skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Response of multiscale turbulence to electron cyclotron heating in the DIII-D tokamak

Abstract

Small-scale density turbulence (k{sub perpendicular}{rho}{sub i}{approx}4-10) and electron thermal flux are both observed to increase during electron cyclotron heating (ECH) of a high-temperature tokamak plasma (k{sub perpendicular}) is the turbulent wavenumber and {rho}{sub i} the ion gyroradius). In contrast, large- (k{sub perpendicular}{rho}{sub i}{<=}1) and intermediate-scale (k{sub perpendicular}){rho}{sub i}{approx}1-3) turbulence n-tilde/n levels and ion thermal transport remain effectively constant. This implies that the small-scale turbulence is not a remnant or tail of the ubiquitous, large-scale or intermediate-scale turbulence, and also indicates a potentially important role in determining anomalous electron thermal transport. Radial scans of small-scale turbulence during ECH indicate decreased fluctuations in the deep core compared with increased levels towards the edge. This trend is consistent with linear gyrokinetic growth rate predictions for electron temperature gradient driven instabilities.

Authors:
; ; ; ; ;  [1]; ; ; ; ; ; ;  [2];  [3]
  1. University of California-Los Angeles, Los Angeles, California 90095 (United States)
  2. General Atomics, San Diego, California 92186 (United States)
  3. University of Texas-Austin, Austin, Texas 78712 (United States)
Publication Date:
OSTI Identifier:
20975055
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2714019; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; BOUNDARY LAYERS; DOUBLET-3 DEVICE; ECR HEATING; ELECTRON TEMPERATURE; ELECTRONS; FLUCTUATIONS; ION TEMPERATURE; IONS; PLASMA; PLASMA CONFINEMENT; PLASMA DENSITY; PLASMA INSTABILITY; RF SYSTEMS; TEMPERATURE GRADIENTS; TEMPERATURE RANGE 0400-1000 K; TURBULENCE

Citation Formats

Rhodes, T. L., Peebles, W. A., Nguyen, X. V., Doyle, E. J., Zeng, L., Wang, G., Van Zeeland, M. A., De Grassie, J. S., Burrell, K. H., DeBoo, J. C., Lohr, J., Petty, C. C., Greenfield, C. M., and Bravenec, R. V.. Response of multiscale turbulence to electron cyclotron heating in the DIII-D tokamak. United States: N. p., 2007. Web. doi:10.1063/1.2714019.
Rhodes, T. L., Peebles, W. A., Nguyen, X. V., Doyle, E. J., Zeng, L., Wang, G., Van Zeeland, M. A., De Grassie, J. S., Burrell, K. H., DeBoo, J. C., Lohr, J., Petty, C. C., Greenfield, C. M., & Bravenec, R. V.. Response of multiscale turbulence to electron cyclotron heating in the DIII-D tokamak. United States. doi:10.1063/1.2714019.
Rhodes, T. L., Peebles, W. A., Nguyen, X. V., Doyle, E. J., Zeng, L., Wang, G., Van Zeeland, M. A., De Grassie, J. S., Burrell, K. H., DeBoo, J. C., Lohr, J., Petty, C. C., Greenfield, C. M., and Bravenec, R. V.. Tue . "Response of multiscale turbulence to electron cyclotron heating in the DIII-D tokamak". United States. doi:10.1063/1.2714019.
@article{osti_20975055,
title = {Response of multiscale turbulence to electron cyclotron heating in the DIII-D tokamak},
author = {Rhodes, T. L. and Peebles, W. A. and Nguyen, X. V. and Doyle, E. J. and Zeng, L. and Wang, G. and Van Zeeland, M. A. and De Grassie, J. S. and Burrell, K. H. and DeBoo, J. C. and Lohr, J. and Petty, C. C. and Greenfield, C. M. and Bravenec, R. V.},
abstractNote = {Small-scale density turbulence (k{sub perpendicular}{rho}{sub i}{approx}4-10) and electron thermal flux are both observed to increase during electron cyclotron heating (ECH) of a high-temperature tokamak plasma (k{sub perpendicular}) is the turbulent wavenumber and {rho}{sub i} the ion gyroradius). In contrast, large- (k{sub perpendicular}{rho}{sub i}{<=}1) and intermediate-scale (k{sub perpendicular}){rho}{sub i}{approx}1-3) turbulence n-tilde/n levels and ion thermal transport remain effectively constant. This implies that the small-scale turbulence is not a remnant or tail of the ubiquitous, large-scale or intermediate-scale turbulence, and also indicates a potentially important role in determining anomalous electron thermal transport. Radial scans of small-scale turbulence during ECH indicate decreased fluctuations in the deep core compared with increased levels towards the edge. This trend is consistent with linear gyrokinetic growth rate predictions for electron temperature gradient driven instabilities.},
doi = {10.1063/1.2714019},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • The multi-field/multi-scale core ({rho}{approx} 0.5-0.8) turbulence response to electron cyclotron heating (ECH) of DIII-D Ohmic plasmas is reported for the first time. Long wavelength (low-k) electron temperature (T-tilde{sub e}/T{sub e}) and high-k density turbulence levels (n-tilde{sub e}/n{sub e}) are observed to strongly increase during ECH. In contrast, low-k and intermediate-k n-tilde{sub e}/n{sub e} showed little change, whereas the cross-phase between local low-k electron temperature and density fluctuations ({alpha}{sub n{sub eT{sub e}}}) was significantly modified. The increase in the electron thermal diffusivity determined from power balance is consistent with the increased turbulent transport correlated with the measured increases in low-k T-tilde{submore » e}/T{sub e} and high-k n-tilde{sub e}/n{sub e}. Linear stability analysis using the trapped gyro-Landau fluid (TGLF) model indicates an enhanced growth rate for electron modes [e.g., trapped electron mode (TEM)] at low-k consistent with the observed modifications in T-tilde{sub e}/T{sub e} and {alpha}{sub n{sub eT{sub e}}}. TGLF also predicts an increase in high-k electron mode growth rates for normalized wavenumbers k{sub {theta}}{rho}{sub s} > 7, where electron temperature gradient (ETG) modes exist, which is consistent with the observed increase in high-kn-tilde{sub e}/n{sub e} turbulence.« less
  • We report the first observation of increased edge electron temperature turbulence correlated with changes in gradients and the ELM suppression time which occurs after the application of resonant magnetic perturbations (RMP) on DIII-D H-mode plasmas. This increase (T ~ e/T e approximately doubles) occurs in the region extending from the top of the pedestal outward to the upper part of the edge steep gradient region. This is significant as it is consistent with increased turbulence driven transport potentially replacing some part of the edge localized mode (ELM) driven transport. However, temperature turbulence does not change with the initial RMP applicationmore » while ELMs are still present, indicating the turbulence changes are not causative in the development of ELM suppression or initial profile evolution with RMP – but rather a response to these effects. This temperature turbulence is broadband and long wavelength, k θρ s < 0.5, where k θ = poloidal wavenumber, ρ s = ion sound gyroradius. As has been reported previously, long wavelength density turbulence (k θρ s < 1.0) in the same location also increases after ELMs were suppressed by the RMP. Since the decrease of the density starts nearly immediately with RMP application, these results suggest that the so-called RMP “density pump-out” is not linked to these long wavelength turbulent transport changes. Comparison with linear stability analysis finds both consistencies and inconsistencies in this important region.« less
  • Experiments simulating the ITER Baseline Scenario on the DIII-D tokamak show that torque-free pure electron heating, when coupled to plasmas subject to a net co-current beam torque, affects density fluctuations at electron scales on a sub-confinement time scale, whereas fluctuations at ion scales change only after profiles have evolved to a new stationary state. Modifications to the density fluctuations measured by the Phase Contrast Imaging diagnostic (PCI) are assessed by analyzing the time evolution following the switch-off of Electron Cyclotron Heating (ECH), thus going from mixed beam/ECH to pure neutral beam heating at fixed β N . Within 20 msmore » after turning off ECH, the intensity of fluctuations is observed to increase at frequencies higher than 200 kHz; in contrast, fluctuations at lower frequency are seen to decrease in intensity on a longer time scale, after other equilibrium quantities have evolved. Non-linear gyro-kinetic modeling at ion and electron scales scales suggest that, while the low frequency response of the diagnostic is consistent with the dominant ITG modes being weakened by the slow-time increase in flow shear, the high frequency response is due to prompt changes to the electron temperature profile that enhance electron modes and generate a larger heat flux and an inward particle pinch. Furthermore, these results suggest that electron heated regimes in ITER will feature multi-scale fluctuations that might affect fusion performance via modifications to profiles.« less
    Cited by 1
  • Experiments simulating the ITER Baseline Scenario on the DIII-D tokamak show that torque-free pure electron heating, when coupled to plasmas subject to a net co-current beam torque, affects density fluctuations at electron scales on a sub confinement time scale, whereas fluctuations at ion scales change only after profiles have evolved to a new stationary state. Modifications to the density fluctuations measured by the Phase Contrast Imaging diagnostic (PCI) are assessed by analyzing the time evolution following the switch-off of Electron Cyclotron Heating (ECH), thus going from mixed beam/ECH to pure neutral beam heating at fixed βN. Within 20ms after turningmore » off ECH, the intensity of fluctuations is observed to increase at frequencies higher than 200 kHz; in contrast, fluctuations at lower frequency are seen to decrease in intensity on a longer time scale, after other equilibrium quantities have evolved. Non-linear gyro-kinetic modeling at ion and electron scales scales suggest that, while the low frequency response of the diagnostic is consistent with the dominant ITG modes being weakened by the slow-time increase in flow shear, the high frequency response is due to prompt changes to the electron temperature profile that enhance electron modes and generate a larger heat flux and an inward particle pinch. These results suggest that electron heated regimes in ITER will feature multi-scale fluctuations that might affect fusion performance via modifications to profiles.« less
    Cited by 1
  • The first observation of H-mode confinement with electron cyclotron heating as the sole auxiliary heating method has been made in divertor discharges in the DIII-D tokamak. These discharges exhibit the usual characteristics of the H mode, including improved confinement of particles and energy, when electron cyclotron heating is added at a power level above 0.7 MW. The H-mode transition is accompanied by the development of an electron temperature pedestal of 0.25 keV and a dramatic steepening of the density gradient near the separatrix.