IAEA FEC 2002, Paper EX/P1-17 Experimental Study of the Stability of Alfvén Eigenmodes on JET

Abstract. Over the last few years, experiments have been performed on JET to study the dependence of the AE stability limits on the main plasma parameters in different operating scenarios. The measurements are compared with theoretical models with the aim of improving the prediction capabilities for burning plasma experiments, such as ITER. An increase in the edge magnetic shear provides a significant stabilising contribution for AEs in plasmas characterised by a monotonic q-profile. Conversely, with non-monotonic q-profiles and Internal Transport Barriers, multiple weakly damped modes exist in the Alfvén frequency range even in the presence of a high edge magnetic shear, with possible negative implications for the AE stability in ITER. The dependence of the frequency and damping rate of n=1 TAEs on the bulk plasma β was also analysed using NBI heating in limiter plasmas. The mode frequency decreases for increasing β, in agreement with fluid and gyrokinetic predictions. Conversely, contrary to fluid predictions for intermediate and high-n TAEs, for low NBI powers we observe a splitting in the n=1 TAE frequency spectrum, accompanied with a reduction of the mode damping rate. The dependence of the damping rate for n=1 TAEs on the Larmor radius ρi has been investigated for plasmas characterised by a low magnetic shear in the core. In these plasmas the coupling between AEs and kinetic Alfvén waves is predicted to contribute significantly to the total damping rate for n=1 TAEs. This mechanism is accounted for by the radiative damping model in the NOVA-K code, and is a strong function of ρi. Wherea