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Title: Electron energy transport inferences from modulated electron cyclotron heating in DIII-D

Abstract

Electron thermal energy transport in high-temperature plasmas is poorly understood. A useful characterization of the transport as energy flux versus temperature gradient may be obtained for a modest range of fluxes and gradients about an equilibrium by using modulated electron cyclotron heating. The transport is rapid and stiff (high {chi}{sub inc}) at the outside, becoming slower and more diffusive toward the center.

Authors:
; ; ; ;  [1];  [2]
  1. Fusion Research Center, University of Texas, Austin, Texas 78712 (United States)
  2. (United States)
Publication Date:
OSTI Identifier:
20782442
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 13; Journal Issue: 1; Other Information: DOI: 10.1063/1.2163251; (c) 2006 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; CHARGED-PARTICLE TRANSPORT; DOUBLET-3 DEVICE; ECR HEATING; ELECTRON TEMPERATURE; ELECTRONS; EQUILIBRIUM; HOT PLASMA; ION TEMPERATURE; PLASMA CONFINEMENT; RF SYSTEMS; TEMPERATURE GRADIENTS

Citation Formats

Gentle, K.W., Austin, M.E., DeBoo, J.C., Luce, T.C., Petty, C.C., and General Atomics, San Diego, California 92186. Electron energy transport inferences from modulated electron cyclotron heating in DIII-D. United States: N. p., 2006. Web. doi:10.1063/1.2163251.
Gentle, K.W., Austin, M.E., DeBoo, J.C., Luce, T.C., Petty, C.C., & General Atomics, San Diego, California 92186. Electron energy transport inferences from modulated electron cyclotron heating in DIII-D. United States. doi:10.1063/1.2163251.
Gentle, K.W., Austin, M.E., DeBoo, J.C., Luce, T.C., Petty, C.C., and General Atomics, San Diego, California 92186. Sun . "Electron energy transport inferences from modulated electron cyclotron heating in DIII-D". United States. doi:10.1063/1.2163251.
@article{osti_20782442,
title = {Electron energy transport inferences from modulated electron cyclotron heating in DIII-D},
author = {Gentle, K.W. and Austin, M.E. and DeBoo, J.C. and Luce, T.C. and Petty, C.C. and General Atomics, San Diego, California 92186},
abstractNote = {Electron thermal energy transport in high-temperature plasmas is poorly understood. A useful characterization of the transport as energy flux versus temperature gradient may be obtained for a modest range of fluxes and gradients about an equilibrium by using modulated electron cyclotron heating. The transport is rapid and stiff (high {chi}{sub inc}) at the outside, becoming slower and more diffusive toward the center.},
doi = {10.1063/1.2163251},
journal = {Physics of Plasmas},
number = 1,
volume = 13,
place = {United States},
year = {Sun Jan 15 00:00:00 EST 2006},
month = {Sun Jan 15 00:00:00 EST 2006}
}
  • New measurements show that long-wavelength (k{sub t}hetarho{sub s}<0.5) electron temperature fluctuations can play an important role in determining electron thermal transport in low-confinement mode (L-mode) tokamak plasmas. In neutral beam-heated L-mode tokamak plasmas, electron thermal transport and the amplitude of long-wavelength electron temperature fluctuations both increase in cases where local electron cyclotron heating (ECH) is used to modify the plasma profiles. In contrast, the amplitude of simultaneously measured long-wavelength density fluctuations does not significantly increase. Linear stability analysis indicates that the ratio of the trapped electron mode (TEM) to ion temperature gradient (ITG) mode growth rates increases in the casesmore » with ECH. The increased importance of the TEM drive relative to the ITG mode drive in the cases with ECH may be associated with the increases in electron thermal transport and electron temperature fluctuations.« 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.
  • Neutral beam injection into reversed magnetic shear DIII-D plasmas produces a variety of Alfvenic activity including toroidicity and ellipticity induced Alfven eigenmodes (TAE/EAE, respectively) and reversed shear Alfven eigenmodes (RSAE) as well as their spatial coupling. These modes are studied during the discharge current ramp phase when incomplete current penetration results in a high central safety factor and strong drive due to multiple higher order resonances. It is found that ideal MHD modelling of eigenmode spectral evolution, coupling and structure are in excellent agreement with experimental measurements. It is also found that higher radial envelope harmonic RSAEs are clearly observedmore » and agree with modelling. Some discrepancies with modelling such as that due to up/down eigenmode asymmetries are also pointed out. Concomitant with the Alfvenic activity, fast ion (FIDA) spectroscopy shows large reductions in the central fast ion profile, the degree of which depends on the Alfven eigenmode amplitude. Interestingly, localized electron cyclotron heating (ECH) near the mode location stabilizes RSAE activity and results in significantly improved fast ion confinement relative to discharges with ECH deposition on axis. In these discharges, RSAE activity is suppressed when ECH is deposited near the radius of the shear reversal point and enhanced with deposition near the axis. The sensitivity of this effect to deposition power and current drive phasing as well as ECH modulation are presented.« less