Zonal Jet Creation from Secondary Instability of Drift Waves for Plasma Edge Turbulence

TL;DR

This paper introduces a new mechanism based on secondary instability analysis within the modified Hasegawa-Mima model to explain how zonal flows form and persist in plasma edge turbulence, supported by numerical simulations.

Contribution

It demonstrates that the modified Hasegawa-Mima model captures the creation of zonal jets via secondary instability, unlike the traditional CHM model, providing a new theoretical framework.

Findings

Zonal flows emerge from non-zonal drift waves through secondary instability.

Numerical simulations confirm the formation of dominant zonal flows from initial non-zonal states.

The CHM model does not exhibit instability leading to zonal flow formation.

Abstract

A new strategy is presented to explain the creation and persistence of zonal flows widely observed in plasma edge turbulence. The core physics in the edge regime of the magnetic-fusion tokamaks can be described qualitatively by the one-state modified Hasegawa-Mima (MHM) model, which creates enhanced zonal flows and more physically relevant features in comparison with the familiar Charney-Hasegawa-Mima (CHM) model for both plasma and geophysical flows. The generation mechanism of zonal jets is displayed from the secondary instability analysis via nonlinear interactions with a background base state. Strong exponential growth in the zonal modes is induced due to a non-zonal drift wave base state in the MHM model, while stabilizing damping effect is shown with a zonal flow base state. Together with the selective decay effect from the dissipation, the secondary instability offers a complete characterization of the convergence process to the purely zonal structure. Direct numerical simulations with and without dissipation are carried out to confirm the instability theory. It shows clearly the emergence of a dominant zonal flow from pure non-zonal drift waves with small perturbation in the initial configuration. In comparison, the CHM model does not create instability in the zonal modes and usually converges to homogeneous turbulence.