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(EX/C3-7Ra) Double Transport Barrier Plasmas in Alcator C-Mod

J.E. Rice1), P. Bonoli1), S. Wukitch1), R. Boivin2), D. Ernst3), C. Fiore1), R. Granetz1), M. Greenwald1), A. Hubbard1), J. Hughes1), I. Hutchinson1), Y. In1), J. Irby1), Y. Lin1), E. Marmar1), D. Mossessian1), M. Porkolab1), M. Redi3), G. Schilling3), J. Snipes1), S. Wolfe1)
1) MIT PSFC, Cambridge, MA, USA
2) General Atomics, San Diego, CA, USA
3) Princeton Plasma Physics Laboratory, Princeton, NJ, USA

Abstract.  Double transport barrier plasmas comprised of an edge EDA H-mode pedestal and an ITB have been observed in Alcator C-Mod. The ITB can be routinely produced in ICRF heated plasmas by locating the wave resonance off-axis near r/a $ \sim$ 0.5, provided the target plasma average density is above 1.4×1020m3, and can develop spontaneously in some Ohmic H-mode discharges. The formation of the barrier appears in conjunction with a decrease or reversal in the central (impurity) toroidal rotation velocity. The ITBs can persist for $ \sim$15 energy confinement times, but exhibit a continuous increase of the central electron density, (in the absence of an internal particle source), followed by collapse of the barrier. A significant drop of the core thermal conductivity when the barrier forms is confirmed by modeling. Application of additional on-axis ICRF heating arrests the density and impurity peaking, which occurs along with an increase (co-current) in the core rotation velocity. The density peaking is found to be consistent with an inward neoclassical pinch velocity and a reduced particle diffusivity. Linear growth rate calculations indicate the ITG mode is stabilized in the barrier region.

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IAEA 2003