Measurements of the high confinement anode pedestal region on Alcator C-Mod

Abstract

Measurements of the steep transport barrier at the edge of the Alcator C-Mod tokamak [1. H. Hutchinson et at., Phys. Plasmas 1, 1511 (1994)] are presented. The parameters at the top of this barrier are in the range T-e=150-750 eV and n(e)=0.5-3.3x10(20) m(-3), depending on the confinement regime. Type m edge localized modes (ELMs) have an upper temperature limit. T-e pedestal profiles show a barrier width Delta similar or equal to=8 mm. Soft x-ray emissivity profiles are narrower, with Delta=2-4 mm. Edge currents are calculated to alter the ideal stability boundary favorably, leading to ideally stable pedestal profiles. High frequency, broadband, edge density fluctuations are sometimes observed in H-mode (high-confinement mode) and are associated with enhanced particle transport. Coherent magnetic fluctuations localized near the pedestal are also seen.