Nonlinear mode coupling induced high frequency axisymmetric mode on the HL-2A tokamak

Experimental observations and theoretical interpretations are presented in detail for the high frequency axisymmetric mode driven by nonlinear mode coupling. In the neutral beam injection heating plasma, toroidal Alfvén eigenmodes (TAEs) are found to nonlinearly interact with the tearing mode and result in the generation of other Alfvénic sidebands (Chen et al 2014 Europhys. Lett. 107 25001). Then two TAEs couple together and contribute to the excitation of an axisymmetric mode with frequency in the ellipticity-induced Alfvén eigenmode (EAE) frequency region. Direct evidence suggests the axisymmetric modes play an energy transfer channel role during the nonlinear procedure and cause the growth of magnetohydrodynamic instability with finite toroidal mode number via beating with tearing mode. Finally, the nonlinear process leading to high frequency n _E  =  0 mode generation is demonstrated using nonlinear gyrokinetic theory. Here n _E is toroidal mode number and the subscript E represents the mode frequencies lying in the EAE frequency range. (paper)