Drift wave solitons and zonal flows: implication on staircase formation

TL;DR

This paper investigates the nonlinear interaction between drift waves and zonal flows using gyrokinetic theory, revealing how their interplay leads to soliton and staircase formation, which may influence plasma confinement and transport barriers.

Contribution

It provides a first-principles analysis of drift wave and zonal flow interactions, explaining the formation of staircase structures and micro-barriers in plasma.

Findings

Drift wave solitons are confined between micro-barriers.

Spontaneously excited ZF can generate micro-barriers and solitons.

The staircase pattern in plasma is explained by nonlinear DW-ZF interactions.

Abstract

The self-consistent nonlinear interaction of drift wave (DW) and zonal flow (ZF) is investigated using nonlinear gyrokinetic theory, with both spontaneous excitation and beat-driven of ZF by DW treated on the same footing. DW solitons are formed in the nonlinear DW-ZF interactions and are confined between radially spaced micro-barriers. The resulting radial structures in the nonlinear DW-ZF interactions exhibit similar pattern to the ExB "staircase" observed in numerical simulations. These micro-barriers are generated by the repulsive response due to spontaneously excited ZF, which, as a general property demonstrated in this work, also generate an attractive nonlinear potential in DW equation. Meanwhile, the nonlinear potential due to beat-driven ZF is always attractive and, as such, always serve as potential well to contribute to soliton formation. For spontaneously excited ZF from initial noise, the simultaneous excitation of solitons and micro-barriers is found to be universal, due to the zero frequency nature of ZF and spatial structure of the Reynolds stress. The present analysis, thus, provides a potential first-principle-based interpretation of the ExB staircase observed in simulations, which may contribute to micro transport barriers formation and enhance plasma confinement.