Damping rates of global Alfven waves in tokamaks

The ability to predict the stability of fast-particle-driven Alfven eigenmodes in burning fusion plasmas requires a detailed understanding of the dissipative mechanisms that damp these modes. In order to address this question, the linear gyro-kinetic, electromagnetic code LIGKA is employed to investigate their behaviour in realistic tokamak geometry. Recently, the model and the implementation of LIGKA were extended in order to capture rigorously the coupling of the shear Alfven wave to the drift and sound waves. This coupling becomes important for the investigation of modes like the cascade modes that chirp from the geodesic-acoustic frequency up to the TAE frequency. Especially interesting in this context is the transformation of the Cascade mode into a TAE mode: non-local continuum damping and also radiative damping that depend strongly on the details of the q-profile, can increase the mode damping considerably at the transition point. This may explain, why the experimental mode signal as measured by magnetic pick-up coils or reflectometry often vanishes at a critical value for qmin. Furthermore, reflectometry measurements provide information about the radial Eigenfunction structure. Therefore, the gyrokinetic results can be directly compared to ASDEX-Upgrade measurements for both radial envelope and phase. (author)