Active excitation and damping rate measurement of intermediate-n toroidal Alfvén eigenmodes in JET, C-Mod and MAST plasmas

Abstract

The stability properties of Alfvén eigenmodes (AEs) are investigated directly using external antenna excitation and detection of stable modes in a variety of plasma configurations in different devices. Dedicated methods to measure the AE damping rate separately from the fast ion drive have been pioneered at JET, using low toroidal mode number internal saddle coil antennas. Other experiments have since installed localized in-vessel antennas to drive and detect MHD modes in the Alfvén frequency range, first on C-Mod, then on MAST. Experiments on C-Mod proved for the first time that intermediate-n toroidal AEs can be driven and detected, and point out significant differences with respect to the low-n regime on JET in the values and scaling of the damping rate with plasma parameters, e.g. the edge shape. On JET, a new antenna system, comprising two assemblies of four toroidally spaced coils each, was developed to replace the low-n saddle coil structure and excite AEs in the toroidal mode number range that is expected to be most unstable in ITER, with n up to ∼10. Experiments with the new JET antennas confirm that excitation is possible in a large volume plasma, together with real-time tracking of core modes throughout the limiter and divertor phases of high performance discharges, with significant additional heating. The similarities and differences between the active MHD antenna systems as well as a comparison of the results on C-Mod, JET and MAST are illustrated. Both C-Mod and JET results underline the fact that a precise reconstruction of the mode structure and its spectrum, important for a quantitative comparison with theoretical models, represents a significant challenge in the intermediate-n range and in the presence of several modes.