Edge transport barrier phenomena in Alcator C-Mod

Key phenomena controlling the transport barrier in H-mode plasmas on the Al cator C-Mod tokamak are discussed. Type I edge localized modes (ELMs) are n ot observed. Instead the particle transport barrier is continuously degrade d giving rise to 'enhanced D-alpha' operation which avoids the deleterious impurity accumulation observed in 'ELM-free' H-modes. The dependence of ope rational regime on plasma parameters suggests that the instability responsi ble for barrier degradation is enhanced by higher pressure and longer field -line connection. Detailed analysis of the magnetic structure of fluctuatio ns associated with the different H-mode regimes shows that these field-alig ned fluctuations propagate with large perpendicular velocities believed to represent the large velocity shear responsible for barrier formation. In th e enhanced D-alpha regime the fluctuations are generally more incoherent, b ut have not otherwise yet shown a specific distinctive character.