Experimental characterization of multiscale and multifield turbulence as a critical gradient threshold is surpassed in the DIII-D tokamak

Journal Article · Wed May 15 04:00:00 EDT 2013 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.4807123· OSTI ID:22228087

Hillesheim, J. C.; Peebles, W. A.; Carter, T. A.; Wang, G.; Rhodes, T. L.; Schmitz, L.; Doyle, E. J.; Zeng, L. ^[1]; DeBoo, J. C.; Staebler, G. M.; Burrell, K. H.; Petty, C. C.; Smith, S. P. ^[2]; McKee, G. R.; Yan, Z. ^[3]; Holland, C. ^[4]; White, A. E. ^[5]

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1. University of California Los Angeles, PO Box 957099, Los Angeles, California 90024-1547 (United States)
2. General Atomics, PO Box 85608, San Diego, California 92186-5608 (United States)
3. University of Wisconsin-Madison, 1500 Engineering Dr., Madison, Wisconsin 53706-1687 (United States)
4. University of California San Diego, 9500 Gilman Dr., La Jolla, California 92093-0417 (United States)
5. Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States)

A critical gradient for long wavelength (k{sub θ}ρ{sub s}≲0.4) electron temperature fluctuations has been observed in an experiment in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)], where below a threshold value of L{sub T{sub e}{sup −1}}=|∇T{sub e}|/T{sub e} electron temperature fluctuations are constant and above they steadily increase. Above the critical gradient, the electron heat flux inferred by power balance also increases rapidly. Critical gradients are a predicted attribute of turbulence arising from linear instabilities and are thought to be related to transport stiffness. The presented results are the first direct, systematic demonstration of critical gradient behavior in turbulence measurements in a tokamak. The experiment was performed by changing the deposition location of electron cyclotron heating shot-to-shot to locally scan L{sub T{sub e}{sup −1}} at r/a = 0.6 in L-mode plasmas; rotation was also varied by changing the momentum input from neutral beam injection. Temperature fluctuations were measured with a correlation electron cyclotron emission (CECE) radiometry system. In addition to the CECE measurements, an array of turbulence measurements were acquired to characterize fluctuations in multiple fields and at multiple scales as L{sub T{sub e}{sup −1}} and rotation were modified: long wavelength (k{sub θ}ρ{sub s}≲0.5) density fluctuations were acquired with beam emission spectroscopy, the phase angle between electron temperature and density fluctuations was measured by coupling the CECE system and a reflectometer, intermediate scale (k{sub θ}ρ{sub s}∼0.8) density fluctuations were measured with a Doppler backscattering (DBS) system, and low frequency flows were also measured with DBS. The accumulated measurements and trends constrain identification of the instability responsible for the observed critical gradient to the ∇T{sub e}-driven trapped electron mode.

• OSTI ID: 22228087
• Journal Information: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 5 Vol. 20; ISSN PHPAEN; ISSN 1070-664X
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
BACKSCATTERING
BEAM INJECTION
BEAM-PLASMA SYSTEMS
DOUBLET-3 DEVICE
ECR HEATING
ELECTRON TEMPERATURE
EMISSION SPECTROSCOPY
FLUCTUATIONS
HEAT FLUX
ION TEMPERATURE
L-MODE PLASMA CONFINEMENT
PLASMA DENSITY
PLASMA DIAGNOSTICS
PLASMA INSTABILITY
RF SYSTEMS
TRAPPED ELECTRONS
TURBULENCE