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

High resolution measurements on the Alcator C-Mod tokamak [I. H. Hutchinson et al., 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 $B_{\mathrm{pol}},$ with $f_0\text{∼50–150\hspace{0.05em}}\mathrm{kHz}$ and $k_{\theta }\text{∼4\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ always appears in the barrier during EDA, and drives a large particle flux. Conditions to access the steady-state EDA regime in deuterium include δ>0.35, $q_{95}\text{>3.5,}$ and L-mode target density ${\mathrm{n̄}}_e{\text{>1.2×10}}^{20} {\mathrm{m}}^{\mathrm{-3}}.$ A reduced $q_{95}$ limit is found for hydrogen discharges.