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

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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 ... continued below

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26 p.

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Petrie, T. W.; Maingi, R. & Porter, G. D. August 1, 1998.

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  • Petrie, T. W. General Atomics, San Diego, CA (United States)
  • Maingi, R. Oak Ridge National Lab., TN (United States)
  • Porter, G. D. Lawrence Livermore National Lab., CA (United States)

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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.

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26 p.

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INIS; OSTI as DE98006175

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  • 13. international conference on plasma surface interactions, San Diego, CA (United States), 18-22 May 1998

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  • Other: DE98006175
  • Report No.: GA--A22864
  • Report No.: CONF-980560--
  • Grant Number: AC03-89ER51114;W-7405-ENG-48;AC05-96OR22464;FG03-95ER54294
  • Office of Scientific & Technical Information Report Number: 638253
  • Archival Resource Key: ark:/67531/metadc698243

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  • August 1, 1998

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  • Aug. 14, 2015, 8:43 a.m.

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  • Aug. 1, 2016, 6:39 p.m.

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Petrie, T. W.; Maingi, R. & Porter, G. D. The role of neutrals in the H-L back transition of high density single-null and double-null gas-fueled discharges in DIII-D, article, August 1, 1998; San Diego, California. (digital.library.unt.edu/ark:/67531/metadc698243/: accessed September 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.