Radial Structure and Fluxes Associated with the Low Frequency Edge Oscillation in Alcator C-Mod I-Mode Plasmas

The role of the Low Frequency Edge Oscillation (LFEO) in regulating particle transport in Alcator C-Mod I-mode plasmas has been investigated via direct measurement of the density and poloidal electric field fluctuations, and resulting fluxes, using the scanning Mirror Langmuir Probe (MLP) on Alcator C-Mod. I-mode on Alcator C-Mod is often accompanied by the LFEO below 40 kHz in addition to the ubiquitous Weakly Coherent Mode at ~200 kHz on C-Mod. Understanding its nature and role is of interest as I-Mode’s high energy confinement without the presence of Edge Localized Modes make it an attractive regime for a reactor. The MLP measurements indicate the LFEO is often associated with significant outward particle flux near the separatrix which can be observed in divertor LP measurements. The LFEO’s radial structure has also been investigated by synchronizing a Gas Puff Imaging system and the MLP. This has revealed a radial zonal structure for the electric potential and density fluctuations consistent with a radially propagating GAM, in agreement with work by Cziegler et. al. The LFEO frequency is shown to be modulated by the Sawtooth Cycle, consistent with the GAM’s Te^(1/2) frequency dependence, and impacted by Impurities. Supported by US DoE award SC0014264.