The role of edge plasma parameters in H-mode density limit on the JET-ILW

H. J. Sun, R. J. Goldston, A. Huber, X. Q. Xu, J. Flanagan, D. C. McDonald, E. De La Luna, M. Maslov, J. R. Harrison, F. Militello, J. Fessey, S. Cramp

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Abstract

A study of a dataset of JET H-mode plasma with the Be/W ITER-like wall (JET-ILW) shows that reaching the edge MHD ballooning limit leads to confinement degradation. However, unlike JET plasmas with a carbon wall (JET-C), the JET-ILW plasmas stay in a marginal dithering phase for a relatively long period, associated with a higher (≈20%) H-mode density limit (HDL) than JET-C equivalents. This suggests that ITER could be operated in H-mode with higher density than the scaling based on carbon wall devices, but likely with a dithering phase plasma with lower confinement. A new, reliable estimator for JET E r, min has been derived by combining HRTS measurements of pedestal gradient and edge-SOL decay lengths. JET radial E r ETB wells are observed in the range of -15 to -60 kV m-1 in high performance H-modes, consistent with previous CXRS results in ASDEX Upgrade. The results imply that a higher positive E × B shear in the near SOL plays a role in sustaining a marginal phase in JET-ILW which leads to a higher HDL than that in JET-C. The results of the JET-ILW dataset show agreement with the Goldston finite collisionality HD model for SOL broadening at high collisionality. A hypothesis for the dithering H-mode phase is proposed: as n e,SOL increases, ν ∗,SOL increases, SOL broadens, E r shear decreases, triggers L-mode; n e drops, ν ∗,SOL decreases, SOL becomes narrower, and E r shear increases, triggering H-mode, resulting in a cycle of H-L-H- oscillations. For burning plasma devices, such as ITER, operating just below the MHD limit for the dithering phase could be a promising regime for maximising core density, and fusion performance while minimising plasma-material interaction. The oscillatory signal during the dithering phase could be used as a precursor of undesirable plasma performance for control purposes.

Original languageEnglish (US)
Article number066009
JournalNuclear Fusion
Volume61
Issue number6
DOIs
StatePublished - Jun 2021

All Science Journal Classification (ASJC) codes

  • Nuclear and High Energy Physics
  • Condensed Matter Physics

Keywords

  • H-mode density limit
  • JET-ILW
  • SOL decay length
  • edge ballooning limit
  • radial electrical field

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