The Low Frequency Edge Oscillation in Alcator C-Mod and ASDEX Upgrade I-Mode

The Low Frequency Edge Oscillation (LFEO) is a low frequency fluctuation in many plasma quantities in the pedestal region of the I-Mode confinement regime. It is observed on Alcator C-Mod between 10-30kHz and on ASDEX Upgrade between 5-10kHz. On both tokamaks it has been previously identified as a Geodesic Acoustic Mode (GAM), however the recent discovery of the Edge Temperature Ring Oscillation (ETRO) in a similar frequency and spatial location as the LFEO in I-Modes on EAST has called this identification into question. In this paper we investigate the LFEO on C-Mod and AUG using a variety of different experimental techniques including spectral analysis, magnetic mode number analysis, localization, and direct measurement of the LFEO zonal structure and propagation using a Mirror Langmuir Probe. This investigation has reconfirmed the identification of the LFEO as a GAM and determined that it has several key differences from the ETRO.