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Author: Yang Chen
Requested Type: Pre-Selected Invited
Submitted: 2018-03-01 13:50:38

Co-authors: G.Y. Fu, S. E. Parker

Contact Info:
University of Colorado at Boulder
2000 Colorado Avenue
Boulder, Colorado   80309

Abstract Text:
Alfven eigenmodes in a tokamak plasma can be excited by energetic particles. For near marginal instabilities the main nonlinear saturation mechanism is resonant particle trapping in the wave field. In a numerical study of such modes, the mode structure and frequency can then be taken from linear eigenmode analyses and assumed fixed, and only the mode amplitude or phase evolve in response to the energetic particle drive. However, the validity of the near marginal assumption should be established with a more self-consistent model. The effect of the n = 0 mode is of special interest because it can either be force generated by the unstable n ̸= 0 Alfven eigenmode, or spontaneously generated. As such n = 0 fluctuations naturally accompany any n > 0 instabilities. Here we use the GEM code to study the nonlinear evolution of reverse shear Alfven eigenmodes (RSAE) driven by energetic particles. Ions are gyrokinetic. Electrons are either fluid or drift-kinetic. When only the energetic particle nonlinear effects are included, the saturation amplitude of a single-n RSAE follows the trapping scaling, δB/B ∼ (γ/ω)2. When the nonlinear effects of thermal ions and electrons are included, zonal structures are force generated, but do not affect the saturation amplitude for γ/ω ≤ 0.03. No spontaneous generation of of zonal structures is observed, in contrast to ion-temperature-gradient-driven modes. At larger energetic particle drive, the effects of zonal structures cause a significant reduction in the RSAE saturation amplitude. The reduction is not caused by the zonal flow shearing of the RSAE, but by the force-generated n = 0 component in the thermal ion distribution function and the electron density. These n = 0 perturbations lead to nonlinear evolution of the RSAE mode structure and enhance damping.