Enhanced D-Alpha H-mode Studies in the Alcator C-Mod Tokamak

A favorable regime of H-mode confinement, seen on the Alcator C-Mod tokamak is described. Following a brief period of ELM-free H-mode, the plasma evolves into the Enhanced D-Alpha (EDA) H-mode which is characterized by very good energy confinement, the complete absence of large, intermittent type I ELMs, finite impurity and majority species confinement, and low radiated power fraction. Accompanying the EDA H-mode, a quasi-coherent (QC) edge mode is observed, and found to be responsible for particle transport through the edge confinement barrier. The QC-mode is localized within the strong density gradient region, and has poloidal wavenumber kθ≈5 cm and lab-frame frequency of ≈100 kHz. Parametric studies show that the conditions which promote EDA include moderate safety factor (q95>3.5), high triangularity (δ>0.35) and high target density (ne>1.2x20 m). EDA H-mode is readily obtained in purely ohmic and well as in ICRF auxiliaryheated discharges.