Tokamaks are currently the leading concept for thermonuclear fusion reactors, using magnetic fields to toroidally confine a hot plasma and achieve the conditions necessary for sustained fusion. However, experiments reveal anomalous heat and particle losses which far exceed collisional transport predictions and significantly degrade confinement. The anomalous radial transport is now known to result from drift-wave microturbulence, driven unstable in the plasma by the intense pressure gradients involved. Understanding these turbulent transport mechanisms is critical for predicting and maintaining steady-state energy production. After extensive development efforts, gyrokinetic theory and numerical modeling have emerged as essential tools for studying the complex nonlinear dynamics of tokamak microturbulence.

Gyrokinetic simulations at ion gyroradius scales have successfully reproduced transport characteristics in agreement with experiment, but often underestimate electron thermal transport levels. The electron-temperature-gradient (ETG) mode, arising at electron gyroradius scales, is a key candidate to explain excess electron heat losses. ETG transport is expected to be particularly important in reactor-relevant plasmas like ITER, where ETG turbulence can interact with ion-scale turbulence through complex multiscale processes which are sensitive to small variations in equilibrium parameters. Direct simulation of these interactions remains computationally prohibitive, even on exascale computing platforms, thereby motivating the development of the subgrid model presented here. The subgrid model captures electron-scale effects in a reduced form suitable for whole-device modeling of future burning plasmas.

Quasilinear modeling offers an efficient method for predicting turbulent transport spectra by leveraging linear gyrokinetic simulation results. Quaslinear theory is introduced here for modeling ion-scale turbulence in DIII-D shot #162940 using linear gyrokinetic simulation. The quasilinear predictions show good agreement with nonlinear flux spectra, and analysis is successfully extended to negative triangularity shaping - a plasma configuration which has reported reduced turbulent transport levels. Quasilinear models are further compared against nonlinear gyrokinetic ETG simulations and considered for reduced modeling of local electron-scale turbulence effects in global ion-scale simulation.

A key mechanism of instability regulation is by perpendicular shearing from zonal flows (ZFs), which break up radially-elongated drift wave eddies. These are self-generated shear flows which are driven by growing primary instabilities as nonlinear effects become significant. Intermediate-scale gyrokinetic theory, encompassing wavelengths much shorter than the ion gyroradius but much longer than the electron gyroradius, predicts strong ETG-ZF coupling which is expected to drive significant ZF generation and ETG mode regulation. Zonal flow generation due to a single ETG mode is investigated in local single-mode gyrokinetic simulations and intermediate-scale results are found to be in agreement with the gyrokinetic theory. Full-spectrum results are then presented and explained qualitatively in terms of the single-mode results. The resulting intermediate-scale zonal flows have been reported to help regulate ion-scale turbulence levels in multiscale gyrokinetic simulation.

A subgrid ETG model is then demonstrated which averages local electron-scale turbulence over intermediate scales in space and time to include in global ion-temperature-gradient (ITG) simulations. This approach results in ion-scale equations which incorporate the electron heat transport from ETG turbulence and effects of electron-scale turbulence on the ion scale. Local ETG simulations are performed at different radial locations and a kinetic form of the flux is added to global ion-scale simulations as a source term. Analytic radial profiles of ETG heat flux are constructed and compared to flux-tube results at multiple radial locations. Different ratios of ITG to ETG heat flux levels are considered and the results of capturing ETG heat transport in global ion-scale simulations are discussed. Potential coupling of the ETG streamer potential and intermediate-scale zonal flows to the ion scale is further addressed.