Identifying Alfvén eigenmodes in the early phase of advanced tokamak plasmas

Abstract

We examine the fast-ion driven Alfvén eigenmodes (AEs) in the frequency range 0–500 kHz during the current ramp-up phase of advanced JET plasma before the main heating starts (the preheating phase). Modelling of AEs using equilibrium reconstruction and the ideal MHD spectral code MISHKA shows that three types of mode commonly found during the preheating phase correspond to: global toroidal Alfvén eigenmodes (TAEs) found in regions of moderate to high magnetic shear; core-localized TAEs found in regions of low magnetic shear; and Alfvén cascades found in regions of zero shear. MHD spectroscopy diagnosis of the time evolution of the plasma current within the central region of the plasma and of the minimum safety factor value based upon the frequency evolution of these unstable AEs is discussed.