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Title: Investigation of edge pedestal structure in DIII-D

Abstract

A calculation based on the requirements of particle, momentum and energy conservation, conductive heat transport, and atomic physics resulting from a recycling and fueling neutral influx was employed to investigate the experimental density, temperature, rotation velocities, and radial electric field profiles in the edge of three DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] high-confinement-mode plasmas. The calculation indicated that the cause of the pedestal structure in the density was a momentum balance requirement for a steep negative pressure gradient to balance the forces associated with an edge peaking in the inward pinch velocity (caused by the observed edge peaking in the radial electric field and rotation velocity profiles) and, to a lesser extent, in the outward radial particle flux (caused by the ionization of recycling neutrals). Thermal and angular momentum transport coefficients were inferred from experiment and compared with theoretical predictions, indicating that thermal transport coefficients were of the magnitude predicted by neoclassical and ion-temperature-gradient theories (ions) and electron-temperature-gradient theory (electrons), but that neoclassical gyroviscous theory plus atomic physics effects combined were not sufficient to explain the inferred angular momentum transfer rate throughout the edge region.

Authors:
;  [1];  [2]
  1. Fusion Research Center, Georgia Tech, Atlanta, Georgia 30332-0425 (United States)
  2. (United States)
Publication Date:
OSTI Identifier:
20782455
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 13; Journal Issue: 1; Other Information: DOI: 10.1063/1.2167310; (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; ANGULAR MOMENTUM; ANGULAR MOMENTUM TRANSFER; BOUNDARY LAYERS; CHARGED-PARTICLE TRANSPORT; DOUBLET-3 DEVICE; ELECTRIC FIELDS; ELECTRON TEMPERATURE; ELECTRONS; HEAT TRANSFER; ION TEMPERATURE; IONS; NEOCLASSICAL TRANSPORT THEORY; PLASMA; PLASMA CONFINEMENT; PLASMA DENSITY; PLASMA PRESSURE; PRESSURE GRADIENTS; ROTATION; TEMPERATURE GRADIENTS

Citation Formats

Stacey, W.M., Groebner, R.J., and Energy Group, General Atomics, San Diego, California 92186-5608. Investigation of edge pedestal structure in DIII-D. United States: N. p., 2006. Web. doi:10.1063/1.2167310.
Stacey, W.M., Groebner, R.J., & Energy Group, General Atomics, San Diego, California 92186-5608. Investigation of edge pedestal structure in DIII-D. United States. doi:10.1063/1.2167310.
Stacey, W.M., Groebner, R.J., and Energy Group, General Atomics, San Diego, California 92186-5608. Sun . "Investigation of edge pedestal structure in DIII-D". United States. doi:10.1063/1.2167310.
@article{osti_20782455,
title = {Investigation of edge pedestal structure in DIII-D},
author = {Stacey, W.M. and Groebner, R.J. and Energy Group, General Atomics, San Diego, California 92186-5608},
abstractNote = {A calculation based on the requirements of particle, momentum and energy conservation, conductive heat transport, and atomic physics resulting from a recycling and fueling neutral influx was employed to investigate the experimental density, temperature, rotation velocities, and radial electric field profiles in the edge of three DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] high-confinement-mode plasmas. The calculation indicated that the cause of the pedestal structure in the density was a momentum balance requirement for a steep negative pressure gradient to balance the forces associated with an edge peaking in the inward pinch velocity (caused by the observed edge peaking in the radial electric field and rotation velocity profiles) and, to a lesser extent, in the outward radial particle flux (caused by the ionization of recycling neutrals). Thermal and angular momentum transport coefficients were inferred from experiment and compared with theoretical predictions, indicating that thermal transport coefficients were of the magnitude predicted by neoclassical and ion-temperature-gradient theories (ions) and electron-temperature-gradient theory (electrons), but that neoclassical gyroviscous theory plus atomic physics effects combined were not sufficient to explain the inferred angular momentum transfer rate throughout the edge region.},
doi = {10.1063/1.2167310},
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}
}
  • Direct measurements of the pedestal recovery during an edge-localized mode cycle provide evidence that quasi-coherent fluctuations (QCFs) play a role in the inter-ELM pedestal dynamics. Using fast Thomson scattering measurements, the pedestal density and temperature evolutions are probed on sub-millisecond time scales to show a fast recovery of the density gradient compared to the temperature gradient. The temperature gradient appears to provide a drive for the onset of quasi-coherent fluctuations (as measured with the magnetic probe and the density diagnostics) localized in the pedestal. The amplitude evolution of these QCFs tracks the temperature gradient evolution including its saturation. Such correlationmore » suggests that these QCFs play a key role in limiting the pedestal temperature gradient. The saturation of the QCFs coincides with the pressure gradient reaching the kinetic-ballooning mode (KBM) critical gradient as predicted by EPED1. Furthermore, linear microinstability analysis using GS2 indicates that the steep gradient is near the KBM threshold. Furthermore, the modeling and the observations together suggest that QCFs are consistent with dominant KBMs (although microtearing cannot be excluded as subdominant).« less
  • A calculation of edge density and temperature profiles based on 'classical' physics - particle, momentum, and energy balances, heat conduction closure relations, and neutral particle transport - yielded a pedestal structure that is qualitatively and quantitatively similar to that found experimentally in five DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] discharges, when experimental radial electric field and rotation profiles and experimentally inferred heat transport coefficients were used. The principal cause of the density pedestal was a peaking of the inward pinch velocity just inside the separatrix caused by the negative well in the experimental electric field, and the secondarymore » cause was a peaking of the radial particle flux caused by the ionization of incoming neutrals. There is some evidence that this peaking of the radial particle flux just inside the separatrix may also be responsible in part for the negative electric field in that location.« less
  • Using temperature and density profiles averaged over the same subinterval of several successive inter-edge-localized-mode (ELM) periods, the ion and electron thermal diffusivities in the edge pedestal were inferred between ELMs for two DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] discharges. The inference procedure took into account the effects of plasma reheating and density buildup between ELMs, radiation and atomic physics cooling, neutral beam heating and ion-electron equilibration, and recycling neutral and beam ionization particle sources in determining the conductive heat flux profiles used to infer the thermal diffusivities in the edge pedestal. Comparison of the inferred thermal diffusivities withmore » theoretical formulas based on various transport mechanisms was inconclusive insofar as identifying likely transport mechanisms.« less
  • Evolution of measured profiles of densities, temperatures, and velocities in the edge pedestal region between successive ELM (edge-localized mode) events are analyzed and interpreted in terms of the constraints imposed by particle, momentum and energy balance in order to gain insights regarding the underlying evolution of transport processes in the edge pedestal between ELMs in a series of DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] discharges. The data from successive inter-ELM periods during an otherwise steady-state phase of the discharges were combined into a composite inter-ELM period for the purpose of increasing the number of data points in themore » analysis. Variation of diffusive and non-diffusive (pinch) particle, momentum, and energy transport over the inter-ELM period are interpreted using the GTEDGE code for discharges with plasma currents from 0.5 to 1.5 MA and inter-ELM periods from 50 to 220 ms. Diffusive transport is dominant for ρ < 0.925, while non-diffusive and diffusive transport are very large and nearly balancing in the sharp gradient region 0.925 < ρ < 1.0. During the inter-ELM period, diffusive transport increases slightly more than non-diffusive transport, increasing total outward transport. Both diffusive and non-diffusive transport have a strong inverse correlation with plasma current.« less
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