Observations of Reduced Electron Gyroscale Fluctuations in National Spherical Torus Experiment<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>H</mml:mi></mml:math>-Mode Plasmas with Large<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>E</mml:mi><mml:mo>×</mml:mo><mml:mi>B</mml:mi></mml:math>Flow Shear

Smith, D. R.; Kaye, S. M.; Lee, William; Mazzucato, E.; Hk, Park; Bell, R. E.; Domier, C.W.; Bp, Leblanc; Fm, Levinton; Nc, Luhmann; Ménard, J.; Yuh, H.

doi:10.1103/physrevlett.102.225005

Cited by 42 publications

(30 citation statements)

References 32 publications

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“…22 These simulations illustrate that ETG transport is susceptible to suppression via E ϫ B shear, consistent with indications from recent turbulence measurements. 12 To address the second scenario, an additional simulation was run for = 30 using ␥ E ͑a / v te ͒ = 0.015, or equivalently, ␥ E ͑a / c s ͒ = 0.45͒. The resulting e is 0.934 GBi -about twice as large as the = 60 simulation but nearly identical in electron gyroBohm units: 28 GBe .…”

Section: B Reduced Ion Massmentioning

confidence: 99%

“…Because they are driven at high frequencies in the range v te / a, where a is the plasma minor radius, ETG modes are not expected to be completely suppressed by E ϫ B shear, although recent experimental evidence suggests that this is possible. 12 In addition to turbulence measurements, there has been some limited success in predicting electron temperature profiles using theory-based transport models [13][14][15] illustrating that ETG is likely a relevant transport mechanism, at least in some ST regimes. These two observations lead us to further pursue physically comprehensive ETG simulations for STs.…”

Section: Introductionmentioning

confidence: 99%

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Resolving electron scale turbulence in spherical tokamaks with flow shear

Guttenfelder

Candy

2011

Physics of Plasmas

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This paper presents nonlinear gyrokinetic simulations of electron temperature gradient (ETG) turbulence based on spherical tokamak (ST) parameters. Most significantly the simulations include the strong toroidal flow and flow shear present in STs that suppress ion-scale turbulence while using kinetic ions at full mass ratio (m(i)/m(e) = 3600). The flow shear provides a physical long-wavelength cutoff mechanism that aids saturation of the simulations, which has previously been demonstrated to be problematic depending on magnetic shear. As magnetic shear varies widely in STs we systematically demonstrate saturation and convergence of the ETG simulations with respect to grid resolution, physical domain size, and boundary conditions. While using reduced ion mass or adiabatic ions can lessen computational expense they do not always provide reliable results. The resulting spectra from converged simulations are anisotropic everywhere in contrast to previous ETG simulations without flow shear. These results have implications for interpreting turbulence measurements, and represent an important step in determining when and where ETG turbulence is expected to be relevant in ST plasmas. They are also important in the context of validating simulations with both experimental transport analysis and turbulence measurements. (C) 2011 American Institute of Physics. [doi:10.1063/1.3551701

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Section: B Reduced Ion Massmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Resolving electron scale turbulence in spherical tokamaks with flow shear

Guttenfelder

Candy

2011

Physics of Plasmas

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“…Several mechanisms have recently been reported to affect ETG turbulence. The E Â B shear has been observed to reduce electron scale turbulence (measured with a high-k scattering system 1,14,15 ). Negative magnetic shear configurations can also affect ETG turbulence through the formation of internal transport barriers as reported in Refs.…”

Section: Introductionmentioning

confidence: 99%

Stabilization of electron-scale turbulence by electron density gradient in national spherical torus experiment

et al. 2015

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Theory and experiments have shown that electron temperature gradient (ETG) turbulence on the electron gyro-scale, k ? q e Շ 1, can be responsible for anomalous electron thermal transport in NSTX. Electron scale (high-k) turbulence is diagnosed in NSTX with a high-k microwave scattering system [D. R. Smith et al., Rev. Sci. Instrum. 79, 123501 (2008)]. Here we report on stabilization effects of the electron density gradient on electron-scale density fluctuations in a set of neutral beam injection heated H-mode plasmas. We found that the absence of high-k density fluctuations from measurements is correlated with large equilibrium density gradient, which is shown to be consistent with linear stabilization of ETG modes due to the density gradient using the analytical ETG linear threshold in F. Jenko et al. [Phys. Plasmas 8, 4096 (2001)] and linear gyrokinetic simulations with GS2 [M. Kotschenreuther et al., Comput. Phys. Commun. 88, 128 (1995)]. We also found that the observed power of electron-scale turbulence (when it exists) is anti-correlated with the equilibrium density gradient, suggesting density gradient as a nonlinear stabilizing mechanism. Higher density gradients give rise to lower values of the plasma frame frequency, calculated based on the Doppler shift of the measured density fluctuations. Linear gyrokinetic simulations show that higher values of the electron density gradient reduce the value of the real frequency, in agreement with experimental observation. Nonlinear electron-scale gyrokinetic simulations show that high electron density gradient reduces electron heat flux and stiffness, and increases the ETG nonlinear threshold, consistent with experimental observations. V C 2015 AIP Publishing LLC.

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“…This discrepancy is correlated with the absence of ion-scale, (k h q s < 1.0) Trapped Electron Mode (TEM) turbulence and is thought to be linked to electron-scale turbulence (high-k TEM and Electron Temperature Gradient (ETG)) not included in standard low-k (ion-scale) gyrokinetic simulation. 4,12 There exists in the literature both experimental 13,14 and computational 10,[15][16][17][18] evidence of electron-scale turbulence in tokamaks. However, the exact role of electron-scale turbulence in conventional tokamak discharges is not well understood, as simulation work generally focuses on single-scale simulation that covers the electron scale, while ignoring long wavelength, ion-scale turbulence.…”

Section: Introductionmentioning

confidence: 99%

Multi-scale gyrokinetic simulation of Alcator C-Mod tokamak discharges

et al. 2014

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Alcator C-Mod tokamak discharges have been studied with nonlinear gyrokinetic simulation simultaneously spanning both ion and electron spatiotemporal scales. These multi-scale simulations utilized the gyrokinetic model implemented by GYRO code [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] and the approximation of reduced electron mass (l ¼ (m D /m e ) .5 ¼ 20.0) to qualitatively study a pair of Alcator C-Mod discharges: a low-power discharge, previously demonstrated (using realistic mass, ion-scale simulation) to display an under-prediction of the electron heat flux and a high-power discharge displaying agreement with both ion and electron heat flux channels [N. T. Howard et al., Nucl. Fusion 53, 123011 (2013)]. These multi-scale simulations demonstrate the importance of electron-scale turbulence in the core of conventional tokamak discharges and suggest it is a viable candidate for explaining the observed under-prediction of electron heat flux. In this paper, we investigate the coupling of turbulence at the ion (k h q s $ Oð1:0Þ) and electron (k h q e $ Oð1:0Þ) scales for experimental plasma conditions both exhibiting strong (high-power) and marginally stable (low-power) low-k (k h q s < 1.0) turbulence. It is found that reduced mass simulation of the plasma exhibiting marginally stable low-k turbulence fails to provide even qualitative insight into the turbulence present in the realistic plasma conditions. In contrast, multi-scale simulation of the plasma condition exhibiting strong turbulence provides valuable insight into the coupling of the ion and electron scales. V C 2014 AIP Publishing LLC.

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Observations of Reduced Electron Gyroscale Fluctuations in National Spherical Torus ExperimentH-Mode Plasmas with LargeE×BFlow Shear

Cited by 42 publications

References 32 publications

Resolving electron scale turbulence in spherical tokamaks with flow shear

Resolving electron scale turbulence in spherical tokamaks with flow shear

Stabilization of electron-scale turbulence by electron density gradient in national spherical torus experiment

Multi-scale gyrokinetic simulation of Alcator C-Mod tokamak discharges

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