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OT09 · Nonlinear MHD Effects on the Alfvén Eigenmode Evolution

Y. Todo¹, H. L. Berk², B. N. Breizman²

¹National Institute for Fusion Science, Toki, Japan
²Institute for Fusion Studies, University of Texas at Austin, Austin, United States

Abstract:Two types of hybrid simulations of MHD fluid and energetic particles were carried out to investigate MHD nonlinear effects on Alfvén eigenmode evolution. The first type contains fully nonlinear effects of both the MHD fluid and the energetic particles. The second type of the simulation is similar to the first type but different in that the MHD equations are linearized. Comparison between the results of the two types of simulations clarifies the MHD nonlinear effects. A tokamak plasma, where a toroidal Alfvén eigenmode (TAE) with toroidal mode number n=4 is the most unstable, was investigated. When the saturation level is
δB/B ~ 2 x 10^-2 in the linear MHD simulation results, we found that the saturation & level is δB/B ~ 8 x 10^-8 in the nonlinear MHD simulation results. The MHD nonlinear effects suppress the saturation level of the TAE. Detailed analyses indicate that the suppression effect arises from the change in n=0 harmonics of the magnetic field that is generated by the nonlinear electric field -v_TAE x dB_TAE , a product of the velocity field and the magnetic field of the TAE. Axisymmetric velocity fields are also generated in the nonlinear run, although the change in the n=0 magnetic field plays the dominant role in the suppression of TAE.

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