Author: Garth G Whelan
Requested Type: Pre-Selected Invited
Submitted: 2016-02-15 15:34:23
1150 University Ave.
Madison, WI 53726
In gyrokinetic simulations of ion-temperature-gradient-driven turbulence (ITG), zonal flows have been shown to have a major role in saturating the instability. Being linearly stable, zonal flows are driven by nonlinear energy transfer, which conservatively transfer energy within wavenumber triplets. Using a new GENE diagnostic, we resolve energy transfer within triplets and show that the main role of the zonal flow is to catalyze energy transfer to higher radial wavenumber rather than dissipate it. Triplet interactions around the toroidal wavenumber corresponding to the peak in turbulent energy contribute most of the energy to the zonal flow. Interestingly, interactions at higher toroidal wavenumber can transfer energy out of the zonal flow. While this does not directly contribute appreciable energy to the nonzonal modes, it indirectly contributes by changing the zonal flow amplitude. Increased temperature gradients decrease nonlinear energy transfer out of the zonal mode while increasing collisionality increases transfer in. Furthermore, we investigate the effect of an artificial damping that only affects the zonal mode. The triplet diagnostic also splits energy transfers between the unstable and stable modes at a single wavenumber. We find that typically 20% of the energy is transferred to the stable branch in any given triplet. Because this occurs at any given interaction, it compounds to a significant net effect on the turbulent amplitude and transport. Regarding the phenomenon of nonlinearly enhanced finite-beta stabilization, we find that as beta is increased, proportionally more energy is transferred to stable modes; the involvement of the zonal flow coupling is consistent with the previous observation that the Dimits shift is increased at higher beta values.