Simulation of the eigenmode spectrum below the Toroidicity-induced Alfvén eigenmode gap generated by the coupling of Alfvén and slow-magnetosonic waves in tokamaks

The discrete Alfvén Eigenmode spectrum below the TAE frequency is studied for hybrid and sawtooth scenarios in tokamaks whereby the full coupling between the Alfvén and slow magnetosonic waves are taken into account. In this frequency regime the usally applied slow-sound approximation is not valid and modes in this frequency regime were recently called Alfvén-Slow Eigenmodes (ASE). For the hybrid scenarios (central q ≈ 2 up to ρ ≈ 0.6) a rich spectrum of eigenmodes was found that depend on the plasma pressure and magnetic shear. It was found that the number of modes below the TAE gap is the highest for weakly reversed q profiles while the number of modes increases with pressure. For zero central shear q-profiles the number of modes is lower than in the reversed shear case while for the normal shear hybrid q-profiles the number of eigenmode first increased with increasing pressure followed by a decrease in the number of eigenmodes. For plasma scenarios with central (sawtoothing plasmas) modes were found outside the q = 1 surface that are consistent with observations in contrast to the conventional BAE which is located at the q = 1 surface. The frequency behaviour of the modes below the TAE gap was studied for a reversed shear q-profile in which was varied and it was found that ASEs frequencies increase and/or decrease as a function of thereby emerging from and/or disappearing into the continuum.