Experimental Evidence of Turbulent Transport Regulation by Zonal Flows

The regulation of turbulent transport by zonal flows is studied experimentally on a flux surface of the stellarator experiment TJ-K. Data of 128 Langmuir probes at different toroidal and poloidal positions on a single flux surface enable us to measure simultaneously the zonal flow activity and the turbulent transport in great detail. A reduction of turbulent transport by 30% during the zonal flow phase is found. It is shown that the reduction process is initiated by a modification in the cross phase between density and electric field followed by a reduction in the fluctuation levels, which sustain low transport levels on larger time scales than the zonal flow lifetime.

Figure 1

Poloidal power spectra of the floating-potential (c), (d), and ion-saturation current fluctuations (e), (f) acquired with the top port array (TPA) (a) and the outer port array (OPA) (b).

Figure 2

Time series of total turbulent transport (a) and zonal potential (b). Cross correlation (c) and conditional averaged transport (solid line) and ZF (blue dashed line) indicate a predator-prey dynamics (d).

Figure 3

Characteristic evolution of scale resolved ion-saturation current (a), (b), floating potential fluctuations (c), (d), and turbulent transport (e), (f). During different phases of the ZF (g), the coherence (h) and cross phase (j) is shown. In phase space, ZF and total turbulent transport establish a limit cycle (i).