2006 Coupled ITG/TEM-ETG gyrokinetic simulations

Abstract. This work reports on the first realistic numerical studies of small-scale electron-temperature-gradient (ETG) turbulence embedded in large-scale ion-temperature-gradient plus trapped-electron-mode turbulence. To begin with, we find that the simplified adiabatic-ion model of ETG does not always saturate nonlinearly, suggesting that nonadiabatic (gyrokinetic) ions are required for robust saturated states of electron energy flux. Our results also qualita-tively confirm a prediction of Holland and Diamond that the back-reaction of ETG on ITG turbulence is insignificant. For the parameters studied we also find that – as a mechanism for ETG turbulence supression – ITG modulation of the electron temperature gradient appears to dominate ITG random shearing. An important practical result of this work is the finding that nearly all of the electron energy transport arises from ion scales; for the Cyclone base case parameters, only 16 % of the energy diffusivity, χe, arises from the spectral region kθρi> 1.0. 1