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

Dependence of density fluctuations on shape and collisionality in positive- and negative-triangularity tokamak plasmas

The effects of plasma shaping, in particular of the triangularity delta, on plasma turbulence, in terms of relative density fluctuations, have been studied in the TCV tokamak. It has been found that for inner wall limited L-mode plasmas, negative triangularity leads to a substantial reduction of turbulence amplitude, as well as of the spectral index and correlation length, consistent with the beneficial effect on energy confinement. Crucially, this reduction extends deep in the core, where the local triangularity becomes vanishingly small. A stabilizing effect of effective collisionality on trapped electron mode turbulence was also observed. These observations are consistent with previous experimental results on the effects of triangularity and collisionality on electron heat transport, as well as with global gyrokinetic GENE simulation results

Experimental observations of modes with geodesic acoustic character from the core to the edge in the TCV tokamak

International audienceThe geodesic acoustic mode is a coherently oscillating mode, related to the zonal flows that can regulate turbulence in magnetized toroidal plasmas. Modes possessing geodesic acoustic character have been widely observed in the TCV tokamak. A transition has been observed in the course of a single discharge from a continuum regime to a radially extended single-frequency regime. The mode has been also observed and characterized for the first time in the scrape-off layer, primarily by Langmuir probes, suggesting a particle flow to the edge modulated at the mode frequency. These experimental observations are consistent with nonlinear global gyrokinetic simulations, reported in a companion paper (Merlo et al 2017 Plasma Phys. Control. Fusion). These also suggest a possible coupling with radial avalanche phenomena

Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution

\u3cp\u3eIntegrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal and SOL parameters are not achievable simultaneously in present day devices. A two prong approach is adopted. On the one hand, scenarios with tolerable transient heat and particle loads, including active edge localised mode (ELM) control are developed. On the other hand, divertor solutions including advanced magnetic configurations are studied. Considerable progress has been made on both approaches, in particular in the fields of: ELM control with resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control, as well as filamentary scrape-off-layer transport. For example full ELM suppression has now been achieved on AUG at low collisionality with n = 2 RMP maintaining good confinement . Advances have been made with respect to detachment onset and control. Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor) shed new light on SOL physics. Cross field filamentary transport has been characterised in a wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the SOL also play a crucial role for ELM stability and access to small ELM regimes.\u3c/p\u3