Speaker
Description
See full abstract here:
http://ocs.ciemat.es/EPS2019ABS/pdf/P2.1091.pdf
The interaction of Geodesic Acoustic Modes (GAMs) and Zonal Flows with small-scale turbulence is an important topic in magnetic confinement studies. Much progress has been made in recent years on the measurement, interpretation and numerical simulation of the GAM-turbulence interaction. Using microwave reflectometry diagnostics, we present here recent results from the ASDEX Upgrade tokamak on the temporal and spatial behaviour of coherent edge GAMs and their interaction with both the background incoherent flow fluctuations and the ambient density n_e turbulence. Experimentally, the L-mode GAM intensity displays a low frequency (few hundred Hz) intermittency or `breathing' modulation, of up to 50% of the p.t.p. flow amplitude, phase-shifted to a corresponding modulation of the GAM frequency, the RMS δn_e, and the incoherent flow perturbation levels. The GAM spectral peak width (indicative of the GAM lifetime) is narrow (Δf_(fwhm) few hundred Hz) but often displays splitting and/or a low frequency modulation, demonstrated by double (short and long) auto-correlation times. Envelope analysis reveals the high frequency (short wavelength) n_e fluctuations can also be modulated at the GAM frequency, indicating a non-linear energy (spectral) coupling to the GAM. This is also supported by auto and cross-bispectral analysis. From cross-phase analysis the GAM generally leads the turbulence, but the phase delay shifts and reverses in both time and space. The extent of these effects depend strongly on the plasma parameters and the radial location. Structurally, the GAM is zonal with radial widths of a few cms and inward radial propagation velocities of a few hundred ms^(-1). Radially, the GAM peak intensity coincides with maximal summed bicoherence (non-linear transfer) and with the maximal plasma pressure gradient. These experimental results are further supported by numerical simulations.
