Studies of EDA H-mode in Alcator C-Mod

Studies of the enhanced D-alpha H-mode (EDA) have been extended to include ohmic plasmas. No clear difference in the EDA/ELMfree boundary or in other phenomenology are seen between ohmic and ICRF-heated plasmas, suggesting th at neither the effect of ion tails nor direct RF/edge plasma interaction pl ays a role in EDA. Edge safety factor (q(95)) is the principal variable whi ch determines which regime a discharge will be in. When q(95) is greater th an 4.0 for standard-shaped plasmas, the discharge is almost always EDA, whi le when it is less than 3.5, the plasma is almost always ELMfree. New edge diagnostics have allowed measurement of pedestal profiles with resolution o f the order of 1 mm. Sudden changes in profile widths are not seen when the plasma makes a transition from EDA to ELMfree; however, the widths do vary with the same parameters that determine the EDA/ELMfree boundary. Strong e dge-density fluctuations are observed to accompany EDA and may be responsib le for the change in particle transport which is observed. The fluctuations have a quasi-coherent component whose frequency varies inversely with the pedestal width as measured by a visible continuum diagnostic.