Observation and theory of nonlinear mode couplings between shear Alfvén wave and magnetic island in tokamak plasmas

Experimental observation and theoretical analysis are presented for the nonlinear mode couplings between shear Alfven wave and magnetic island. In the sub-Alfven frequency range, two kind axi-symmetry magnetic activities with n = 0 have been observed during NBI on HL-2A. One kind has been identified, and belongs to EGAM. Another kind is found for the first time, its frequency lies in the range of TAE frequency. The Fourier bicoherence analysis suggests these axi-symmetry modes are generated by the nonlinear mode coupling via the decay process between Alfven eigenmodes and low-frequency MHD modes. The experimental results indicate that the nonlinear mode coupling is one of mechanisms of the energy cascade in energetic-particle turbulence or Alfven turbulence.