The role of neutrals in the H-L back transition of high density single-null and double-null gas-fueled discharges in DIII-D

The role of neutrals in triggering the H-L back transition in high density ELMing H-mode plasmas is explored in double-null (DN) and single-null (SN) divertors. The authors propose that the neutral particle buildup below the X-point may play an important role in triggering the H-L transition at high density. Neutral pressure in the private flux region is, in fact, significant near the H-L backtransition. High density formation inside the separatrix near the X-point may also be a factor in triggering the H-L backtransition. They have observed that the ELMing H-mode density limit in SN divertors normally occurred at or near the H-L back transition. The radiated power coming from inside the separatrix at the H-L transition did not appear sufficient by itself to produce this back transition, since it is only ∼15--30% of Pin. Poloidally-localized neutrals may explain two important differences in SN and double-null (DN) plasmas near their respective H-L backtransitions. First, electron pressure along the separatrix between the X-point and the outboard strike point decreased only modestly for DN divertors, even at densities comparable to the Greenwald density limit bar ne,G, in contrast to SN plasmas. Second, no divertor (or core) MARFEs were detected in the DNs as bar ne approaches bar ne,G, in contrast to SNs, where divertor MARFEs can form at bar ne/bar ne,G as low as ∼0.6. High X-point DNs achieved density limits well above those of comparably-prepared SNs, e.g., bar ne/ne,G ∼ 0.9--1.0 for DNs versus 0.75--0.80 for SNs. These differences result from a lower neutral pressure in the private flux regions of DNs than in comparable SNs at the same bar ne, since neutrals impact both pressure balance and MARFEing behavior