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Title: Nonlinear excitation of geodesic acoustic modes by drift waves

Abstract

In this paper, two mode-coupling analyses for the nonlinear excitation of the geodesic acoustic modes (GAMs) in tokamak plasmas by drift waves are presented. The first approach is a coherent parametric process, which leads to a three-wave resonant interaction. This investigation allows for the drift waves and the GAMs to have comparable scales. The second approach uses the wave-kinetic equations for the drift waves, which then couples to the GAMs. This requires that the GAM scale length be large compared to the wave packet associated with the drift waves. The resonance conditions for these two cases lead to specific predictions of the radial wave number of the excited GAMs.

Authors:
; ; ;  [1];  [2];  [3]
  1. Saha Institute of Nuclear Research, 1/AF Bidhannagar, Kolkata 700064 (India)
  2. (India)
  3. (United States)
Publication Date:
OSTI Identifier:
20974978
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2732167; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; EXCITATION; FORECASTING; INTERACTIONS; KINETIC EQUATIONS; NONLINEAR PROBLEMS; PARAMETRIC INSTABILITIES; PLASMA; PLASMA DRIFT; RESONANCE; TOKAMAK DEVICES; WAVE PACKETS; WAVE PROPAGATION

Citation Formats

Chakrabarti, N., Singh, R., Kaw, P. K., Guzdar, P. N., Institute for Plasma Research, Bhat, Gandhinagar, 382428, and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742. Nonlinear excitation of geodesic acoustic modes by drift waves. United States: N. p., 2007. Web. doi:10.1063/1.2732167.
Chakrabarti, N., Singh, R., Kaw, P. K., Guzdar, P. N., Institute for Plasma Research, Bhat, Gandhinagar, 382428, & Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742. Nonlinear excitation of geodesic acoustic modes by drift waves. United States. doi:10.1063/1.2732167.
Chakrabarti, N., Singh, R., Kaw, P. K., Guzdar, P. N., Institute for Plasma Research, Bhat, Gandhinagar, 382428, and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742. Tue . "Nonlinear excitation of geodesic acoustic modes by drift waves". United States. doi:10.1063/1.2732167.
@article{osti_20974978,
title = {Nonlinear excitation of geodesic acoustic modes by drift waves},
author = {Chakrabarti, N. and Singh, R. and Kaw, P. K. and Guzdar, P. N. and Institute for Plasma Research, Bhat, Gandhinagar, 382428 and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742},
abstractNote = {In this paper, two mode-coupling analyses for the nonlinear excitation of the geodesic acoustic modes (GAMs) in tokamak plasmas by drift waves are presented. The first approach is a coherent parametric process, which leads to a three-wave resonant interaction. This investigation allows for the drift waves and the GAMs to have comparable scales. The second approach uses the wave-kinetic equations for the drift waves, which then couples to the GAMs. This requires that the GAM scale length be large compared to the wave packet associated with the drift waves. The resonance conditions for these two cases lead to specific predictions of the radial wave number of the excited GAMs.},
doi = {10.1063/1.2732167},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • Geodesic Acoustic Modes (GAMs) have been predicted and subsequently observed in many toroidal plasma devices. Bicoherence studies on various devices have suggested three-wave mode coupling processes between GAMs and high frequency turbulence. Thus the parametric coupling of GAMS to drift waves and/or ion temperature gradient(ITG{r_brace} modes is a potential candidate for excitation of these modes. In this paper we discuss the resonant three-wave coupling mechanism for the excitation of GAMs by ITG and finite beta drift waves in homogeneous and inhomogeneous plasmas and compare theoretical predictions with observed characteristics of the GAMs.
  • Effects of system nonuniformities and kinetic dispersiveness on the spontaneous excitation of Geodesic Acoustic Mode (GAM) by Drift Wave (DW) turbulence are investigated based on nonlinear gyrokinetic theory. The coupled nonlinear equations describing parametric decay of DW into GAM and DW lower sideband are derived and then solved both analytically and numerically to investigate the effects on the parametric decay process due to system nonuniformities, such as nonuniform diamagnetic frequency, finite radial envelope of DW pump, and kinetic dispersiveness. It is found that the parametric decay process is a convective instability for typical tokamak parameters when finite group velocities ofmore » DW and GAM associated with kinetic dispersiveness and finite radial envelope are taken into account. When, however, nonuniformity of diamagnetic frequency is taken into account, the parametric decay process becomes, time asymptotically, a quasi-exponentially growing absolute instability.« less
  • Presented in this paper is a mode-coupling analysis for the nonlinear excitation of the geodesic acoustic modes (GAMs) in tokamak plasmas by finite beta drift waves. The finite beta effects give rise to a strong stabilizing influence on the parametric excitation process. The dominant finite beta effect is the combination of the Maxwell stress, which has a tendency to cancel the primary drive from the Reynolds stress, and the finite beta modification of the drift waves. The zonal magnetic field is also excited at the GAM frequency. However, it does not contribute to the overall stability of the three-wave processmore » for parameters of relevance to the edge region of tokamaks.« less
  • The geodesic acoustic modes (GAMs) are typically observed in the edge region of toroidal plasmas. Drift waves have been identified as a possible cause of excitation of GAMs by a resonant three wave parametric process. A nonlocal theory of excitation of these modes in inhomogeneous plasmas typical of the edge region of tokamaks is presented in this paper. The continuum GAM modes with coupling to the drift waves can create discrete ''global'' unstable eigenmodes localized in the edge 'pedestal' region of the plasma. Multiple resonantly driven unstable radial eigenmodes can coexist on the edge pedestal.
  • A two fluid model with parallel viscosity is employed to derive the dispersion relation for electromagnetic geodesic acoustic modes (GAMs) in the presence of drift (diamagnetic) effects. Concerning the influence of the electron dynamics on the high frequency GAM, it is shown that the frequency of the electromagnetic GAM is independent of the equilibrium parallel current but, in contrast with purely electrostatic GAMs, significantly depends on the electron temperature gradient. The electromagnetic GAM may explain the discrepancy between the f ∼ 40 kHz oscillation observed in tokamak TCABR [Yu. K. Kuznetsov et al., Nucl. Fusion 52, 063044 (2012)] and the former prediction formore » the electrostatic GAM frequency. The radial wave length associated with this oscillation, estimated presently from this analytical model, is λ{sub r} ∼ 25 cm, i.e., an order of magnitude higher than the usual value for zonal flows (ZFs)« less