Infoscience

Thesis

Experimental study of plasma turbulence in the TCV tokamak

This thesis presents an experimental investigation of electron density fluctuations in the TCV tokamak, primarily with the tangential phase contrast imaging (TPCI) diagnostic. The focus is to study the small-scale broadband turbulence in the plasma, which has long been believed to be the primary cause of anomalous transport, one of the most important issues for magnetic confinement fusion research. Nonlinear interactions within the turbulence can drive zonal flows, which in turn modulate and suppress the turbulence; geodesic acoustic modes (GAMs) are associated to zonal flows and easier to detect thanks to their finite frequency. A characterization of the GAMs in TCV with multiple diagnostics constitutes a central part of this work. The study includes spatial profile measurements as well as frequency and wavenumber spectra of turbulence and GAMs in various plasma conditions. The first part of this work is dedicated to investigating the improvement in energy confinement when the triangularity at the edge of the plasma poloidal cross-section changes from positive to negative. Measurements of turbulent density fluctuations show a substantial reduction of the turbulence amplitude, decorrelation time and radial correlation length from positive to negative triangularity. In addition, the turbulence amplitude decreases with the effective collisionality of the plasma in the Trapped Electron Mode (TEM) regime. This result is consistent with the experimental observation of transport reduction in TCV and with nonlinear gyrokinetic simulations. The second part of this thesis is focused on the characterization of the GAMs in TCV. A transition has been observed in the course of a single discharge from a local regime, with frequency varying along the radius according to a theoretically predicted linear dependence on the sound speed, to a radially extended regime, in which the oscillations are at a constant frequency over a large fraction of the minor radius. Gyrokinetic simulations succeed in reproducing the transition. The coupling between the GAM and the broadband turbulence was studied in a density ramp-up using bispectral techniques. The GAM has also been observed and characterized for the first time in the scrape-off layer (SOL), primarily by magnetic probes and Langmuir probes but also by Dalpha emission.

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