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Y. Todo, H. L. Berk, B. N. Breizman
Simulation Study of Nonlinear Magnetohydrodynamic Effects on Alfven Eigenmode Evolution and Zonal Flow Generation
Date of publication:
22 IAEA Fusion Energy Conference, THW/2-3Ra
Nonlinear magnetohydrodymamic (MHD) effects on Alfven eigenmode instability and on Alfven eigenmode bursts were investigated via hybrid simulations of an MHD fluid interacting with energetic particles. The investigation of the Alfven eigenmode instability focused on the evolution of an n=4 toroidal Alfven eigenmode (TAE) which is destabilized by energetic particles in a tokamak. In addition to fully nonlinear code, a linear-MHD code was used for comparison. The only nonlinearity in that linear code is from the energetic particle dynamics. No significant difference was found in the results of the two codes for low saturation levels, deltaB/ B ~ 10^-3 . In contrast, when the TAE saturation level predicted by the linear code is deltaB/ B ~ 10^-2 , the saturation amplitude in the fully nonlinear simulation was reduced by a factor of 2 due to the generation of zonal (n=0) and higher-n (n ? 8) modes. This reduction is attributed to the increased dissipation arising from the non-linearly generated modes. The fully nonlinear simulations also show that geodesic acoustic mode is excited by the MHD nonlinearity after the TAE mode saturation. Furthermore, energetic-particle source, loss, and collisions are implemented in the hybrid simulation code. The energetic particles are simulated using the deltaf particle-in-cell method with a time-dependent equilibrium distribution function f_0. The first numerical demonstration of Alfven eigenmode bursts with parameters similar to the TFTR experiment and with MHD nonlinearity retained is presented.
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