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Title: Excitation of macromagnetohydrodynamic mode due to multiscale interaction in a quasi-steady equilibrium formed by a balance between microturbulence and zonal flow

Abstract

This is the first numerical simulation demonstrating that a macromagnetohydrodynamic (macro-MHD) mode is excited as a result of multi-scale interaction in a quasi-steady equilibrium formed by a balance between microturbulence and zonal flow based on a reduced two-fluid model. This simulation of a macro-MHD mode, a double tearing mode, is accomplished in a reversed shear equilibrium that includes zonal flow and turbulence due to kinetic ballooning modes. In the quasi-steady equilibrium, a macroscale fluctuation that has the same helicity as the double tearing mode is a part of the turbulence. After a certain period of time, the macro-MHD mode begins to grow. It effectively utilizes free energy of the equilibrium current density gradient and is destabilized by a positive feedback loop between zonal flow suppression and magnetic island growth. Thus, once the macro-MHD appears from the quasi-equilibrium, it continues to grow steadily. This simulation is more comparable with experimental observations of growing macro-MHD activity than earlier MHD simulations starting from linear macroinstabilities in a static equilibrium.

Authors:
;  [1]
  1. National Institute for Fusion Science, Toki 509-5292 (Japan)
Publication Date:
OSTI Identifier:
20974908
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 4; Other Information: DOI: 10.1063/1.2716669; (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; BALLOONING INSTABILITY; CURRENT DENSITY; EQUILIBRIUM; EQUILIBRIUM PLASMA; EXCITATION; FLUCTUATIONS; FREE ENERGY; INTERACTIONS; MAGNETIC ISLANDS; MAGNETOHYDRODYNAMICS; PLASMA SIMULATION; REVERSED SHEAR; TEARING INSTABILITY; TURBULENCE

Citation Formats

Ishizawa, A., and Nakajima, N. Excitation of macromagnetohydrodynamic mode due to multiscale interaction in a quasi-steady equilibrium formed by a balance between microturbulence and zonal flow. United States: N. p., 2007. Web. doi:10.1063/1.2716669.
Ishizawa, A., & Nakajima, N. Excitation of macromagnetohydrodynamic mode due to multiscale interaction in a quasi-steady equilibrium formed by a balance between microturbulence and zonal flow. United States. doi:10.1063/1.2716669.
Ishizawa, A., and Nakajima, N. Sun . "Excitation of macromagnetohydrodynamic mode due to multiscale interaction in a quasi-steady equilibrium formed by a balance between microturbulence and zonal flow". United States. doi:10.1063/1.2716669.
@article{osti_20974908,
title = {Excitation of macromagnetohydrodynamic mode due to multiscale interaction in a quasi-steady equilibrium formed by a balance between microturbulence and zonal flow},
author = {Ishizawa, A. and Nakajima, N.},
abstractNote = {This is the first numerical simulation demonstrating that a macromagnetohydrodynamic (macro-MHD) mode is excited as a result of multi-scale interaction in a quasi-steady equilibrium formed by a balance between microturbulence and zonal flow based on a reduced two-fluid model. This simulation of a macro-MHD mode, a double tearing mode, is accomplished in a reversed shear equilibrium that includes zonal flow and turbulence due to kinetic ballooning modes. In the quasi-steady equilibrium, a macroscale fluctuation that has the same helicity as the double tearing mode is a part of the turbulence. After a certain period of time, the macro-MHD mode begins to grow. It effectively utilizes free energy of the equilibrium current density gradient and is destabilized by a positive feedback loop between zonal flow suppression and magnetic island growth. Thus, once the macro-MHD appears from the quasi-equilibrium, it continues to grow steadily. This simulation is more comparable with experimental observations of growing macro-MHD activity than earlier MHD simulations starting from linear macroinstabilities in a static equilibrium.},
doi = {10.1063/1.2716669},
journal = {Physics of Plasmas},
number = 4,
volume = 14,
place = {United States},
year = {Sun Apr 15 00:00:00 EDT 2007},
month = {Sun Apr 15 00:00:00 EDT 2007}
}
  • The effect of zonal flow shear on the double tearing mode is investigated by solving the linear reduced two-fluid equations for the equilibrium including zonal flow. The zonal flow caused by microturbulence is obtained from nonlinear simulation results presented by A. Ishizawa and N. Nakajima [Phys. Plasmas 14, 040702 (2007)]. There is no clear evidence that could indicate whether the double tearing mode is stabilized or destabilized by the zonal flow.
  • During the past years the understanding of the multiscale interaction problems have increased significantly. However, at present there exists a range of different analytical models for investigating multiscale interactions and hardly any specific comparisons have been performed among these models. In this work, two different models for the generation of zonal flows from ion-temperature-gradient (ITG) background turbulence are discussed and compared. The methods used is the coherent mode coupling model and the wave kinetic equation model (WKE). It is shown that the two models qualitatively give the same results even though the assumption on the spectral difference is used inmore » the WKE approach.« less
  • Collisionless trapped ion modes (CTIMs) turbulence exhibits a rich variety of zonal flow physics. The coupling of CTIMs with shear flow driven by the Kelvin-Helmholtz (KH) instability has been investigated. The work explores the parametric excitation of zonal flow modified by wave-particle interactions leading to a new type of resonant low-frequency zonal flow. The KH-CTIM interaction on zonal flow growth and its feedback on turbulence is investigated using semi-Lagrangian gyrokinetic Vlasov simulations based on a Hamiltonian reduction technique, where both fast scales (cyclotron plus bounce motions) are gyro-averaged.
  • We address the mechanisms underlying low-frequency zonal flow generation in turbulent system and the associated intermittent regime of ion-temperature-gradient (ITG) turbulence. This model is in connection with the recent observation of quasi periodic zonal flow oscillation at a frequency close to 2 kHz, at the low-high transition, observed in the ASDEX Upgrade [Conway et al., Phys. Rev. Lett. 106, 065001 (2011)] and EAST tokamak [Xu et al., Phys. Rev. Lett 107, 125001 (2011)]. Turbulent bursts caused by the coupling of Kelvin-Helmholtz (KH) driven shear flows with trapped ion modes (TIMs) were investigated by means of reduced gyrokinetic simulations. It was foundmore » that ITG turbulence can be regulated by low-frequency meso-scale zonal flows driven by resonant collisionless trapped ion modes (CTIMs), through parametric-type scattering, a process in competition with the usual KH instability.« less
  • Global three-dimensional magnetohydrodynamic (MHD) simulations of turbulent accretion disks are presented which start from fully equilibrium initial conditions in which the magnetic forces are accounted for and the induction equation is satisfied. The local linear theory of the magnetorotational instability (MRI) is used as a predictor of the growth of magnetic field perturbations in the global simulations. The linear growth estimates and global simulations diverge when nonlinear motions-perhaps triggered by the onset of turbulence-upset the velocity perturbations used to excite the MRI. The saturated state is found to be independent of the initially excited MRI mode, showing that once themore » disk has expelled the initially net flux field and settled into quasi-periodic oscillations in the toroidal magnetic flux, the dynamo cycle regulates the global saturation stress level. Furthermore, time-averaged measures of converged turbulence, such as the ratio of magnetic energies, are found to be in agreement with previous works. In particular, the globally averaged stress normalized to the gas pressure <{alpha}{sub P}>bar = 0.034, with notably higher values achieved for simulations with higher azimuthal resolution. Supplementary tests are performed using different numerical algorithms and resolutions. Convergence with resolution during the initial linear MRI growth phase is found for 23-35 cells per scale height (in the vertical direction).« less