Kinetic-Ballooning-Bifurcation in Tokamak Pedestals Across Shaping and Aspect-Ratio

TL;DR

This paper introduces a gyrokinetic threshold model predicting a bifurcation in tokamak pedestal scalings influenced by plasma shaping and aspect-ratio, revealing new stable pedestal configurations with implications for ELM-free operation.

Contribution

It presents a novel gyrokinetic model linking kinetic-ballooning-modes to pedestal bifurcations, expanding understanding of pedestal stability and shape effects in tokamaks.

Findings

Bifurcation depends on plasma shaping and aspect-ratio.

Wide pedestal branch may enable ELM-free operation.

Steeper pedestals predicted for negative triangularity at low aspect-ratio.

Abstract

We use a new gyrokinetic threshold model to predict a bifurcation in tokamak pedestal width-height scalings that depends strongly on plasma shaping and aspect-ratio. The bifurcation arises from the first and second stability properties of kinetic-ballooning-modes that yields wide and narrow pedestal branches, expanding the space of accessible pedestal widths and heights. The wide branch offers potential for edge-localized-mode-free pedestals with high core pressure. For negative triangularity, low-aspect-ratio configurations are predicted to give steeper pedestals than conventional-aspect-ratio. Both wide and narrow branches have been attained in tokamak experiments.