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

TL;DR

This paper investigates how zonal flows influence the transport crossphase in dissipative trapped-electron mode turbulence, revealing a new stabilization mechanism affecting particle flux without suppressing turbulence intensity.

Contribution

It introduces a novel mechanism where zonal flows directly modify the transport crossphase, affecting particle flux independently of turbulence amplitude suppression.

Findings

Zonal flows can induce limit-cycle oscillations in the transport crossphase.

The transport crossphase dynamics can be modeled as a predator-prey system.

Instantaneous growth rates show quasi-periodic relaxations deviating from linear predictions.

Abstract

For H-mode, standard decorrelation theory predicts that it is the turbulence intensity $|\phi_k|^2$ 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 $|\phi_k|^2$ 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 $\delta_k$ 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 $\phi_p$, the pump crossphase $\delta_p$, the zonal amplitude $\phi_z$ and the triad phase-mismatch $\Delta \delta$. 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.