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

Experimental observations and theoretical interpretations are detailedly presented for the high frequency axisymmetric mode driven by nonlinear mode coupling. In the neutral beam injection heating plasma, toroidal Alfven eigenmodes (TAEs) are found to nonlinearly interact with tearing mode and result in the generations of other Alfvenic sidebands (W. Chen et al. EPL 107(2014) 25001). Then two TAEs couple together and contribute to the excitation of axisymmetric mode with frequency in the ellipticity-induced Alfven eigenmode (EAE) frequency region. Direct evi- dences suggest the axisymmetric modes play an energy transfer channel role during the nonlinear procedure and cause growth of magnetohydrodynamic (MHD) 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 represent the mode frequencies lying in EAE frequency range.