The Influence of Plasma Shaping Effects on the Damping of Toroidal Alfven Eigenmodes

Full text: The dependence of the damping of toroidal Alfven Eigenmodes (TAEs) on various plasma parameters and shapes is an important field of research in view of the prediction and control of fast particle transport in future fusion experiments like ITER and DEMO. Experimentally, the measurement of TAE damping rates by active in-vessel antennas at JET and Alcator C-mod allows for a direct comparison of both frequency and damping rate with theory. However, it is well known that uncertainties in the equilibrium quantities and profiles can influence the damping significantly, since the damping is exponentially sensitive to these background parameters. Therefore, parameter scans in well diagnosed discharges are a very promising way to benchmark and validate the theoretical and numerical models against the experiment. For this reason dedicated experiments were proposed at the Joint European Torus (JET). On the one hand, a slowly relaxing q-profile changes the gap structure significantly and therefore the relation between core and edge damping. On the other hand, an elongation scan was carried out while keeping the density and the central safety factor value fixed. A series of equilibria was carefully reconstructed that are the basis for both a detailed physics analysis of the effect of q-profile relaxation and shaping on the TAE stability as well as a world-wide code benchmark. The models range from perturbative MHD codes like CASTOR-K and NOVA-K via a warm dielectric tensor model like LEMan and TASK/WM to gyrofluid (TAEFL) and linear gyrokinetic codes (LIGKA). Also codes designed for stellarator geometries like CAS3D-K and AE3D-K are planning to contribute to this benchmark in the tokamak limit. Secondly, the mode frequency, the damping and the mode structure can be compared. Although the TAE mode frequency is a very robust quantity and therefore easy to identify, several modes of the same toroidal mode number can be present in the same gap requiring also a comparison of the mode structure. Finally, after documenting the code-code comparisons in detail, the limits and caveats of comparing the simulation results to the experiment are discussed. A sensitivity study will be crucial for this point. Also the coupling between the vacuum region and the plasma that is neglected by some codes may account for differences between measured and calculated damping rates. (author)