Pedestal profiles and fluctuations in C-Mod enhanced D-alpha H-modes

High resolution measurements on the Alcator C-Mod tokamak [I. H. Hutchinson , Phys. Plasmas 1, 1551 (1994)] of the transport barrier in the "Enhanced D-alpha" (EDA) regime, which has increased particle transport without large edge localized modes, show steep density and temperature gradients over a region of 2-5 mm, with peak pressure gradients up to 12 MPa/m. Evolution of the pedestal at the L-H transition is consistent with a large, rapid drop in thermal conductivity across the barrier. A quasi-coherent fluctuation in density, potential, and B-pol, with f(0)similar to 50-150 kHz and k(theta)similar to4 cm(-1), always appears in the barrier during EDA, and drives a large particle flux. Conditions to access the steady-state EDA regime in deuterium include delta >0.35, q(95)>3.5, and L-mode target density (n) over bar (e)>1.2x10(20) m(-3). A reduced q(95) limit is found for hydrogen discharges. (C) 2001 American Institute of Physics.