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Title: Experimental study of parametric dependence of electron-scale turbulence in a spherical tokamak

Abstract

Electron-scale turbulence is predicted to drive anomalous electron thermal transport. However, experimental study of its relation with transport is still in its early stage. On the National Spherical Tokamak Experiment (NSTX), electron-scale density fluctuations are studied with a novel tangential microwave scattering system with high radial resolution of {+-}2 cm. Here, we report a study of parametric dependence of electron-scale turbulence in NSTX H-mode plasmas. The dependence on density gradient is studied through the observation of a large density gradient variation in the core induced by an edge localized mode (ELM) event, where we found the first clear experimental evidence of density gradient stabilization of electron-gyro scale turbulence in a fusion plasma. This observation, coupled with linear gyro-kinetic calculations, leads to the identification of the observed instability as toroidal electron temperature gradient (ETG) modes. It is observed that longer wavelength ETG modes, k{sub Up-Tack }{rho}{sub s} Less-Than-Or-Equivalent-To 10 ({rho}{sub s} is the ion gyroradius at electron temperature and k{sub Up-Tack} is the wavenumber perpendicular to local equilibrium magnetic field), are most stabilized by density gradient, and the stabilization is accompanied by about a factor of two decrease in electron thermal diffusivity. Comparisons with nonlinear ETG gyrokinetic simulations show ETG turbulencemore » may be able to explain the experimental electron heat flux observed before the ELM event. The collisionality dependence of electron-scale turbulence is also studied by systematically varying plasma current and toroidal field, so that electron gyroradius ({rho}{sub e}), electron beta ({beta}{sub e}), and safety factor (q{sub 95}) are kept approximately constant. More than a factor of two change in electron collisionality, {nu}{sub e}{sup *}, was achieved, and we found that the spectral power of electron-scale turbulence appears to increase as {nu}{sub e}{sup *} is decreased in this collisonality scan. However, both linear and nonlinear simulations show no or weak dependence with the electron-ion collision frequency, {nu}{sup e/i}. Instead, other equilibrium parameters (safety factor, electron density gradient, for example) affect ETG linear growth rate and electron thermal transport more than {nu}{sup e/i} does. Furthermore, electron heat flux predicted by the simulations is found to have an order-of-magnitude spatial variation in the experimental measurement region and is also found to be much smaller than experimental levels except at one radial location we evaluated. The predicted electron heat flux is shown to be strongly anti-correlated with density gradient, which varies for a factor of three in the measurement region, which is in agreement with the density gradient dependence study reported in this paper.« less

Authors:
; ; ; ; ; ;  [1]; ;  [2];  [3];  [4]
  1. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543 (United States)
  2. University of California at Davis, Davis, California 95616 (United States)
  3. University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
  4. Nova Photonics, Inc., Princeton, New Jersey 08540 (United States)
Publication Date:
OSTI Identifier:
22072403
Resource Type:
Journal Article
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 19; Journal Issue: 5; Other Information: (c) 2012 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 1070-664X
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; DENSITY; EDGE LOCALIZED MODES; ELECTRIC CURRENTS; ELECTRON DENSITY; ELECTRON TEMPERATURE; ELECTRON-ION COLLISIONS; ELECTRONS; FLUCTUATIONS; HEAT FLUX; H-MODE PLASMA CONFINEMENT; MAGNETIC FIELDS; MICROWAVE RADIATION; NONLINEAR PROBLEMS; NSTX DEVICE; PLASMA SIMULATION; RESOLUTION; TEMPERATURE GRADIENTS; TURBULENCE; WAVELENGTHS

Citation Formats

Ren, Y, Guttenfelder, W, Kaye, S M, Mazzucato, E, Bell, R E, Diallo, A, LeBlanc, B P, Domier, C W, Lee, K C, Smith, D R, and Yuh, H. Experimental study of parametric dependence of electron-scale turbulence in a spherical tokamak. United States: N. p., 2012. Web. doi:10.1063/1.4719689.
Ren, Y, Guttenfelder, W, Kaye, S M, Mazzucato, E, Bell, R E, Diallo, A, LeBlanc, B P, Domier, C W, Lee, K C, Smith, D R, & Yuh, H. Experimental study of parametric dependence of electron-scale turbulence in a spherical tokamak. United States. doi:10.1063/1.4719689.
Ren, Y, Guttenfelder, W, Kaye, S M, Mazzucato, E, Bell, R E, Diallo, A, LeBlanc, B P, Domier, C W, Lee, K C, Smith, D R, and Yuh, H. Tue . "Experimental study of parametric dependence of electron-scale turbulence in a spherical tokamak". United States. doi:10.1063/1.4719689.
@article{osti_22072403,
title = {Experimental study of parametric dependence of electron-scale turbulence in a spherical tokamak},
author = {Ren, Y and Guttenfelder, W and Kaye, S M and Mazzucato, E and Bell, R E and Diallo, A and LeBlanc, B P and Domier, C W and Lee, K C and Smith, D R and Yuh, H},
abstractNote = {Electron-scale turbulence is predicted to drive anomalous electron thermal transport. However, experimental study of its relation with transport is still in its early stage. On the National Spherical Tokamak Experiment (NSTX), electron-scale density fluctuations are studied with a novel tangential microwave scattering system with high radial resolution of {+-}2 cm. Here, we report a study of parametric dependence of electron-scale turbulence in NSTX H-mode plasmas. The dependence on density gradient is studied through the observation of a large density gradient variation in the core induced by an edge localized mode (ELM) event, where we found the first clear experimental evidence of density gradient stabilization of electron-gyro scale turbulence in a fusion plasma. This observation, coupled with linear gyro-kinetic calculations, leads to the identification of the observed instability as toroidal electron temperature gradient (ETG) modes. It is observed that longer wavelength ETG modes, k{sub Up-Tack }{rho}{sub s} Less-Than-Or-Equivalent-To 10 ({rho}{sub s} is the ion gyroradius at electron temperature and k{sub Up-Tack} is the wavenumber perpendicular to local equilibrium magnetic field), are most stabilized by density gradient, and the stabilization is accompanied by about a factor of two decrease in electron thermal diffusivity. Comparisons with nonlinear ETG gyrokinetic simulations show ETG turbulence may be able to explain the experimental electron heat flux observed before the ELM event. The collisionality dependence of electron-scale turbulence is also studied by systematically varying plasma current and toroidal field, so that electron gyroradius ({rho}{sub e}), electron beta ({beta}{sub e}), and safety factor (q{sub 95}) are kept approximately constant. More than a factor of two change in electron collisionality, {nu}{sub e}{sup *}, was achieved, and we found that the spectral power of electron-scale turbulence appears to increase as {nu}{sub e}{sup *} is decreased in this collisonality scan. However, both linear and nonlinear simulations show no or weak dependence with the electron-ion collision frequency, {nu}{sup e/i}. Instead, other equilibrium parameters (safety factor, electron density gradient, for example) affect ETG linear growth rate and electron thermal transport more than {nu}{sup e/i} does. Furthermore, electron heat flux predicted by the simulations is found to have an order-of-magnitude spatial variation in the experimental measurement region and is also found to be much smaller than experimental levels except at one radial location we evaluated. The predicted electron heat flux is shown to be strongly anti-correlated with density gradient, which varies for a factor of three in the measurement region, which is in agreement with the density gradient dependence study reported in this paper.},
doi = {10.1063/1.4719689},
journal = {Physics of Plasmas},
issn = {1070-664X},
number = 5,
volume = 19,
place = {United States},
year = {2012},
month = {5}
}