New Characteristics of Zonal Flows in Multi-scale Plasma Turbulence

The evolution of multi-scale plasma turbulence including resistive MHD and micro-instability is studied based on a 5-field gyrofluid simulation aiming to understand complex nonlinear interaction and turbulent transport. Here we report two new findings on the zonal flow (ZF) characteristics: (1) A robust oscillatory ZF with finite frequency is created in slab geometry for the first time due to the multi-scale interaction so that the ion heat transport is not efficiently suppressed. It is identified that the finite frequency ZF results from a magnetic island seesaw pivoted around the singular layer due to a net oscillatory electromagnetic torque, which is sustained by micro-instability through multi-scale interaction. These results imply probable relaxation of locking mode and degeneration of the favorable role of ITG ZFs in transport suppression in tokamak plasmas. (2) On the other hand, a global ZF eigenmode is found in Hasegawa-Mima(HM) model due to the radial spectral effect of ZFs, in which all components have the same enhanced growth rate. It is found that the global ZF eigenmode originates from successive cross coupling between ZFs and turbulent sidebands.