A Subgrid Model for Electron-Scale Turbulence in Global Ion-Scale Gyrokinetic Simulations

Ion-scale gyrokinetic simulations are now able to accurately predict transport levels and power spectra; however, they can often underestimate the electron thermal transport. Recent multiscale simulations have shown that including electron-scale turbulence can lead to better agreement with experimental heat flux levels and that capturing cross-scale dynamics is important. However, multiscale simulations require resolving the electron gyroradius thereby limiting simulation domain size, while accounting for global effects on the ion scale requires radial domains of hundreds of ion-gyroradii. Therefore, a subgrid model of electron-scale turbulence is of interest for modeling future burning plasma experiments using global ion-scale simulation. Here we demonstrate a subgrid ETG model that averages electron-scale turbulence from GENE over intermediate scales in space and time to input into global ITG GEM simulations. This approach results in ion-scale equations which include the electron heat transport from ETG turbulence and the effects of electron-scale turbulence on the ion scale. Flux-tube ETG Cyclone Base Case simulations are carried out using GENE at different radial locations and a kinetic form of the flux is added to ion-scale global GEM simulations as a source term. Analytic radial profiles of ETG heat flux are constructed and tested against local runs at multiple radial locations. The results of capturing ETG heat transport in global GEM simulations are discussed.