Investigation of triangularity effects on tokamak edge turbulence through multi-fidelity gyrokinetic simulations

TL;DR

This study uses multi-fidelity gyrokinetic simulations to analyze how triangularity influences edge turbulence in tokamaks, highlighting the importance of kinetic effects for accurate modeling.

Contribution

It introduces a gyro-moment hierarchy as an efficient multi-fidelity tool to explore triangularity effects on tokamak edge turbulence, comparing models and validating their ranges of validity.

Findings

TEMs drive turbulent heat transport under nominal conditions.

Reduced models are valid only when TEMs are absent and turbulence is ITG-driven.

Positive triangularity destabilizes TEMs, affecting model accuracy.

Abstract

This paper uses the gyro-moment (GM) approach as a multi-fidelity tool to explore the effect of triangularity on tokamak edge turbulence. Considering experimental data from an L-mode DIII-D discharge, we conduct gyrokinetic (GK) simulations with realistic plasma edge geometry parameters at $\rho=0.95$. We find that employing ten GMs effectively captures essential features of both trapped electron mode (TEM) and ion temperature gradient (ITG) turbulence. By comparing electromagnetic GK simulations with adiabatic electron GK and reduced fluid simulations, we identify the range of validity of the reduced models. We observe that TEMs drive turbulent heat transport under nominal discharge conditions, hindering accurate transport level estimates by both simplified models. However, when TEMs are absent, and turbulence is ITG-driven, an agreement across the different models is observed. Finally, a parameter scan shows that the positive triangularity scenario destabilizes the TEM, therefore, the adiabatic electron model tends to show agreement with the electromagnetic simulations in zero and negative triangularity scenarios. On the other hand, the reduced fluid simulations exhibit limited sensitivity to triangularity changes, shedding light on the importance of retaining kinetic effects to accurately model the impact of triangularity turbulence in the tokamak edge. In conclusion, our multi-fidelity study suggests that a GM hierarchy with a limited number of moments is an ideal candidate for efficiently exploring triangularity effects on micro-scale turbulence.