Nonlinear entropy transfer via zonal flows in gyrokinetic plasma turbulent transport

Nakata, Motoki; Watanabe, Tomohiko*; Sugama, Hideo*; Idomura, Yasuhiro   

Nonlinear entropy transfer processes among zonal and non-zonal modes are carefully examined for the toroidal ITG and ETG turbulence by means of the local gyrokinetic turbulence simulations. The entropy transfer from non-zonal to zonal modes is substantial in the instability-saturation phase of the ITG turbulence, while, once the strong zonal flow is generated, the entropy transfer to the zonal modes becomes quite weak in the steady state. Instead, the entropy of non-zonal modes with low radial-wavenumbers is successively transferred to the other non-zonal modes with higher radial-wavenumbers (with less contribution to the heat flux) via the triad-interaction with zonal flows. The successive entropy transfer is closely associated with the regulation of ITG-driven transport. On the other hand, in both the saturation and steady phases of the ETG turbulence, the entropy transfer processes among low-wavenumber non-zonal modes are dominant, then the higher transport level is sustained.