Gyrokinetic modeling of electron temperature gradient turbulence in a tokamak pedestal

In H-mode tokamak plasmas, transport barriers in the pedestal allow for better confinement and heating of core plasmas. Additionally, the steep gradients of the pedestal region provide a source of free energy to drive microscale instabilities and turbulence. Understanding the operation of these instabilities and their saturation in the nonlinear regime is key to discovering better performing equilibria.

This work uses gyrokinetic simulations to study in detail the linear and nonlinear (turbulent) states driven by the electron temperature gradient instability (ETG) in a tokamak pedestal.

The system is dominated by modes with fine parallel structure, requiring large resolution in order to correctly predict the heat flux. In contrast to core plasmas, the nonlinear phase of the system has fewer characteristics of the linear model. E.g. the dominant linear modes are not prominent in the nonlinear state. Heat-flux spectra are downshifted to smaller bi-normal wavenumbers, which is relevant for quasilinear modeling. The system saturates via an inverse cascade of energy to large scales, while critical balance is maintained over a wide range of scales, particularly at small scales.