Infoscience

Thesis

Flux-tube and global grid-based gyrokinetic simulations of plasma microturbulence and comparisons with experimental TCV measurements

In magnetic fusion devices, the radial transport of heat and particle largely exceeds predictions based on collisional processes. This is widely understood as a consequence of small-scale turbulence which results from the nonlinear behaviour of so-called microinstabilities. The complexity of such nonlinear phenomena allows one to address microturbulence only with a numerical description, carried out here within the gyrokinetic framework. This reduced kinetic model describes the evolution of the particle distribution functions and of the self-consistently generated electromagnetic fields neglecting the fast gyromotion. In this work we applied the grid-based gyrokinetic code GENE, using both its local and global versions, to model some of the experimental observations made in the Tokamak à Configuration Variable (TCV) at the Swiss Plasma Center. All simulations are performed considering realistic magnetic geometries, in turn provided by the MHD equilibrium solver CHEASE. In order to verify the interface of GENE with CHEASE, a series of benchmarks have been developed and successfully carried out in the linear local limit. These tests have then been extended to the global version of the code and a good agreement found with results obtained with the gyrokinetic Particle In Cell code ORB5. A significant part of this work deals with the electron heat confinement improvement observed when the shape of the plasma is modified by changing the sign of the edge triangularity, from positive to negative. In the latter case, half the heating power is required to maintain the same electron profiles compared to the former, which was experimentally interpreted as a better confinement at all radial locations, even though triangularity has a finite radial penetration depth. A series of local runs were carried out to investigate the dependence of profile stiffness on shaping, failing at reproducing both the absolute level of transport as well as the ratio between the two shapes. Global gradient-driven simulations have then been performed, showing a very high sensitivity of the electron heat flux with respect to the density gradient. These runs, carried out neglecting carbon impurities, are compatible with the experiments when using parameters from an experimentally well diagnosed discharge. In this case, strong global effects which lower the heat flux compared to local runs are seen. Local and global simulations have then been performed looking at axisymmetric dynamics in the frequency range of the Geodesic Acoustic Mode (GAM). Experimentally, the GAM is almost always observed as a radially coherent mode, i.e. an oscillation at constant frequency over a main fraction of the plasma minor radius. The only exception is for very large values of the edge safety factor q, where the mode looses its coherence becoming dispersive. A density ramp-up was studied with local simulation, already obtaining a reasonable agreement with measurements of heat transport as well as GAM frequency and amplitude. The coherent GAM was then investigated with global runs. Simulations qualitatively agree with experiments and a good match is recovered with ORB5 results when the same physical model is used. Finally, the hypothesis of a coherent-dispersive GAM transition related to the safety factor profile is addressed. It is found that changing only q is not sufficient to induce a regime transition, which thus appears to be due to other parameters, including finite machine size effects.

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