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

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Chinese Physics B 2022年第4期31卷 449-467页

作者：刘朝阳章扬忠 Swadesh Mitter Mahajan 刘阿娣周楚谢涛 Institute for Fusion Theory and Simulation and Department of Physics Zhejiang UniversityHangzhou 310027China Center for Magnetic Fusion Theory Chinese Academy of SciencesHefei 230026China Institute for Fusion Studies University of Texas at AustinAustinTexas 78712USA School of Nuclear Science and Technology University of Science and Technology of ChinaHefei 230026China School of Science Sichuan University of Science and EngineeringZigong 643000China

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.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 composite drift wave-zonal flow system evolves at the two well-separate scales:the micro-scale and the meso-scale.The eigenmode equation of the model defines the zero-order(micro-scale) variation;it is solved by making use of the two-dimensional(2 D) weakly asymmetric ballooning theory(WABT),a theory suitable for modes localized to rational surface like drift waves,and then refined by shifted inverse power method,an iterative finite difference method.The next order is the equation of electron drift wave(EDW) envelope(containing group velocity of EDW) which is modulated by the zonal flow generated by Reynolds stress of EDW.This equation is coupled to the zonal flow equation,and numerically solved in spatiotemporal representation;the results are displayed in self-explanatory graphs.One observes a strong correlation between the Caviton-Instanton transition and the zero-crossing of radial group velocity of EDW.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 brought in by the nonlinear Instanton 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.

关键词： zonal flow drift wave Instanton L-H mode

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

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Plasma Science and Technology 2021年第3期23卷 30-50页

作者：刘朝阳章扬忠 Swadesh Mitter Mahajan 刘阿娣谢涛周楚兰涛谢锦林李弘庄革刘万东 School of Nuclear Science and Technology University of Science and Technology of ChinaHefei 230026People's Republic of China Center for Magnetic Fusion Theory Chinese Academy of SciencesHefei 230026People's Republic of China Institute for Fusion Studies University of Texas at AustinAustinTX 78712United States of America School of Science Sichuan University of Science and EngineeringZigong 643000People's Republic of China

Through a systematically developed theory,we demonstrate that the motion of Instanton identified in Zhang et al(2017 Phys.Plasmas 24122304)is highly correlated to the intermittent excitation and propagation of geodesic acoustic mode(GAM)that is 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 mode,and slab-based phenomenological model of zonal flow.When full toroidal effect is 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 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.

关键词： zonal flow drift wave intermittency GAM L–H mode

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