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 D α experiments on Alcator C-Mod observed pedestal degradation and loss in confinement below a critical value of net power crossing the separatrix, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>net</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>net</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>crit</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo>≈</mml:mo> <mml:mn>2.3</mml:mn> </mml:mrow> </mml:math> MW, in the absence of any external fueling. New analysis of ionization and particle flux profiles reveal saturation of the pedestal electron density, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ped</mml:mi> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> , despite continuous increases in ionization throughout the pedestal, inversely related to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>net</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . A limit to the pedestal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">∇</mml:mi> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> emerges as the particle flux, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">Γ</mml:mi> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , continues to grow, implying increases in the effective particle diffusivity, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>D</mml:mi> <mml:mrow> <mml:mi>eff</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . This is well-correlated with the separatrix collisionality, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>ν</mml:mi> <mml:mrow> <mml:mi>sep</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>∗</mml:mo> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>net</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>crit</mml:mi> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> . 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>D</mml:mi> <mml:mrow> <mml:mi>eff</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , consistent with experimental findings, and additionally suggests even larger growth in χ e at the same <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>net</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>crit</mml:mi> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> .