Changes in Core Electron Temperature Fluctuations Across the Ohmic Energy Confinement Transition in Alcator C-Mod Plasmas

The first measurements of long wavelength (kyρs < 0.3) electron temperature fluctuations in Alcator C-Mod made with a new Correlation Electron Cyclotron Emission (CECE) diagnostic support a long-standing hypothesis regarding the confinement transition from Linear Ohmic Confinement (LOC) to Saturated Ohmic Confinement (SOC). Electron temperature fluctuations decrease significantly (∼40%) crossing from LOC to SOC, consistent with a change from Trapped Electron Mode (TEM) turbulence domination to Ion Temperature Gradient (ITG) turbulence as the density is increased. Linear stability analysis performed with the GYRO code [Candy and Waltz 2003 J. Comput. Phys. 186 545] shows that TEMs are dominant for long wavelength turbulence in the LOC regime and ITG modes are dominant in the SOC regime at the radial location (ρ ∼ 0.8) where the changes in electron temperature fluctuations are measured. In contrast, deeper in the core (ρ < 0.8), linear stability analysis indicates that ITG modes remain dominant across the LOC/SOC transition. This radial variation suggests that the robust global changes in confinement of energy and momentum occurring across the LOC/SOC transition are correlated to local changes in the dominant turbulent mode near the edge. Changes in Core Te Fluctuations Across the Ohmic Energy Confinement Transition 2