Soliton generation and drift wave turbulence spreading via geodesic acoustic mode excitation

TL;DR

This paper derives nonlinear equations for drift wave and GAM interactions, revealing soliton formation that enhances turbulence spreading and core-edge interactions in tokamak plasmas.

Contribution

It develops a nonlinear gyrokinetic framework capturing soliton formation and turbulence spreading via GAM excitation, advancing understanding of nonlocal transport mechanisms.

Findings

Spontaneous GAM excitation is confirmed through numerical solutions.

Soliton structures form due to self-trapping and dispersiveness balance.

Turbulence spreading is enhanced by soliton propagation, affecting core-edge dynamics.

Abstract

The two-field equations governing fully nonlinear dynamics of the drift wave (DW) and geodesic acoustic mode (GAM) in the toroidal geometry are derived in nonlinear gyrokinetic framework. Two stages with distinctive features are identified and analyzed. In the linear growth stage, the set of nonlinear equations can be reduced to the intensively studied parametric decay instability (PDI), accounting for the spontaneous resonant excitation of GAM by DW. The main results of previous works on spontaneous GAM excitation, e.g., the much enhanced GAM group velocity and the nonlinear growth rate of GAM, are reproduced from numerical solution of the two-field equations. In the fully nonlinear stage, soliton structures are observed to form due to the balancing of the self-trapping effect by the spontaneously excited GAM and kinetic dispersiveness of DW. The soliton structures enhance turbulence spreading from DW linearly unstable to stable region, exhibiting convective propagation instead of typical linear dispersive process, and is thus, expected to induce core-edge interaction and nonlocal transport.