The intermittent excitation of geodesic acoustic mode by nonlinear Instanton of electron drift wave envelope in L-mode discharge near tokamak edge

TL;DR

This paper models the nonlinear dynamics of electron drift wave envelopes in tokamaks, revealing how instanton phenomena can intermittently excite geodesic acoustic modes through rapid radial motions.

Contribution

It introduces a model for electron drift wave turbulence that captures instanton dynamics and their role in intermittent GAM excitation in tokamak edge plasmas.

Findings

Instanton exhibits a transition from linear to nonlinear stages with frequencies up to 20 kHz.

Intermittent GAM excitation is linked to the rapid radial motion of Instanton.

Resonant excitation of GAM occurs due to modulation of Reynolds stress by Instanton dynamics.

Abstract

There are two distinct phases in the evolution of drift wave envelope in the presence of zonal flow. A long-lived standing wave phase, which we call the Caviton, and a short-lived traveling wave phase (in radial direction) we call the Instanton. For drift wave turbulence driven by ion temperature gradient mode (ITG), these two stages of dynamics were displayed in [Zhang Y Z, Liu Z Y, Xie T, Mahajan S M and Liu J 2017 Physics of Plasmas 24 122304]. In this paper we show that the dynamical attributes of ITG turbulence are readily replicated when the turbulence rotates in the electron direction; our model calculation deals specifically with the toroidal electron drift waves (EDW) in the well-known \delta_e model. While the basic calculations are presented in parallel to the ITG counterpart, more emphasis is laid here on the motion of Instanton; several abrupt phenomena observed in tokamaks, such as intermittent excitation of geodesic acoustic mode (GAM) shown in this paper, could be attributed to the sudden and fast radial motion of Instanton. The calculation brings out the defining characteristics of the Instanton: it begins as a linear traveling wave right after the transition. Then, it evolves to a nonlinear stage with increasing frequency all the way to 20 kHz. The modulation to Reynolds stress in zonal flow equation will cause resonant excitation to GAM. The intermittency is shown due to the random phase mixing between multiple central rational surfaces in the reaction region.