Numerical simulation of the multiple reversed shear Alfvén eigenmodes associated with the triangularity Alfvén gap

It was found that there are multiplicity of low shear toroidicity-induced Alfvén eigenmodes in a zero beta limit if the inverse aspect ratio is larger than the magnetic shear at the mode location (Candy 1996 Phys. Lett. A 215 299). Because the reversed shear Alfvén eigenmode (RSAE) and even the RSAE associated with the non-circular triangularity-induced Alfvén eigenmode (NAE) gap (NAE-RSAE) usually reside near the shear-reversal point, the condition that the inverse aspect ratio is larger than the magnetic shear is naturally satisfied. For this reason, we numerically investigate the existence of multiplicity of core-localized NAE-RSAEs and mode characteristics in the present work. We firstly verify the existence of the multiplicity for zero beta plasma by using a D-shaped equilibrium. It is pointed out that, for a given toroidal mode number, the Alfvén cascade spectrum accommodates down-sweeping and up-sweeping modes above and below the NAE range of frequencies. An analytical model for the existence of multiple RSAE modes is in good agreement with the simulation results. One notices that the triangularity has a greater effect on the odd-type modes than that on the even-type modes:the odd-type modes come into existence because of the plasma triangularity.