Impact of wall materials and seeding gases on the pedestal and on core plasma performance
Plasmas in machines with all metal plasma facing components have a lower Zeff, less radiation cooling in the scrape-off layer and divertor regions and are prone to impurity accumulation in the core. Higher gas puff and the seeding of low-Z impurities are applied to prevent impurity accumulation, to...
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doaj-0f384f20f8ac43108d0b9aa1e51c7c5a2020-11-25T00:31:53ZengElsevierNuclear Materials and Energy2352-17912017-08-01121827Impact of wall materials and seeding gases on the pedestal and on core plasma performanceE. Wolfrum0M. Beurskens1M.G. Dunne2L. Frassinetti3X. Gao4C. Giroud5J. Hughes6T. Lunt7R. Maingi8T. Osborne9M. Reinke10H. Urano11Max-Planck-Institute for Plasma Physics, Boltzmannstr. 2, D - 85748 Garching, Germany; Corresponding author.Max-Planck-Institute for Plasma Physics, Wendelsteinstraße 1, D-17491 Greifswald, GermanyMax-Planck-Institute for Plasma Physics, Boltzmannstr. 2, D - 85748 Garching, GermanyDivision of Fusion Plasma Physics, KTH Royal Institute of Technology, SE-10044 Stockholm, SwedenInstitute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People's Republic of ChinaEURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon, Oxon OX14 3DB, UKMIT Plasma Science and Fusion Center, 175 Albany St., Cambridge, MA 02139, United StatesMax-Planck-Institute for Plasma Physics, Boltzmannstr. 2, D - 85748 Garching, GermanyPrinceton Plasma Physics Laboratory, PO Box 451, Princeton, NJ 08543, USAGeneral Atomics, PO Box 85608, San Diego, CA 92186-5608, USAOak Ridge National Laboratory, Oak Ridge, TN 37831, USAJapan Atomic Energy Agency, Naka Fusion Institute, Naka, Ibaraki 311-0193, JapanPlasmas in machines with all metal plasma facing components have a lower Zeff, less radiation cooling in the scrape-off layer and divertor regions and are prone to impurity accumulation in the core. Higher gas puff and the seeding of low-Z impurities are applied to prevent impurity accumulation, to increase the frequency of edge localised modes and to cool the divertor. A lower power threshold for the transition from low-confinement mode to high confinement mode has been found in all metal wall machines when compared to carbon wall machines. The application of lithium before or during discharges can lead to ELM free H-modes. The seeding of high-Z impurities increases core radiation, reduces the power flux across the separatrix and, if applied in the right amount, does not lead to deterioration of the confinement. All these effects have in common that they can often be explained by the shape or position of the density profile. Not only the peakedness of the density profile in the core but also the position of the edge pressure gradient influences global confinement. It is shown how (i) ionisation in the pedestal region due to higher reflection of deuterium from high-Z walls, (ii) reduced recycling in consequence of lithium wall conditioning, (iii) the fostering of edge modes with lithium dropping, (iv) increased gas puff and (v) the cooling of the scrape-off layer by medium-Z impurities such as nitrogen affect the edge density profile. The consequence is a shift in the pressure profile relative to the separatrix, leading to improved pedestal stability of H-mode plasmas when the direction is inwards. Keywords: Plasma, Pedestal, Confinement, Wall material, Reflection, Lithium, Edge localised modes, Impurities, Stabilityhttp://www.sciencedirect.com/science/article/pii/S2352179116302290 |
collection |
DOAJ |
language |
English |
format |
Article |
sources |
DOAJ |
author |
E. Wolfrum M. Beurskens M.G. Dunne L. Frassinetti X. Gao C. Giroud J. Hughes T. Lunt R. Maingi T. Osborne M. Reinke H. Urano |
spellingShingle |
E. Wolfrum M. Beurskens M.G. Dunne L. Frassinetti X. Gao C. Giroud J. Hughes T. Lunt R. Maingi T. Osborne M. Reinke H. Urano Impact of wall materials and seeding gases on the pedestal and on core plasma performance Nuclear Materials and Energy |
author_facet |
E. Wolfrum M. Beurskens M.G. Dunne L. Frassinetti X. Gao C. Giroud J. Hughes T. Lunt R. Maingi T. Osborne M. Reinke H. Urano |
author_sort |
E. Wolfrum |
title |
Impact of wall materials and seeding gases on the pedestal and on core plasma performance |
title_short |
Impact of wall materials and seeding gases on the pedestal and on core plasma performance |
title_full |
Impact of wall materials and seeding gases on the pedestal and on core plasma performance |
title_fullStr |
Impact of wall materials and seeding gases on the pedestal and on core plasma performance |
title_full_unstemmed |
Impact of wall materials and seeding gases on the pedestal and on core plasma performance |
title_sort |
impact of wall materials and seeding gases on the pedestal and on core plasma performance |
publisher |
Elsevier |
series |
Nuclear Materials and Energy |
issn |
2352-1791 |
publishDate |
2017-08-01 |
description |
Plasmas in machines with all metal plasma facing components have a lower Zeff, less radiation cooling in the scrape-off layer and divertor regions and are prone to impurity accumulation in the core. Higher gas puff and the seeding of low-Z impurities are applied to prevent impurity accumulation, to increase the frequency of edge localised modes and to cool the divertor. A lower power threshold for the transition from low-confinement mode to high confinement mode has been found in all metal wall machines when compared to carbon wall machines. The application of lithium before or during discharges can lead to ELM free H-modes. The seeding of high-Z impurities increases core radiation, reduces the power flux across the separatrix and, if applied in the right amount, does not lead to deterioration of the confinement. All these effects have in common that they can often be explained by the shape or position of the density profile. Not only the peakedness of the density profile in the core but also the position of the edge pressure gradient influences global confinement. It is shown how (i) ionisation in the pedestal region due to higher reflection of deuterium from high-Z walls, (ii) reduced recycling in consequence of lithium wall conditioning, (iii) the fostering of edge modes with lithium dropping, (iv) increased gas puff and (v) the cooling of the scrape-off layer by medium-Z impurities such as nitrogen affect the edge density profile. The consequence is a shift in the pressure profile relative to the separatrix, leading to improved pedestal stability of H-mode plasmas when the direction is inwards. Keywords: Plasma, Pedestal, Confinement, Wall material, Reflection, Lithium, Edge localised modes, Impurities, Stability |
url |
http://www.sciencedirect.com/science/article/pii/S2352179116302290 |
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