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Title: Strong ExB Shear Flows in the Transport-Barrier Region in H-Mode Plasma

Abstract

We report the first experimental observation of stationary zonal flow in the transport-barrier region of the H-mode plasma. Strong peaks in E{sub r} shear mark the width of this region. A strong m=n=0 low-frequency (f<0.6 kHz) zonal flow is observed in regions of increased E{sub r}, suggesting a substantial contribution of zonal flow to the spatial modulation of E{sub r} radial profiles. Radial localization of the zonal flow is correlated with a region of zero magnetic shear and low-order (7/5) rational surfaces.

Authors:
; ;  [1]
  1. Plasma Research Laboratory, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200 (Australia)
Publication Date:
OSTI Identifier:
20861529
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Letters; Journal Volume: 97; Journal Issue: 25; Other Information: DOI: 10.1103/PhysRevLett.97.255003; (c) 2006 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; ELECTROMAGNETIC FIELDS; H-MODE PLASMA CONFINEMENT; MODE RATIONAL SURFACES; MODULATION; PLASMA

Citation Formats

Xia, H., Shats, M. G., and Punzmann, H.. Strong ExB Shear Flows in the Transport-Barrier Region in H-Mode Plasma. United States: N. p., 2006. Web. doi:10.1103/PHYSREVLETT.97.255003.
Xia, H., Shats, M. G., & Punzmann, H.. Strong ExB Shear Flows in the Transport-Barrier Region in H-Mode Plasma. United States. doi:10.1103/PHYSREVLETT.97.255003.
Xia, H., Shats, M. G., and Punzmann, H.. Fri . "Strong ExB Shear Flows in the Transport-Barrier Region in H-Mode Plasma". United States. doi:10.1103/PHYSREVLETT.97.255003.
@article{osti_20861529,
title = {Strong ExB Shear Flows in the Transport-Barrier Region in H-Mode Plasma},
author = {Xia, H. and Shats, M. G. and Punzmann, H.},
abstractNote = {We report the first experimental observation of stationary zonal flow in the transport-barrier region of the H-mode plasma. Strong peaks in E{sub r} shear mark the width of this region. A strong m=n=0 low-frequency (f<0.6 kHz) zonal flow is observed in regions of increased E{sub r}, suggesting a substantial contribution of zonal flow to the spatial modulation of E{sub r} radial profiles. Radial localization of the zonal flow is correlated with a region of zero magnetic shear and low-order (7/5) rational surfaces.},
doi = {10.1103/PHYSREVLETT.97.255003},
journal = {Physical Review Letters},
number = 25,
volume = 97,
place = {United States},
year = {Fri Dec 22 00:00:00 EST 2006},
month = {Fri Dec 22 00:00:00 EST 2006}
}
  • Long-distance toroidal correlations of potential and density fluctuations have been investigated at the TEXTOR tokamak [H. Soltwisch et al., Plasma Phys. Controlled Fusion 26, 23 (1984)] in edge electrode-biasing experiments. During the biasing-induced H-mode, the dc ExB shear flow triggers a zonal flow structure and hence long-distance correlation in potential fluctuations, whereas for density fluctuations there is nearly no correlation. These results indicate an intimate interaction between the mean and zonal flows, and the significance of long range correlations in improved-confinement regimes.
  • Long-distance correlations of density and potential fluctuations are investigated on the poloidal circumference of flux surface in the TJ-K plasmas. Shear flow as induced by biasing causes a strong increase in the long-distance correlation of potential and density fluctuations. In the potential fluctuations, zonal-flow-like modes are excited. The density long-distance correlation is dominated by an m=3 mode. As a new feature a coherency analysis reveals also the existence of m=0 density fluctuations. As an explanation, background profile fluctuations or a turbulent zonal density drive are proposed.
  • A significant inward flux of toroidal momentum is found in global gyrokinetic simulations of ion temperature gradient turbulence, leading to core plasma rotation spin-up. The underlying mechanism is identified to be the generation of residual stress due to the k{sub parallel} symmetry breaking induced by global quasistationary zonal flow shear. Simulations also show a significant off-diagonal element associated with the ion temperature gradient in the neoclassical momentum flux, while the overall neoclassical flux is small. In addition, the residual turbulence found in the presence of strong ExB flow shear may account for neoclassical-level ion heat and anomalous momentum transport widelymore » observed in experiments.« less
  • Poloidal ExB shear flows are widely accepted as a trigger mechanism of transport barriers in the edge of fusion plasmas. Strong ExB flows can act on turbulence and turbulent transport through the shear decorrelation mechanism, which can reduce the radial size of turbulent structures or change the phase relation between density and potential fluctuations. In this contribution the influence of ExB shear flows on the microscopic structure of turbulence is investigated. The experiments have been carried out on the toroidally confined low-temperature plasma of the torsatron TJ-K. The plasma is dimensionally similar to fusion edge plasmas and accessible throughout formore » Langmuir probes. Multi-probe arrays are used to resolve the turbulent dynamics perpendicular to the confining magnetic field in high detail.Strong ExB flows are externally generated by core plasma biasing. It is shown that the fluctuations are dominated by large-scale coherent structures even though strong flow shear is present. These structures reveal increased correlation lengths. It is found that these structures can contribute to improved confinement through inwards transport due to cross-phase modifications. Furthermore, the response of the turbulent Reynolds stress, which is supposed to drive zonal flows as internally ExB shear flows, is investigated. The externally generated flow shear leads to a redistribution of the Reynolds stress with increased poloidal symmetry.« less
  • Electron gyroscale fluctuation measurements in National Spherical Torus Experiment H-mode plasmas with large toroidal rotation reveal fluctuations consistent with electron temperature gradient (ETG) turbulence. Large toroidal rotation in National Spherical Torus Experiment plasmas with neutral beam injection generates ExB flow shear rates comparable to ETG linear growth rates. Enhanced fluctuations occur when the electron temperature gradient is marginally stable with respect to the ETG linear critical gradient. Fluctuation amplitudes decrease when the ExB flow shear rate exceeds ETG linear growth rates. The observations indicate that ExB flow shear can be an effective suppression mechanism for ETG turbulence.