Kinetic ballooning modes as a constraint on plasma triangularity in commercial spherical tokamaks

R. Davies, D. Dickinson, H. Wilson

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

Abstract

To be economically competitive, spherical tokamak (ST) power plant designs require a high β (plasma pressure/magnetic pressure) and sufficiently low turbulent transport to enable steady-state operation. A novel approach to tokamak optimisation is for the plasma to have negative triangularity, with experimental results indicating this reduces transport. However, negative triangularity is known to close access to the 'second stability' region for ballooning modes, and thus impose a hard β limit. Second stability access is particularly important in ST power plant design, and this raises the question as to whether negative triangularity is feasible. A linear gyrokinetic study of three hypothetical high β ST equilibria is performed, with similar size and fusion power in the range 500-800 MW. By closing the second stability window, the negative triangularity case becomes strongly unstable to long-wavelength kinetic ballooning modes (KBMs) across the plasma, likely driving unacceptably high transport. By contrast, positive triangularity can completely avoid the ideal ballooning unstable region whilst having reactor-relevant β, provided the on-axis safety factor is sufficiently high. Nevertheless, the dominant instability at long wavelength still appears to be the KBM, though it could be stabilised by flow shear.

Original languageEnglish
Article number105001
JournalPlasma Physics and Controlled Fusion
Volume64
Issue number10
DOIs
StatePublished - Oct 2022
Externally publishedYes

Funding

The authors are grateful for productive discussions with M Anastopoulos-Tzanis, B Patel, C M Roach, B F McMillan and S Biggs-Fox. This work was supported by the Engineering and Physical Sciences Research Council (EP/L01663X/1) and (EP/R034737/1). This project was undertaken on the Viking Cluster, which is a high performance compute facility provided by the University of York. We are grateful for computational support from the University of York High Performance Computing service, Viking and the Research Computing team. This research used the GS2 version latest commit 142c78781c1bc4c06d234b32fee70eaf100d68e4 (8.1).

Keywords

  • kinetic ballooning mode
  • negative triangularity
  • plasma microinstability
  • positive triangularity
  • spherical tokamak

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