Abstract
Numerical simulations of Alfvén modes driven by energetic particles are performed with the gyrokinetic (GK) global particle-in-cell code ORB5. A reversed shear equilibrium magnetic field is adopted. A simplified configuration with circular flux surfaces and large aspect ratio is considered. The nonlinear saturation of beta-induced Alfvén eigenmodes (BAE) is investigated. The roles of the wave–particle nonlinearity of the different species, i.e. thermal ions, electrons and energetic ions are described, in particular for their role in the saturation of the BAE and the generation of zonal flows. The nonlinear redistribution of the electron population is found to be important in increasing the BAE saturation level and the zonal flow amplitude.
Highlights
Tokamak plasmas often present a supra-thermal species, due to external heating or to the product of fusion reactions
We investigate the separate contributions of the wave–particle nonlinearity of the different species, with emphasis on the importance of the nonlinear kinetic dynamics of the electrons
The choice of the magnetic equilibrium, thermal plasma profiles and energetic particle (EP) profiles and distribution function, is done for reasons of continuity with previous papers where results of Alfvén modes (AM) with ORB5 were presented (Biancalani et al 2016; Könies et al 2018) as the main goal of this paper is to study the nonlinear dynamics of the electrons in a simplified case, where the effect is isolated and clearly visible
Summary
Tokamak plasmas often present a supra-thermal species, due to external heating or to the product of fusion reactions. The importance of the electron Landau damping in the dynamics of AMs has been emphasized in the past mainly by means of analytical theory (Fu & Van Dam 1989; Betti & Freidberg 1992; Candy 1996), and recently global gyrokinetic simulations have shown how this can be the dominant damping mechanism of AMs in experimentally relevant configurations, like ASDEX Upgrade plasmas (Vannini et al 2020) Another possible saturation mechanism of AMs is given by the wave–wave coupling, i.e. the energy exchange among different modes (Chen & Zonca 2013).
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