Mode Structure, Coherence, and Locality of Edge Modes on Alcator C-Mod

Persistent edge fluctuations are implicated as the impurity exhaust mechanism that sustains steady-state, non-ELMing confinement regimes. These modes appear in a limited frequency band, and a global character with long coherence length is often assumed. Here, we examine the coherence time and length scales for two edge fluctuations: the Quasi-Coherent Mode (QCM, f~30-200 kHz, 5<n<30, k_θρ_s<0.1) of EDA H-mode, and the Weakly Coherent Mode of I-mode (WCM, f~100-500 kHz, 5<n<25, k_θρ_s<0.1). Using data from Mirnov coils, phase contrast imaging, two-color interferometry, and reflectometry, we find that both modes have relatively short coherence times of several wave periods, and similarly short coherence lengths of several wavelengths. This suggests a wavelet interpretation of the fluctuation, wherein short, uncorrelated bursts of characteristic wavelets appear at the edge, and is reminiscent of WCM behavior observed on AUG, as well as "blobby" scrape-off-layer turbulence. The intermittent and localized nature of these bursts permits spatial asymmetries in the power spectra. This can manifest itself in a surprising way: the QCM peak frequency can occasionally vary with toroidal angle for a portion of a discharge.