Observation of Coherent Sheared Turbulence Flows in the DIII-D Tokamak

University of Wisconsin-Madison, 1500 Engineering Drive, Madison, Wisconsin 53706-1687, USA.
Physical Review Letters (Impact Factor: 7.51). 12/2002; 89(26):265003. DOI: 10.1103/PhysRevLett.89.265003
Source: PubMed


Time-resolved measurements of the turbulent density flow field in a tokamak plasma reveal low-frequency ( approximately 15 KHz), coherent oscillations in the poloidal flow, v(theta). These flow oscillations have a long poloidal wavelength (m<3) and narrow radial extent (k(r)rho(i) approximately 0.2). The estimated flow-shearing rate is of the same order of magnitude as the turbulence decorrelation rate and may thus regulate the turbulence amplitude. These features are consistent with theoretically predicted axisymmetric, self-regulating, sheared flows recognized as geodesic acoustic modes.

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    ABSTRACT: Two-dimensional measurements of density fluctuations are obtained in the radial and poloidal plane of the DIII-D tokamak with the Beam Emission Spectroscopy (BES) diagnostic system. The goals are to visualize the spatial structure-and time evolution of turbulent eddies, as well as to obtain the 2D statistical properties of turbulence. The measurements are obtained with an array of localized BES spatial channels configured to image a midplane region of the plasma. 32 channels have been deployed, each with a spatial resolution of about 1 cm in the radial and poloidal directions, thus providing measurements of turbulence in the wave number range 0 < k(&BOTTOM;) &LE; 3 cm(-1). A 5 (radial) X 6 (poloidal) channel grid provides time-resolved images near the outer midplane at the sampling frequency of 1 MHz, thus providing a modest spatial resolution, high throughput, high time resolution turbulence imaging system. The images and resulting movies have broad application to a wide variety of fundamental turbulence studies: imaging of the highly complex, nonlinear turbulent eddy interactions, measurement of the 2D correlation function, and S(k(r),k(θ)) wave number spectra, and direct measurement of the equilibrium and time-dependent turbulence flow field. The time-dependent, two-dimensional turbulence velocity flow-field is obtained with time-delay-estimation techniques. (C) 2003 American Institute of Physics.
    Preview · Article · Mar 2003 · Review of Scientific Instruments
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    ABSTRACT: The application of time-delay-estimation techniques to two-dimensional measurements of density fluctuations, obtained with beam emission spectroscopy in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] plasmas, has provided temporally and spatially resolved measurements of the turbulence flow-field. Features that are characteristic of self-generated zonal flows are observed in the radial region 0.85less than or equal tor/aless than or equal to1.0. These features include a coherent oscillation (approximately 15 kHz) in the poloidal flow of density fluctuations that has a long poloidal wavelength, possibly m=0, narrow radial extent (k(r)rho(I)<0.2), and whose frequency varies monotonically with the local temperature. The approximate effective shearing rate, dv(theta)/dr, of the flow is of the same order of magnitude as the measured nonlinear decorrelation rate of the turbulence, and the density fluctuation amplitude is modulated at the frequency of the observed flow oscillation. Some phase coherence is observed between the higher wavenumber density fluctuations and low frequency poloidal flow fluctuations, suggesting a Reynolds stress contribution. These characteristics are consistent with predicted features of zonal flows, specifically identified as geodesic acoustic modes, observed in 3-D Braginskii simulations of core/edge turbulence. (C) 2003 American Institute of Physics.
    No preview · Article · May 2003 · Physics of Plasmas
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