Stabilization of electron-scale turbulence by electron density gradient in national spherical torus experiment
Name
Anne White Ruiz - Phys of Plasmas 22 12 2015.pdf
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2.54 MB
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Author(s)
Kaye, S. M.
Leblanc, B. P.
Mazzucato, E.
Lee, K. C.
Domier, C. W.
Smith, D. R.
Yuh, H.
Ruiz Ruiz, Juan
Ren, Yang
Guttenfelder, Walter
Date Issued
December 2015
Journal
Physics of Plasmas
Publisher
American Institute of Physics (AIP)
Citation
Ruiz Ruiz, J., Y. Ren, W. Guttenfelder, A. E. White, S. M. Kaye, B. P. Leblanc, E. Mazzucato, et al. “Stabilization of Electron-Scale Turbulence by Electron Density Gradient in National Spherical Torus Experiment.” Physics of Plasmas 22, no. 12 (December 2015): 122501.
Version
Final published version
Abstract
Theory and experiments have shown that electron temperature gradient (ETG) turbulence on the electron gyro-scale, k⊥ρe ≲ 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.
MIT Department
Massachusetts Institute of Technology. Department of Aeronautics and Astronautics
Massachusetts Institute of Technology. Department of Nuclear Science and Engineering
Massachusetts Institute of Technology. Plasma Science and Fusion Center
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DOI of Published Version
https://doi.org/10.1063/1.4936110
