Observation of bi-directional global Alfven Eigenmodes modes in the MAST-U tokamak
The first observations and classification of sub-cyclotron frequency range Alfvén eigenmodes (AEs) are presented for the MAST Upgrade tokamak. Sets of observed eigenmodes are separated by approximately 200 kHz. We observe a low frequency separation of ~10 kHz within the set, and the toroidal mode number n increases with frequency sequentially. The ~200 kHz step between the sets coincides with the step between consecutive lines of the Shear Alfvén continuum according to linear, ideal MHD modeling. The sub-cyclotron frequency AEs are the global Alfvén Eigenmodes (GAE) localized in continuum minima according to our modeling. Each set contains the same range of n. These observations are consistent with our GAE modeling. In low plasma current (Ip) discharges, we observe only counter-propagating GAEs. These GAEs are located near the magnetic axis. We observe counter and co-propagating GAEs simultaneously in high Ip discharges. The co-propagating GAEs occur because of the formation of second continuum minima. Very flat safety factor profiles in high Ip MAST-U discharges form these minima. These minima are located at half of the plasma radius. We observe GAEs with properties very different to compressional (CAE) Alfvén Eigenmodes previously described in the MAST tokamak [S. E. Sharapov et al 2014 Physics of Plasmas 21 082501] before the upgrade.