Measurement and physical interpretation of the mean motion of turbulent density patterns detected by the beam emission spectroscopy system on the mega amp spherical tokamak

Ghim, Y-c; Field, A. R.; Dunai, D.; Zoletnik, S.; Bardóczi, L.; Schekochihin, A. A.

doi:10.1088/0741-3335/54/9/095012

Cited by 28 publications

(106 citation statements)

References 55 publications

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“…To estimate the correlation time τ c , we use the fact that the fluctuating density patterns are advected poloidally past the BES array with an apparent velocity v BES = U φ tan α due to the toroidal rotation velocity U φ [29]. We fit C (∆x = 0, ∆Z, ∆t = ∆t peak (∆Z)) taken at the time delay ∆t peak (∆Z) when the correlation function is maximum at a given ∆Z [30], to the function f τ (∆Z) = exp [− |∆t peak (∆Z)| /τ c ].…”

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confidence: 99%

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Experimental Signatures of Critically Balanced Turbulence in MAST

et al. 2013

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Beam Emission Spectroscopy (BES) measurements of ion-scale density fluctuations in the MAST tokamak are used to show that the turbulence correlation time, the drift time associated with ion temperature or density gradients, the particle (ion) streaming time along the magnetic field and the magnetic drift time are consistently comparable, suggesting a "critically balanced" turbulence determined by the local equilibrium. The resulting scalings of the poloidal and radial correlation lengths are derived and tested. The nonlinear time inferred from the density fluctuations is longer than the other times; its ratio to the correlation time scales as ν −0.8±0.1 * i , where ν * i = ion collision rate/streaming rate. This is consistent with turbulent decorrelation being controlled by a zonal component, invisible to the BES, with an amplitude exceeding the drift waves' by ∼ ν −0.8 * i .Introduction. Microscale turbulence hindering energy confinement in magnetically confined hot plasmas is driven by gradients of equilibrium quantities such as temperature and density. These gradients give rise to instabilities that inject energy into plasma fluctuations ("drift waves") at scales just above the ion Larmor scale. The most effective of these is believed to be the iontemperature-gradient (ITG) instability [1][2][3]. A turbulent state ensues, giving rise to "anomalous transport" of energy [4]. It is of interest, both for practical considerations of improving confinement and for the fundamental understanding of multiscale plasma dynamics, what the structure of this turbulence is and how its amplitude, scale(s) and resulting transport depend on the equilibrium parameters: ion and electron temperatures, density, angular velocity, magnetic geometry, etc.Fluctuations in a magnetized toroidal plasma are subject to a number of distinct physical effects, which can be thought about in terms of various time scales such as the drift times associated with the temperature and density gradients, the particle streaming time along the magnetic field as it takes them around the torus toroidally and poloidally, the magnetic (∇B and curvature) drift times of particles moving across the field, the nonlinear time of the fluctuations being advected across the field by the fluctuating E × B velocity, the time between collisions, the shear time associated with plasma rotation. Some of these time scales and, consequently, the corresponding physics may be irrelevant, while others play a crucial role for the saturation of the linearly unstable fluctuations. There has been a growing understanding [5], driven largely by theory [6][7][8][9], observations [10][11][12] and simulations of magnetohydrodynamic [13][14][15] and kinetic [7,16] plasma turbulence in space, that if a medium can

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confidence: 99%

“…The last two exclusion criteria pick out the cases when MHD modes are too strong; they are known to degrade the reliability of the BES data [29]. The remaining database contains 448 points.…”

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Experimental Signatures of Critically Balanced Turbulence in MAST

et al. 2013

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“…are the turbulence length scales parallel and perpendicular, respectively, to the magnetic field line within the flux surface). The poloidal velocity of the elongated turbulent structures measured by multichannel diagnostics (dubbed as "apparent poloidal velocity" in this paper) is given by 17,18 …”

Section: E 3 B Flow Velocity Deduced From the Apparent Poloidal Rmentioning

confidence: 99%

“…The meaning of the poloidal motion of turbulent structures measured by multichannel diagnostics is more precisely explained in Ref. 18. To determine the apparent poloidal velocity from time-dependent multichannel data, a time delayed cross correlation (TDCC) analysis is applied.…”

Section: E 3 B Flow Velocity Deduced From the Apparent Poloidal Rmentioning

confidence: 99%

E × B flow velocity deduced from the poloidal motion of fluctuation patterns in neutral beam injected L-mode plasmas on KSTAR

et al. 2016

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Articles you may be interested inA method for direct assessment of the equilibrium E Â B flow velocity (E Â B flow shear is responsible for the turbulence suppression and transport reduction in tokamak plasmas) is investigated based on two facts. The first one is that the apparent poloidal rotation speed of density fluctuation patterns is close to the turbulence rotation speed in the direction perpendicular to the local magnetic field line within the flux surface. And the second "well-known" fact is that the turbulence rotation velocity consists of the equilibrium E Â B flow velocity and intrinsic phase velocity of turbulence in the E Â B flow frame. In the core region of the low confinement (L-mode) discharges where a strong toroidal rotation is induced by neutral beam injection, the apparent poloidal velocities (and turbulence rotation velocities) are good approximations of the E Â B flow velocities since linear gyrokinetic simulations suggest that the intrinsic phase velocity of the dominant turbulence is significantly lower than the apparent poloidal velocity. In the neutral beam injected L-mode plasmas, temporal and spatial scales of the measured turbulence are studied by comparing with the local equilibrium parameters relevant to the ion-scale turbulence. Published by AIP Publishing. [http://dx

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“…16,38 This occurs when the blobs are elliptical and not circular, and are moving at least somewhat radially. Shape does not affect measurements when the blobs move only vertically.…”

Section: A Fourier Techniquementioning

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Comparison of velocimetry techniques for turbulent structures in gas-puff imaging data

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Cziegler²,

Terry

et al. 2016

Review of Scientific Instruments

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Recent analysis of Gas Puff Imaging (GPI) data from Alcator C-Mod found blob velocities with a modified tracking time delay estimation (TDE). These results disagree with velocity analysis performed using direct Fourier methods. In this paper, the two analysis methods are compared. The implementations of these methods are explained, and direct comparisons using the same GPI data sets are presented to highlight the discrepancies in measured velocities. In order to understand the discrepancies, we present a code that generates synthetic sequences of images that mimic features of the experimental GPI images, with user-specified input values for structure (blob) size and velocity. This allows quantitative comparison of the TDE and Fourier analysis methods, which reveals their strengths and weaknesses. We found that the methods agree for structures of any size as long as all structures move at the same velocity and disagree when there is significant nonlinear dispersion or when structures appear to move in opposite directions. Direct Fourier methods used to extract poloidal velocities give incorrect results when there is a significant radial velocity component and are subject to the barber pole effect. Tracking TDE techniques give incorrect velocity measurements when there are features moving at significantly different speeds or in different directions within the same field of view. Finally, we discuss the limitations and appropriate use of each of methods and applications to the relationship between blob size and velocity. C 2016 AIP Publishing LLC. [http://dx

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Measurement and physical interpretation of the mean motion of turbulent density patterns detected by the beam emission spectroscopy system on the mega amp spherical tokamak

Cited by 28 publications

References 55 publications

Experimental Signatures of Critically Balanced Turbulence in MAST

Experimental Signatures of Critically Balanced Turbulence in MAST

E × B flow velocity deduced from the poloidal motion of fluctuation patterns in neutral beam injected L-mode plasmas on KSTAR

Comparison of velocimetry techniques for turbulent structures in gas-puff imaging data

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