Effect of plasma squareness on gyrokinetic pedestal width-height scaling and prospects for ELM-free operation*

Strongly shaped plasmas with high squareness could present attractive core-edge integrated scenarios, featuring wide pedestals, high core fusion power, and improved ion heat transport [1,2,3,4]. We calculate the effect of plasma squareness on the pedestal width-height scaling [5] for multiple tokamaks using a new kinetic ballooning mode (KBM) gyrokinetic threshold model [6]. It is found that more square plasmas tend to have wider pedestals with lower average pressure gradients, which could be characterized by ELM-free operation [7]. Pedestal width-height scaling expressions are also found across aspect ratio, showing that squareness could be particularly beneficial at low aspect ratio. These benefits are explained by applying nonlinear regression and a basic machine learning model to the geometric metric coefficients in the pedestal. Combined with peeling ballooning mode (PBM) stability [8], our model will calculate a maximum inter-ELM pedestal width and height based on KBM and non-ideal PBM stability.

  

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[3] Imada, K et al, 2024. Observation of a new pedestal stability regime in MAST Upgrade H-mode plasmas. Nuclear Fusion.

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[5] Snyder, P. B. et al, 2009. Development and validation of a predictive model for the pedestal height. Physics of Plasmas 16(5).

[6] Parisi, J.F. et al, 2024. Kinetic-ballooning-limited pedestals in spherical tokamak plasmas. Nuclear Fusion, 64(5), p.054002.

[7] Nelson, A.O. et al, 2023. Robust avoidance of edge-localized modes alongside gradient formation in the negative triangularity tokamak edge. Physical Review Letters, 131(19), p.195101.

[8] Kleiner, A. et al, 2021. Importance of resistivity on edge-localized mode onset in spherical tokamaks. Nuclear Fusion, 61(6), p.064002.