Turbulent Transport-Limited Pedestals in Tokamaks

TL;DR

This paper develops a transport threshold model to understand the scaling of pedestal width and height in tokamak plasmas, focusing on turbulent transport mechanisms like ETG turbulence and their role in ELM-free operation.

Contribution

It introduces a novel transport threshold model for turbulent transport-limited pedestals and applies it to various turbulence types and experimental data.

Findings

Pressure buildup mainly through temperature favors transport-limited, ELM-free pedestals.

Relative inward shift of temperature to density pedestal promotes transport-limitation.

Flow shear is necessary to saturate pedestal growth.

Abstract

H-mode operation of tokamak fusion plasmas free of dangerous Type 1 edge-localized-modes (ELMs) requires a non-ELM mechanism for saturating the edge pedestal growth. One possible mechanism is turbulent transport. We introduce a transport threshold model to find pedestal width-height scalings for turbulent transport-limited pedestals. The model is applied to electron heat transport resulting from electron-temperature-gradient (ETG) turbulence. The width-height scalings are highly sensitive to the relative contribution of density and temperature to the pedestal pressure. Pressure that builds up mainly through temperature is more likely to be transport-limited, and hence ELM-free. A relative radial inward shift of the temperature to density pedestal location is also more likely to transport-limit the pedestal. A second constraint such as flow shear is required to saturate pedestal growth. We also calculate width-height transport scalings resulting from particle and heat transport arising from ETG and kinetic-ballooning-mode turbulence. Comparisons are performed for ELMy and ELM-free experiments in MAST-U, NSTX, and DIII-D. This is a first step towards a pedestal width-height scaling for transport-limited ELM-free pedestals.