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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. Wolfe^1)
 
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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