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The role of carbon and nitrogen on the H-mode confinement in ASDEX Upgrade with a metal wall
KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.ORCID iD: 0000-0002-9546-4494
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2016 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 56, no 5, 056014Article in journal (Refereed) PublishedText
Abstract [en]

Carbon (CD4) and nitrogen (N2) have been seeded in ASDEX Upgrade (AUG) with a tungsten wall and have both led to a 20-30% confinement improvement. The reference plasma is a standard target plasma with I p /B T = 1 MA/2.5 T, total input power P tot ∼ 12 MW and normalized pressure of β N ∼ 1.8. Carbon and nitrogen are almost perfectly exchangeable for the core, pedestal and divertor plasma in this experiment where impurity concentrations of C and N of 2% are achieved and Z eff only mildly increases from ∼1.3 to ∼1.7. As the radiation potentials of C and N are similar and peak well below 100 eV, both impurities act as divertor radiators and radiate well outside the pedestal region. The outer divertor is purposely kept in an attached state when C and N are seeded to avoid confinement degradation by detachment. As reported in earlier publications for nitrogen, carbon is also seen to reduce the high field side high density (the so-called HFSHD) in the scrape off layer above the inner divertor strike point by about 50%. This is accompanied by a confinement improvement for both low (δ ∼ 0.25) and high (δ ∼ 0.4) triangularity configurations for both seeding gases, due to an increase of pedestal temperature and stiff core temperature profiles. The electron density profiles show no apparent change due to the seeding. As an orthogonal effect, increasing the triangularity leads to an additionally increased pedestal density, independent of the impurity seeding. This experiment further closes the gap in understanding the confinement differences observed in carbon and metal wall devices; the absence of carbon can be substituted by nitrogen which leads to a similar confinement benefit. So far, no definite physics explanation for the confinement enhancement has been obtained, but the experimental observations in this paper provide input for further model development.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2016. Vol. 56, no 5, 056014
Keyword [en]
carbon, confinement, impurity seeding, metal wall, nitrogen, radiation, tungsten, Delta wing aircraft, Fusion reactor divertors, Heat radiation, Impurities, Plasma confinement, Plasma simulation, Carbon and nitrogen, Confinement benefits, Divertor radiators, Electron density profiles, Impurity concentration, Metal walls, Model development, Radiation potentials, Nitrogen plasma
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:kth:diva-187002DOI: 10.1088/0029-5515/56/5/056014ISI: 000375229300017ScopusID: 2-s2.0-84964621919OAI: oai:DiVA.org:kth-187002DiVA: diva2:928795
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QC 20160516

Available from: 2016-05-16 Created: 2016-05-16 Last updated: 2016-05-30Bibliographically approved

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Frassinetti, Lorenzo
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