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Role of trapped electron mode turbulence in internal transport barrier control in the Alcator C-Mod Tokamak

Phys. Plasmas 11, 2637 (2004); doi:10.1063/1.1705653

Published 23 April 2004

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D. R. Ernst, P. T. Bonoli, P. J. Catto, W. Dorland, C. L. Fiore, R. S. Granetz, M. Greenwald, A. E. Hubbard, M. Porkolab, M. H. Redi, J. E. Rice, K. Zhurovich, and Alcator C-Mod Group
Plasma Science and Fusion Center, Massachusetts Institute of Technology, 167 Albany Street, NW16-258, Cambridge, Massachusetts 02139
Nonlinear gyrokinetic simulations of trapped electron mode (TEM) turbulence, within an internal particle transport barrier, are performed and compared with experimental data. The results provide a mechanism for transport barrier control with on-axis radio frequency heating, as demonstrated in Alcator C-Mod experiments [S. J. Wukitch et al., Phys. Plasmas 9, 2149 (2002)]. Off-axis heating produces an internal particle and energy transport barrier after the transition to enhanced Dalpha high confinement mode. The barrier foot reaches the half-radius, with a peak density 2.5 times the edge density. While the density profile peaks, the temperature profile remains relatively unaffected. The peaking and concomitant impurity accumulation are controlled by applying modest central heating power late in the discharge. Gyrokinetic turbulence simulations of the barrier formation phase, using the GS2 code [W. Dorland et al., Phys. Rev. Lett. 85, 5579 (2000)] show that toroidal ion temperature gradient driven modes are suppressed inside the barrier foot, but continue to dominate in the outer half-radius. As the density gradient steepens further, trapped electron modes are driven unstable. The onset of TEM turbulence produces an outflow that strongly increases with the density gradient, upon exceeding a new nonlinear critical density gradient, which significantly exceeds the linear critical density gradient. The TEM turbulent outflow ultimately balances the inward Ware pinch, leading to steady state. Moreover, the simulated turbulent particle diffusivity matches that inferred from particle balance using measured density profile data and the calculated Ware pinch. This turbulent diffusivity exhibits a strong unfavorable temperature dependence that allows control with central heating. ©2004 American Institute of Physics.
History: Received 3 November 2003; accepted 23 February 2004; published 23 April 2004
Permalink: http://link.aip.org/link/?PHPAEN/11/2637/1
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KEYWORDS and PACS

Keywords
PACS
  • 52.35.Ra
    Plasma turbulence
  • 52.25.Fi
    Plasma transport properties
  • 52.25.Vy
    Impurities in plasmas
  • 52.58.Lq
    Z-pinches, plasma focus, and other pinch devices
  • 52.40.Hf
    Plasma–material interactions; boundary layer effects
  • 52.35.Qz
    Plasma microinstabilities including ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron instabilities, etc
  • 52.50.Qt
    Plasma heating by radio-frequency fields including ICR, ICP, helicons
  • 52.65.Tt
    Gyrofluid and gyrokinetic plasma simulations
  • 52.25.Dg
    Plasma kinetic equations
  • YEAR: 2004

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ISSN:
1070-664X (print)   1089-7674 (online)
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