Developing predictive capability for the H-mode pedestal: Experimental contributions from Alcator C-Mod

Experiments on Alcator C-Mod characterize the edge pedestal in several high confinement regimes, as part of a community experimental and theoretical effort to improve predictive capability for the pedestal. Pedestal structure in both transiently evolving and stationary enhanced D$_\alpha$ H-modes is studied and examined alongside edge fluctuations ({\it e.g.} quasi-coherent modes), allowing us to seek evidence for transport-driven mechanisms limiting pedestal width and gradients. The pedestal structure in H-modes with edge-localized modes (ELMs) is likely to be governed both by transport and by intermittent ELM relaxation. ELMy H-modes are diagnosed and used to test directly models for pedestal height/width, {\it e.g.} EPED. Pedestal profiles, fluctuations and ELM activity in these H-mode regimes are compared to those in I-mode, in which a temperature and a pressure pedestal are maintained in the absence of a particle barrier ({\it i.e.} edge $L_n/L_T\gg 1$), and which can provide additional insight into physics governing pedestal formation and structure.