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