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

Theory and experiments have shown that electron temperature gradient (ETG) turbulence on the electron gyro-scale, k{sub ⊥}ρ{sub e} ≲ 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. Linearmore » 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.« less
Authors:
;  [1] ; ; ; ; ;  [2] ;  [3] ;  [4] ;  [5] ;  [6]
  1. MIT-Plasma Science and Fusion Center, Cambridge, Massachusetts 02139 (United States)
  2. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543 (United States)
  3. National Fusion Research Institute, Daejeon (Korea, Republic of)
  4. University of California at Davis, Davis, California 95616 (United States)
  5. University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
  6. Nova Photonics, Inc., Princeton, New Jersey 08540 (United States)
Publication Date:
OSTI Identifier:
22489923
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 22; Journal Issue: 12; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; BEAM INJECTION; DOPPLER EFFECT; ELECTRON DENSITY; ELECTRON TEMPERATURE; FLEXIBILITY; FLUCTUATIONS; HEAT FLUX; HEAT TRANSFER; H-MODE PLASMA CONFINEMENT; MICROWAVE RADIATION; NSTX DEVICE; PLASMA; PLASMA SIMULATION; SCATTERING; SPHERICAL CONFIGURATION; TEMPERATURE GRADIENTS; TURBULENCE