Fluctuating zonal flows in the I-mode regime in Alcator C-Mod

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

DOI:https://doi.org/10.1063/1.4803914· OSTI ID:22228069

Cziegler, I.; Diamond, P. H.; Fedorczak, N.; Manz, P.; Tynan, G. R.; Xu, M. ^[1]; Churchill, R. M.; Hubbard, A. E.; Lipschultz, B.; Sierchio, J. M.; Terry, J. L.; Theiler, C. ^[2]

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1. Center for Momentum Transport and Flow Organization, University of California, San Diego, La Jolla, California 92093 (United States)
2. Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02138 (United States)

Velocity fields and density fluctuations of edge turbulence are studied in I-mode [F. Ryter et al., Plasma Phys. Controlled Fusion 40, 725 (1998)] plasmas of the Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] tokamak, which are characterized by a strong thermal transport barrier in the edge while providing little or no barrier to the transport of both bulk and impurity particles. Although previous work showed no clear geodesic-acoustic modes (GAM) on C-Mod, using a newly implemented, gas-puff-imaging based time-delay-estimate velocity inference algorithm, GAM are now shown to be ubiquitous in all I-mode discharges examined to date, with the time histories of the GAM and the I-mode specific [D. Whyte et al., Nucl. Fusion 50, 105005 (2010)] Weakly Coherent Mode (WCM, f = 100–300 kHz, Δf/f≈0.5, and k{sub θ}≈1.3 cm{sup −1}) closely following each other through the entire duration of the regime. Thus, the I-mode presents an example of a plasma state in which zero frequency zonal flows and GAM continuously coexist. Using two-field (density-velocity and radial-poloidal velocity) bispectral methods, the GAM are shown to be coupled to the WCM and to be responsible for its broad frequency structure. The effective nonlinear growth rate of the GAM is estimated, and its comparison to the collisional damping rate seems to suggest a new view on I-mode threshold physics.

• OSTI ID: 22228069
• Journal Information: Physics of Plasmas, Vol. 20, Issue 5; Other Information: (c) 2013 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
ALCATOR DEVICE
ALGORITHMS
BOUNDARY LAYERS
COMPARATIVE EVALUATIONS
DAMPING
ELECTRIC DISCHARGES
FLUCTUATIONS
ION ACOUSTIC WAVES
KHZ RANGE
NONLINEAR PROBLEMS
PLASMA CONFINEMENT
PLASMA DENSITY
PLASMA DIAGNOSTICS
PLASMA IMPURITIES
TIME DELAY
TURBULENCE
WALL EFFECTS