Low-frequency Alfvén eigenmodes during the sawtooth cycle at ASDEX Upgrade

Abstract

The confinement of fast particles, present in tokamak plasmas as nuclear fusion products and through external heating, will be essential for any future reactor. Fast particles can be expelled from the plasma through their interaction with Alfvén eigenmode (AE) instabilities. AEs can exist in gaps in the Alfvén continuum created by plasma equilibrium non-uniformities. In ASDEX Upgrade low-frequency modes in the Alfvén-acoustic frequency regime, including beta-induced Alfvén eigenmodes (BAEs) and lower frequency modes with mixed Alfvén and acoustic polarizations, have been observed. They exist in gaps in the Alfvén continuum opened up by geodesic curvature and finite plasma compressibility. In this paper a kinetic dispersion relation (Lauber et al 2009 Plasma Phys. Control. Fusion 51 124009) is solved numerically to investigate the influence of diamagnetic effects on the evolution of these low-frequency modes during the sawtooth cycle. Other distinct but potentially related modes which sweep significantly upwards in frequency towards the end of the sawtooth cycle are also considered. Using information gained from soft x-ray measurements (Igochine et al 2010 IPP Report 1/338) and electron temperature information from electron cyclotron emission to constrain the safety factor profiles, realistic equilibrium reconstructions for the analysis are obtained using the CLISTE code (Mc Carthy 2012 Plasma Phys. Control. Fusion 54 015010). The results for the mode frequency evolution are then compared with experimental results from ASDEX Upgrade.