The theoretical study on intermittency and propagation of geodesic acoustic mode in L- mode discharge near tokamak edge

TL;DR

This paper develops a theoretical framework linking instanton motion to intermittent excitation and propagation of geodesic acoustic modes (GAM) in tokamaks, revealing the physical mechanisms behind observed GAM intermittency.

Contribution

It introduces a unified theory that explains the physical mechanism of GAM intermittent excitation and propagation, incorporating full toroidal effects and identifying two zonal flow branches.

Findings

Identification of two zonal flow branches: TLFZF and GAM.

Correlation between instanton transition and GAM onset.

Reproduction of intermittent GAM features observed in experiments.

Abstract

Through a systematically developed theory, we demonstrate that the motion of instanton identified in [Y. Z. Zhang, Z. Y. Liu, T. Xie, S. M. Mahajan, and J. Liu, Physics of Plasmas 24, 122304 (2017)] is highly correlated to the intermittent excitation and propagation of geodesic acoustic mode (GAM) that are observed in tokamaks. While many numerical simulations have observed the phenomena, it is the first theory that reveals the physical mechanism behind GAM intermittent excitation and propagation. The preceding work is based on the micro-turbulence associated with toroidal ion temperature gradient (ITG) mode, and slab-based phenomenological model of zonal flow. When full toroidal effect are introduced into the system, two branches of zonal flow emerge: the torus-modified low frequency zonal flow (TLFZF), and GAM, necessitating a unified exploration of GAM and TLFZF. Indeed, we observe that the transition (decay) from the caviton to instanton is triggered by a rapid zero-crossing of radial group velocity of drift wave and is found to be strongly correlated with the GAM onset. Many features peculiar to intermittent GAMs, observed in real machines, are thus identified in the numerical experiment. The results will be displayed in figures and in a movie; first for single central rational surface, and then with coupled multiple central rational surfaces. The periodic bursting first shown disappears as being replaced by irregular one, more similar to the intermittent characteristics observed in GAM experiments.