Characterization of the quasi-coherent mode in QCE in experiment and simulation

The quasi-coherent mode (QCM) observed in the type-I edge-localized mode free quasi-continuous exhaust (QCE) regime of ASDEX Upgrade is studied through a detailed comparison of experimental measurements and linear gyrokinetic simulations using the GENE code. The QCM, detected by the thermal helium beam diagnostic near the separatrix, i.e. at ρ pol ≈ 0.99, exhibits a frequency of approximately 30kHz and a normalized poloidal wavenumber in the range 0.02 < k θ ρ s < 0.08. It propagates in the ion diamagnetic direction with roughly half the ion diamagnetic velocity in the plasma frame and shows electromagnetic characteristics. Cross-phase analyses reveal in-phase density and temperature fluctuations, i.e. α n e, T e ≈ 0, while an interchange-like cross-phase between density and potential fluctuation, i.e. α n e, ϕ ≈ − 0.7 π is observed, indicating a pressure-driven nature. Linear local GENE simulations quantitatively reproduce its location at ρ pol = 0.991, wavenumbers, propagation direction, and cross-phase behavior. Parameter scans demonstrate a strong dependence of the growth rate on the normalized electron pressure β e, confirming the electromagnetic character of the instability, and negligible sensitivity to collisionality ν c. The combined experimental and simulation results consistently identify the underlying QCM instability as a kinetic ballooning mode.