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. Experiments on Alcator C-Mod observed pedestal degradation and loss in confinement below a critical value of net power crossing the separatrix, Pₙet = Pₙetᶜrit ≈ 2.3 MW. New analysis of ionization and particle flux profiles reveal saturation of the pedestal electron density, nₑᵖed despite continuous increases in ionization throughout the pedestal, inversely related to Pₙet. A limit to the pedestal ∇ nₑ emerges as the particle flux, ΓD continues to grow, implying increases in the effective particle diffusivity, Dₑff. This is well-correlated with the separatrix collisionality, ν*ₛep and a turbulence control parameter, αₜ, implying a possible transition in type of turbulence. The transition is well correlated with the experimentally observed value of Pₙetᶜrit. 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 Dₑff, consistent with experimental findings, and additionally suggests even larger growth in χₑ at the same Pₙetᶜrit.