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 10–30 kHz and on ASDEX Upgrade between 5–10 kHz. 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.
This paper investigates a low-frequency edge oscillation (LFEO) observed in the I-mode confinement regime on Alcator C-Mod and ASDEX Upgrade tokamaks. The LFEO was previously identified as a geodesic acoustic mode (GAM), but a new discovery of an edge temperature ring oscillation (ETRO) in a similar frequency and location has raised questions. The study reconfirms the LFEO as a GAM and identifies key differences from the ETRO.