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Investigation into the formation of the scrape-off layer density shoulder in JET ITER-like wall L-mode and H-mode plasmas.

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Title: Investigation into the formation of the scrape-off layer density shoulder in JET ITER-like wall L-mode and H-mode plasmas.
Authors: A. Wynn1,2, B. Lipschultz1,2 bruce.lipschultz@york.ac.uk, I. Cziegler1,2, J. Harrison2,3, A. Jaervinen2,4, G. F. Matthews2,3, J. Schmitz1,2,5, B. Tal2,6, M. Brix2,3, C. Guillemaut2,3,7, D. Frigione2,8, A. Huber2,5, E. Joffrin2,9, U. Kruzei2,3, F. Militello2,3, A. Nielsen2,10, N.r. Walkden2,3, S. Wiesen2,5, Contributors, Jet2,11
Source: Nuclear Fusion. May2018, Vol. 58 Issue 5, p1-1. 1p.
Subjects: Plasma devices, Energy sources for pulp mills, Tokamaks, Ionization (Atomic physics), Langmuir probes
Abstract: The low temperature boundary layer plasma (scrape-off layer or SOL) between the hot core and the surrounding vessel determines the level of power loading, erosion and implantation of material surfaces, and thus the viability of tokamak-based fusion as an energy source. This study explores mechanisms affecting the formation of flattened density profiles, so-called ‘density shoulders’, in the low-field side (LFS) SOL, which modify ion and neutral fluxes to surfaces—and subsequent erosion. There is evidence against local enhancement of ionization inducing shoulder formation. We find that increases in SOL parallel resistivity, Λdiv (=[L||νeiΩi]/csΩe), postulated to lead to shoulder growth through changes in SOL turbulence characteristics, correlates with increases in SOL shoulder amplitude, As, but only under a subset of conditions (D2-fuelled L-mode density scans with outer strike point on the horizontal target). Λdiv fails to correlate with As for cases of N2 seeding or during sweeping of the strike point across the horizontal target. The limited correlation of Λdiv and As is also found for H-mode discharges. Thus, while it may be necessary for Λdiv to be above a threshold of ~1 for shoulder formation and/or growth, another mechanism is required. More significantly, we find that in contrast to parallel resistivity, outer divertor recycling, as quantified by the total outer divertor Balmer Dα emission, I–Dα, does scale with As where Λdiv does and even where Λdiv does not. Divertor recycling could lead to SOL density shoulder formation through: (a) reducing the parallel to the field flow (loss) of ions out of the SOL to the divertor; and (b) changes in radial electric fields which lead to E  ×  B poloidal flows as well as potentially affecting SOL turbulence birth characteristics. Thus, changes in divertor recycling may be the sole process involved in bringing about SOL density shoulders or it may be that it acts in tandem with parallel resistivity. [ABSTRACT FROM AUTHOR]
Copyright of Nuclear Fusion is the property of IOP Publishing and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Investigation into the formation of the scrape-off layer density shoulder in JET ITER-like wall L-mode and H-mode plasmas.
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  Data: <searchLink fieldCode="AR" term="%22A%2E+Wynn%22">A. Wynn</searchLink><relatesTo>1,2</relatesTo><br /><searchLink fieldCode="AR" term="%22B%2E+Lipschultz%22">B. Lipschultz</searchLink><relatesTo>1,2</relatesTo><i> bruce.lipschultz@york.ac.uk</i><br /><searchLink fieldCode="AR" term="%22I%2E+Cziegler%22">I. Cziegler</searchLink><relatesTo>1,2</relatesTo><br /><searchLink fieldCode="AR" term="%22J%2E+Harrison%22">J. Harrison</searchLink><relatesTo>2,3</relatesTo><br /><searchLink fieldCode="AR" term="%22A%2E+Jaervinen%22">A. Jaervinen</searchLink><relatesTo>2,4</relatesTo><br /><searchLink fieldCode="AR" term="%22G%2E+F%2E+Matthews%22">G. F. Matthews</searchLink><relatesTo>2,3</relatesTo><br /><searchLink fieldCode="AR" term="%22J%2E+Schmitz%22">J. Schmitz</searchLink><relatesTo>1,2,5</relatesTo><br /><searchLink fieldCode="AR" term="%22B%2E+Tal%22">B. Tal</searchLink><relatesTo>2,6</relatesTo><br /><searchLink fieldCode="AR" term="%22M%2E+Brix%22">M. Brix</searchLink><relatesTo>2,3</relatesTo><br /><searchLink fieldCode="AR" term="%22C%2E+Guillemaut%22">C. Guillemaut</searchLink><relatesTo>2,3,7</relatesTo><br /><searchLink fieldCode="AR" term="%22D%2E+Frigione%22">D. Frigione</searchLink><relatesTo>2,8</relatesTo><br /><searchLink fieldCode="AR" term="%22A%2E+Huber%22">A. Huber</searchLink><relatesTo>2,5</relatesTo><br /><searchLink fieldCode="AR" term="%22E%2E+Joffrin%22">E. Joffrin</searchLink><relatesTo>2,9</relatesTo><br /><searchLink fieldCode="AR" term="%22U%2E+Kruzei%22">U. Kruzei</searchLink><relatesTo>2,3</relatesTo><br /><searchLink fieldCode="AR" term="%22F%2E+Militello%22">F. Militello</searchLink><relatesTo>2,3</relatesTo><br /><searchLink fieldCode="AR" term="%22A%2E+Nielsen%22">A. Nielsen</searchLink><relatesTo>2,10</relatesTo><br /><searchLink fieldCode="AR" term="%22N%2Er%2E+Walkden%22">N.r. Walkden</searchLink><relatesTo>2,3</relatesTo><br /><searchLink fieldCode="AR" term="%22S%2E+Wiesen%22">S. Wiesen</searchLink><relatesTo>2,5</relatesTo><br /><searchLink fieldCode="AR" term="%22Contributors%2C+Jet%22">Contributors, Jet</searchLink><relatesTo>2,11</relatesTo>
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  Data: <searchLink fieldCode="JN" term="%22Nuclear+Fusion%22">Nuclear Fusion</searchLink>. May2018, Vol. 58 Issue 5, p1-1. 1p.
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  Data: <searchLink fieldCode="DE" term="%22Plasma+devices%22">Plasma devices</searchLink><br /><searchLink fieldCode="DE" term="%22Energy+sources+for+pulp+mills%22">Energy sources for pulp mills</searchLink><br /><searchLink fieldCode="DE" term="%22Tokamaks%22">Tokamaks</searchLink><br /><searchLink fieldCode="DE" term="%22Ionization+%28Atomic+physics%29%22">Ionization (Atomic physics)</searchLink><br /><searchLink fieldCode="DE" term="%22Langmuir+probes%22">Langmuir probes</searchLink>
– Name: Abstract
  Label: Abstract
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  Data: The low temperature boundary layer plasma (scrape-off layer or SOL) between the hot core and the surrounding vessel determines the level of power loading, erosion and implantation of material surfaces, and thus the viability of tokamak-based fusion as an energy source. This study explores mechanisms affecting the formation of flattened density profiles, so-called ‘density shoulders’, in the low-field side (LFS) SOL, which modify ion and neutral fluxes to surfaces—and subsequent erosion. There is evidence against local enhancement of ionization inducing shoulder formation. We find that increases in SOL parallel resistivity, Λdiv (=[L||νeiΩi]/csΩe), postulated to lead to shoulder growth through changes in SOL turbulence characteristics, correlates with increases in SOL shoulder amplitude, As, but only under a subset of conditions (D2-fuelled L-mode density scans with outer strike point on the horizontal target). Λdiv fails to correlate with As for cases of N2 seeding or during sweeping of the strike point across the horizontal target. The limited correlation of Λdiv and As is also found for H-mode discharges. Thus, while it may be necessary for Λdiv to be above a threshold of ~1 for shoulder formation and/or growth, another mechanism is required. More significantly, we find that in contrast to parallel resistivity, outer divertor recycling, as quantified by the total outer divertor Balmer Dα emission, I–Dα, does scale with As where Λdiv does and even where Λdiv does not. Divertor recycling could lead to SOL density shoulder formation through: (a) reducing the parallel to the field flow (loss) of ions out of the SOL to the divertor; and (b) changes in radial electric fields which lead to E  ×  B poloidal flows as well as potentially affecting SOL turbulence birth characteristics. Thus, changes in divertor recycling may be the sole process involved in bringing about SOL density shoulders or it may be that it acts in tandem with parallel resistivity. [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Nuclear Fusion is the property of IOP Publishing and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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        Value: 10.1088/1741-4326/aaad78
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      – SubjectFull: Energy sources for pulp mills
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      – SubjectFull: Tokamaks
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      – SubjectFull: Ionization (Atomic physics)
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