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

Abstract

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 Dα experiments on Alcator C-Mod observed pedestal degradation and loss in confinement below a critical value of net power crossing the separatrix, Pnet = P crit/net ≈ 2.3MW, in the absence of any external fueling. New analysis of ionization and particle flux profiles reveal saturation of the pedestal electron density, npede, 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∗sepand a turbulence control parameter,αt, implying a possible transition in type of turbulence. The transition is well correlated with the experimentally observed value of P crit/net. 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 inχeat the same Pcrit/net.