Reduction of core turbulence in I-mode plasmas in Alcator C-Mod

In this paper, we report observations of reduced core (0.40 < ρ < 0.95) fluctuations in the edge localized mode (ELM)-free high-confinement regime, I-mode, at Alcator C-Mod (Marmar et al2009 Nucl. Fusion49 104014). Long wavelength (kθρs < 0.5) density fluctuation levels are observed to decrease from L-mode levels by up to 30% in I-mode, while long wavelength (kθρs < 0.3) electron temperature fluctuation levels are observed to decrease by up to 70% in I-mode. This reduction in core turbulence is correlated with the increases in confinement in I-mode compared to L-mode. As the pedestal temperature increases across the L–I transition, core density fluctuations (0.40 < ρ < 0.95) are reduced prior to the onset of the edge-localized (ρ ∼ 0.99–1.0) weakly coherent mode (WCM) and prior to the reduction of low-frequency (ρ ∼ 0.99–1.0) turbulence in the edge/pedestal region. This result helps add to our understanding of the dynamics of confinement transitions such as I-mode and H-mode, where changes in edge turbulence are more typically observed to occur prior to changes in core turbulence.