Investigations into interactions between the QCM and broadband turbulence in QCE and EDA H-Modes

Two attractive, ELM free scenarios, the enhanced D_α (EDA) and quasi-continuous exhaust (QCE), have been observed on ASDEX Upgrade, and exhibit an edge fluctuation dubbed the quasi-coherent mode (QCM). It is not clear if these regimes are distinct or some operational limit of the same regime. Recent work using the correlation electron cyclotron emission diagnostic observed differences in the QCM’s spectra [J. Schellpfeffer submitted to PPCF]. Comparing the frequency bandwidth measured using the cross-power spectrum (G_xy) and the coherence spectrum (γ­_c) of a single discharge shows γ­_c to be larger than G_xy, and the ratio of the bandwidth to the QCM’s central frequency increased towards the pedestal top in QCE but not in EDA. These distinctions are thought to be due to differences in how the QCM interacts with surrounding broadband turbulence in each plasma. Bispectral analysis is used to identify if any quadratic coupling could be occurring with the QCM. γ­_c is fit with a Gaussian to capture the QCM. This fit is used to calculate the temperature fluctuations from the QCM and is compared against those from the broadband portion of the spectra. Results are presented using a database of past EDA and QCE discharges and new discharges which transition EDA to QCE to understand how the QCM changes.