Gyrokinetic GENE simulations of DIII-D near-edge L-mode plasmas

TL;DR

This paper uses advanced gyrokinetic simulations with the GENE code to accurately model heat fluxes in the near-edge region of DIII-D tokamak plasmas, demonstrating agreement with experimental data within measurement uncertainties.

Contribution

First multi-scale gyrokinetic simulations of the near-edge region with realistic mass ratio and geometry, showing that ion-scale simulations suffice and reproducing experimental heat fluxes.

Findings

Simulations agree with experimental heat fluxes within uncertainties.

Ion-scale simulations are sufficient due to suppression of electron-scale transport.

Nonlinear simulations reveal a hybrid ITG and TEM mode state.

Abstract

We present gyrokinetic simulations with the GENE code addressing the near-edge region of an L-mode plasma in the DIII-D tokamak. At radial position $\rho=0.80$, simulations with the ion temperature gradient increased by $40\%$ above the nominal value give electron and ion heat fluxes that are in simultaneous agreement with the experiment. This gradient increase is consistent with the combined statistical and systematic uncertainty $\sigma$ of the Charge Exchange Recombination Spectroscopy (CER) measurements at the $1.6 \sigma$ level. Multi-scale simulations are carried out with realistic mass ratio and geometry for the first time in the near-edge. These multi-scale simulations suggest that the highly unstable ion temperature gradient (ITG) modes of the flux-matched ion-scale simulations suppress electron-scale transport, such that ion-scale simulations are sufficient at this location. At radial position $\rho=0.90$, nonlinear simulations show a hybrid state of ITG and trapped electron modes~(TEMs), which was not expected from linear simulations. The nonlinear simulations reproduce the total experimental heat flux with the inclusion of $\mathbf{E} \times \mathbf{B}$ shear effects and an increase in the electron temperature gradient by $\sim 23\%$. This gradient increase is compatible with the combined statistical and systematic uncertainty of the Thomson scattering data at the $1.3 \sigma$ level. These results are consistent with previous findings that gyrokinetic simulations are able to reproduce the experimental heat fluxes by varying input parameters close to their experimental uncertainties, pushing the validation frontier closer to the edge region.