions from UEDGE/DEGAS modeling (similar to 3.75x). The upper cryopump achieved density control of n(e)/n(gw) similar to 0.22 (line density/Greenwald density) with Z(eff) similar to 2 in high-delta plasmas. The measured exhaust is comparable to the loser pump, except at lower core electron densities (n(e) < 5 x 10(19) m(-3)). Efficient impurity exhaust was obtained with deuterium SOL how. Preliminary experiments with DN operation has shown that the particle exhaust to the upper pump depends on the up/down magnetic balance. Preliminary experiments indicate that the DN exhaust is roughly 40-50% of the USN exhaust at n(e) similar to 4 x 10(19) m(-3). (C) 1999 Elsevier Science B.V. All rights reserved.