Nonlinear entropy transfer processes in toroidal ITG/ETG turbulence

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

In this study, the nonlinear interactions among turbulent vortices and turbulence-induced meso-scale flows, e.g., zonal flows or streamers have been investigated by means of gyrokinetic simulations. Signifiant differences of nonlinear fluctuation (or entropy) transfer processes, which are relevant to the resultant transport levels in the toroidal ITG and ETG turbulence, have been clarified. In the ITG turbulence, nonlinearly generated zonal flows enhance the entropy transfer among non-zonal modes, i.e., the entropy of non-zonal modes is successively transferred to the other non-zonal modes with smaller scales (but with less contribution to the heat flux). On the other hand, in the ETG turbulence, the role of zonal flows in the entropy transfer to smaller scales is much weaker than that in the ITG case. Then, the entropy transfer processes are confined within the non-zonal modes with larger scales where the higher transport level is sustained.