Effects of zonal flows on transport crossphase in dissipative trapped-electron mode turbulence in edge plasmas

Abstract

For H-mode, standard decorrelation theory predicts that it is the turbulence intensity $|\ddot{I}{•}_k{|}^2$ that is mainly affected via flow-induced shearing of turbulent eddies. However, for other regimes (e.g. I-mode, characterized by high energy confinement but low particle confinement), this decrease of turbulence amplitude cannot explain the decoupling of particle v.s. thermal flux, since a suppression of turbulence intensity $|\ddot{I}{•}_k{|}^2$ would necessarily affect both fluxes the same way. Here, we explore a possible new stabilizing mechanism: zonal flows may directly affect the transport crossphase. We show the effect of this novel mechanism on the turbulent particle flux, by using a simple fluid model [Baver et al., Phys. Plasmas \textbf{9}, 3318 (2002)] for dissipative trapped-electron mode (DTEM), including zonal flows. We first derive the evolution equation for the transport crossphase $\hat{I}{´}_k$ between density and potential fluctuations, including contributions from the $E\times B$ nonlinearity. By using a parametric interaction analysis, we obtain a predator-prey like system of equations for the pump amplitude $\ddot{I}{•}_p$, the pump crossphase $\hat{I}{´}_p$, the zonal amplitude $\ddot{I}{•}_z$ and the triad phase-mismatch $\hat{I}”\hat{I}´$. The system displays limit-cycle oscillations where the instantaneous DTEM growth rate - proportional to the crossphase - shows quasi-periodic relaxations where it departs from that predicted by linear theory.

12 pages, 10 figures