Detailed structure and role of the Low-Frequency Edge Oscillation in Alcator C-Mod and ASDEX Upgrade I-Modes*

The Low-Frequency Edge Oscillation (LFEO), a coherent low-frequency (5-40 kHz range) fluctuation in I-Mode is seen on Alcator C-Mod and ASDEX Upgrade (AUG). On C-Mod it has been shown to be a radially propagating Geodesic Acoustic Mode (GAM) with eigenmode-like frequency steps. Mode number analysis using a Mirnov coil array has revealed the LFEO has a toroidal mode number <!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>n=0 and poloidal mode number <!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>m=2, consistent with the GAM. A database analysis of frequencies shows that the LFEO frequency is within a factor of 2 of the theoretical frequency of a GAM driven near the last closed flux surface. Analysis of mirror Langmuir probe combined with gas puff imaging has shown the LFEO propagates radially inward, with a ratio of electric potential to density fluctuation amplitudes within a factor of 2 of the expected relationship for a GAM. Separately, a database analysis of I-Modes on C-Mod and AUG has shown that the LFEO is not a ubiquitous feature of I-Mode, indicating it is not necessary for I-Mode. That analysis has revealed I-modes can be roughly categorized into three groups based on LFEO existence and nature of the low-frequency broadband turbulence, suggesting that when present it may play a role in regulating transport.