H-mode pedestal characteristics and MHD stability of the edge plasma in Alcator C-Mod

Under most operational conditions of Alcator C-Mod, the dominant type of H-mode is the steady state enhanced Dα (EDA) mode, characterized by good energy confinement, continuously degraded impurity confinement and absence of regular edge localized modes (ELMs). In this regime, a quasicoherent (QC) electromagnetic mode (kθ~5 cm-1, f~100 kHz) is observed, localized in the region of the density pedestal. Experimental evidence suggests that the mode is responsible for enhancement of particle transport. It is shown experimentally that the QC mode can exist in a well defined region in edge temperature-safety factor space, favouring high edge q values (q95≥3.5) and requiring moderate pedestal temperature Teped ≤ Tc (Tc~400 eV). As edge temperature and pressure gradient increase, the QC mode is replaced by broadband low frequency fluctuations (f