Low frequency m = 1 modes during standard and improved confinement scenarios in W7-X
In the Wendelstein 7-X stellarator (W7-X), two related global low-frequency electromagnetic oscillations are observed on many diagnostic systems (e.g. plasma stored energy, Mirnov coils, Langmuir probes, etc). The activity is related to the presence of magnetic islands at certain radii. However, in...
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IOP Publishing
2025-01-01
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| Series: | Nuclear Fusion |
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| Online Access: | https://doi.org/10.1088/1741-4326/adbb7e |
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| author | Dario Cipciar Carsten Killer Christian Brandt Axel Könies Kian Rahbarnia Neha Chaudhary Jiri Adamek Olaf Grulke Henning Thomsen Torsten Stange Ksenia Aleynikova Matthias Hirsch the W7-X Team |
| author_facet | Dario Cipciar Carsten Killer Christian Brandt Axel Könies Kian Rahbarnia Neha Chaudhary Jiri Adamek Olaf Grulke Henning Thomsen Torsten Stange Ksenia Aleynikova Matthias Hirsch the W7-X Team |
| author_sort | Dario Cipciar |
| collection | DOAJ |
| description | In the Wendelstein 7-X stellarator (W7-X), two related global low-frequency electromagnetic oscillations are observed on many diagnostic systems (e.g. plasma stored energy, Mirnov coils, Langmuir probes, etc). The activity is related to the presence of magnetic islands at certain radii. However, in contrast to initial analyses in earlier works, the mode activity is not localized at the magnetic islands, but rather of an m = 1 type in the plasma outer-core. We observe a strong cross-correlation between electron temperature fluctuations in the confined plasma region, measured with electron cyclotron emission radiometer, and scrape-off layer fluctuations measured with electric probes. Two limiting cases of the mode activity can be observed based on magnetic configuration. A quasi-continuous (QC) mode case, with frequencies of typically some $100 \, \textrm{Hz}$ , is found in configurations that have a $5/5$ magnetic island chain at the last closed flux surface (LCFS). The mode frequency is found to increase linearly with the core electron temperature and decrease with increasing electron density. In contrast, for magnetic configurations with the $5/5$ magnetic island chain just inside the LCFS, the mode activity becomes bursty and intermittent, exhibiting a crash-event like signature. Although these intermittent events are less frequent, more plasma stored energy is lost per event compared to a period of the QC oscillations. The intermittent oscillations are observable as a broadband modulation in turbulence spectra on most diagnostic systems. The presence of these intermittent bursts goes along with an improved energy confinement in the plasma, superficially resembling H-mode and Edge Localized Modes in tokamaks. A self-limiting process is proposed as an explanation of the intermittent oscillations based on observed ejection of particles during each crash event, which leads to steepening of density profiles and temporary suppression of ion temperature gradient turbulence and in turn suppression of turbulence driven m = 1 mode causing the crash. |
| format | Article |
| id | doaj-art-a6c2ee6d22ea49b99d20d3dcacc34153 |
| institution | OA Journals |
| issn | 0029-5515 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
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| series | Nuclear Fusion |
| spelling | doaj-art-a6c2ee6d22ea49b99d20d3dcacc341532025-08-20T01:58:04ZengIOP PublishingNuclear Fusion0029-55152025-01-0165404601010.1088/1741-4326/adbb7eLow frequency m = 1 modes during standard and improved confinement scenarios in W7-XDario Cipciar0https://orcid.org/0000-0002-3798-9524Carsten Killer1https://orcid.org/0000-0001-7747-3066Christian Brandt2https://orcid.org/0000-0002-5455-4629Axel Könies3https://orcid.org/0000-0003-4306-9000Kian Rahbarnia4https://orcid.org/0000-0002-5550-1801Neha Chaudhary5https://orcid.org/0000-0001-5075-2487Jiri Adamek6https://orcid.org/0000-0001-8562-1233Olaf Grulke7Henning Thomsen8Torsten Stange9Ksenia Aleynikova10https://orcid.org/0000-0001-9606-7346Matthias Hirsch11the W7-X TeamMax-Planck-Institut für Plasmaphysik , Wendelsteinstr. 1, Greifswald 17491, GermanyMax-Planck-Institut für Plasmaphysik , Wendelsteinstr. 1, Greifswald 17491, GermanyMax-Planck-Institut für Plasmaphysik , Wendelsteinstr. 1, Greifswald 17491, GermanyMax-Planck-Institut für Plasmaphysik , Wendelsteinstr. 1, Greifswald 17491, GermanyMax-Planck-Institut für Plasmaphysik , Wendelsteinstr. 1, Greifswald 17491, GermanyMax-Planck-Institut für Plasmaphysik , Wendelsteinstr. 1, Greifswald 17491, GermanyInstitute of Plasma Physics of the CAS , U Slovanky 2525/1a, Prague 18200, Czech RepublicMax-Planck-Institut für Plasmaphysik , Wendelsteinstr. 1, Greifswald 17491, Germany; Department of Physics, Technical University of Denmark , Lyngby, DenmarkMax-Planck-Institut für Plasmaphysik , Wendelsteinstr. 1, Greifswald 17491, GermanyMax-Planck-Institut für Plasmaphysik , Wendelsteinstr. 1, Greifswald 17491, GermanyMax-Planck-Institut für Plasmaphysik , Wendelsteinstr. 1, Greifswald 17491, GermanyMax-Planck-Institut für Plasmaphysik , Wendelsteinstr. 1, Greifswald 17491, GermanyIn the Wendelstein 7-X stellarator (W7-X), two related global low-frequency electromagnetic oscillations are observed on many diagnostic systems (e.g. plasma stored energy, Mirnov coils, Langmuir probes, etc). The activity is related to the presence of magnetic islands at certain radii. However, in contrast to initial analyses in earlier works, the mode activity is not localized at the magnetic islands, but rather of an m = 1 type in the plasma outer-core. We observe a strong cross-correlation between electron temperature fluctuations in the confined plasma region, measured with electron cyclotron emission radiometer, and scrape-off layer fluctuations measured with electric probes. Two limiting cases of the mode activity can be observed based on magnetic configuration. A quasi-continuous (QC) mode case, with frequencies of typically some $100 \, \textrm{Hz}$ , is found in configurations that have a $5/5$ magnetic island chain at the last closed flux surface (LCFS). The mode frequency is found to increase linearly with the core electron temperature and decrease with increasing electron density. In contrast, for magnetic configurations with the $5/5$ magnetic island chain just inside the LCFS, the mode activity becomes bursty and intermittent, exhibiting a crash-event like signature. Although these intermittent events are less frequent, more plasma stored energy is lost per event compared to a period of the QC oscillations. The intermittent oscillations are observable as a broadband modulation in turbulence spectra on most diagnostic systems. The presence of these intermittent bursts goes along with an improved energy confinement in the plasma, superficially resembling H-mode and Edge Localized Modes in tokamaks. A self-limiting process is proposed as an explanation of the intermittent oscillations based on observed ejection of particles during each crash event, which leads to steepening of density profiles and temporary suppression of ion temperature gradient turbulence and in turn suppression of turbulence driven m = 1 mode causing the crash.https://doi.org/10.1088/1741-4326/adbb7eW7-XMHD modesstellaratorsoft x-ray tomographyLangmuir probesITG turbulence |
| spellingShingle | Dario Cipciar Carsten Killer Christian Brandt Axel Könies Kian Rahbarnia Neha Chaudhary Jiri Adamek Olaf Grulke Henning Thomsen Torsten Stange Ksenia Aleynikova Matthias Hirsch the W7-X Team Low frequency m = 1 modes during standard and improved confinement scenarios in W7-X Nuclear Fusion W7-X MHD modes stellarator soft x-ray tomography Langmuir probes ITG turbulence |
| title | Low frequency m = 1 modes during standard and improved confinement scenarios in W7-X |
| title_full | Low frequency m = 1 modes during standard and improved confinement scenarios in W7-X |
| title_fullStr | Low frequency m = 1 modes during standard and improved confinement scenarios in W7-X |
| title_full_unstemmed | Low frequency m = 1 modes during standard and improved confinement scenarios in W7-X |
| title_short | Low frequency m = 1 modes during standard and improved confinement scenarios in W7-X |
| title_sort | low frequency m 1 modes during standard and improved confinement scenarios in w7 x |
| topic | W7-X MHD modes stellarator soft x-ray tomography Langmuir probes ITG turbulence |
| url | https://doi.org/10.1088/1741-4326/adbb7e |
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