The role of neutrals in the H-L back transition of high density single-null and double-null gas-fueled discharges in DIII-D
- General Atomics, San Diego, CA (United States)
- Oak Ridge National Lab., TN (United States)
- Lawrence Livermore National Lab., CA (United States); and others
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 {approximately}15--30% of P{sub in}. 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 n}{sub e,G}, in contrast to SN plasmas. Second, no divertor (or core) MARFEs were detected in the DNs as {bar n}{sub e} approaches {bar n}{sub e,G}, in contrast to SNs, where divertor MARFEs can form at {bar n}{sub e}/{bar n}{sub e,G} as low as {approximately}0.6. High X-point DNs achieved density limits well above those of comparably-prepared SNs, e.g., {bar n}{sub e}/n{sub e,G} {approx} 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 n}{sub e}, since neutrals impact both pressure balance and MARFEing behavior.
- Research Organization:
- General Atomics, San Diego, CA (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Univ. of California, San Diego, CA (United States)
- Sponsoring Organization:
- USDOE Office of Energy Research, Washington, DC (United States)
- DOE Contract Number:
- AC03-89ER51114; W-7405-ENG-48; AC05-96OR22464; FG03-95ER54294
- OSTI ID:
- 638253
- Report Number(s):
- GA-A22864; CONF-980560-; ON: DE98006175; TRN: 98:007923
- Resource Relation:
- Conference: 13. international conference on plasma surface interactions, San Diego, CA (United States), 18-22 May 1998; Other Information: PBD: Aug 1998
- Country of Publication:
- United States
- Language:
- English
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