Reversal of turbulent gyroBohm isotope scaling due to nonadiabatic electron drive

Abstract

The influence of kinetic electrons on the isotope scaling of gyrokinetic turbulent energy flux is assessed. A simple framework is used to study the transition from ion-dominated turbulence regimes to regimes where electron and ion transport levels are comparable. In the ion-dominated regime, the turbulent ion energy flux increases as the ion mass increases, in agreement with simple gyroBohm scaling arguments. Conversely, in the latter regime for which the influence of electrons is significant, a strong reversal of the gyroBohm scaling is observed which cannot be captured by mixing length estimates. In this reversed regime, the turbulent ion energy flux decreases as ion mass increases. The reversal is controlled by the finite electron-to-ion mass-ratio dependence of the nonadiabatic electron response. This mass-ratio dependence is dominated by the parallel motion terms in the electron gyrokinetic equation and provides a correction to the bounce-averaged-electron limit which is independent of the mass ratio. The finite-mass correction is larger for light ions and explains the observed gyroBohm reversal for hydrogen plasmas. An implication is that isotope scaling may not be properly described by simplified fluid or bounce-averaged electron equations.