Enhanced pedestal transport driven by edge collisionality on Alcator C-Mod and its role in regulating H-mode pedestal gradients

Experimental measurements of plasma and neutral profiles across the pedestal are used in conjunction with 2D edge modeling to examine pedestal stiffness in Alcator C-Mod H-mode plasmas. Enhanced D_α experiments on Alcator C-Mod observed pedestal degradation and loss in confinement below a critical value of net power crossing the separatrix, P net = P net crit ≈ 2.3 MW, in the absence of any external fueling. New analysis of ionization and particle flux profiles reveal saturation of the pedestal electron density, n e ped , despite continuous increases in ionization throughout the pedestal, inversely related to P net . A limit to the pedestal ∇ n e emerges as the particle flux, Γ D , continues to grow, implying increases in the effective particle diffusivity, D eff . This is well-correlated with the separatrix collisionality, ν sep ∗ and a turbulence control parameter, α_t, implying a possible transition in type of turbulence. The transition is well correlated with the experimentally observed value of P net crit . SOLPS-ITER modeling is performed for select discharges from the power scan, constrained with experimental electron and neutral densities, measured at the outer midplane. The modeling confirms general growth in D eff , consistent with experimental findings, and additionally suggests even larger growth in χ_e at the same P net crit .