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

Abstract

Measurements of the steep transport barrier at the edge of the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. The parameters at the top of this barrier are in the range $T_e\text{=150–750\hspace{0.5em}}\mathrm{eV}$ and $n_e{\text{=0.5−3.3×10}}^{20}\hspace{0.5em}{\mathrm{m}}^{\text{−3}}$ , depending on the confinement regime. Type III edge localized modes (ELMs) have an upper temperature limit. $T_e$ pedestal profiles show a barrier width ${\Delta }_T\text{≃8\hspace{0.5em}}\mathrm{mm}$ . Soft x-ray emissivity profiles are narrower, with $\text{Δ=2–4\hspace{0.5em}}\mathrm{mm}$ . 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.