Abstract
Turbulence in tokamaks generates radially sheared zonal flows. Their oscillatory counterparts, geodesic acoustic modes (GAMs), appear due to the action of the magnetic field curvature. The GAMs can be driven unstable by an anisotropic energetic particle (EP) population leading to the formation of global radial structures, called energetic-particle-driven geodesic acoustic modes (EGAMs). The EGAMs can redistribute EP energy to the bulk plasma through collisionless wave-particle interaction. In such a way, the EGAMs might contribute to the plasma heating. Thus, investigation of EGAM properties, especially in the velocity space, is necessary for precise understanding of the transport phenomena in tokamak plasmas. In this work, the nonlinear dynamics of EGAMs without considering the mode interaction with the turbulence is investigated with the help of a Mode-Particle-Resonance (MPR) diagnostic implemented in the global gyrokinetic particle-in-cell code ORB5. An ASDEX Upgrade discharge is chosen as a reference case for this investigation due to its rich EP nonlinear dynamics. An experimentally relevant magnetic field configuration, thermal species profiles, and an EP density profile are taken for EGAM chirping modeling and its comparison with available empirical data. The same magnetic configuration is used to explore energy transfer by the mode from the energetic particles to the thermal plasma including kinetic electron effects. For a given EGAM level, the plasma heating by the mode can be significantly enhanced by varying the EP parameters. Electron dynamics decreases the EGAM saturation amplitude and consequently reduces the plasma heating, even though the mode transfers its energy to thermal ions much more than to electrons.
Highlights
An energetic particle (EP) beam, injected in tokamak plasma, can excite a variety of modes
The GAMs can be driven unstable by an anisotropic energetic particle (EP) population leading to the formation of global radial structures, called energetic-particle-driven geodesic acoustic modes (EGAMs)
The nonlinear dynamics of EGAMs without considering the mode interaction with the turbulence is investigated with the help of a Mode-Particle-Resonance (MPR) diagnostic implemented in the global gyrokinetic particle-in-cell code ORB5
Summary
An energetic particle (EP) beam, injected in tokamak plasma, can excite a variety of modes. Using two shifted Maxwellians as an EP distribution function, a nonlinear electrostatic (ES) gyrokinetic simulation with the code ORB5 produces a relative EGAM up-chirping close to the experimental one. In this simulation, EP parameters have been taken consistent with previous works Existence of an energy exchange between EPs and thermal particles is emphasized, and dominant resonant thermal species are indicated using the MPR diagnostic Such a kind of plasma heating has been recently demonstrated in Ref. 6 in a realistic 3D equilibrium of the LHD stellarator, using the hybrid code MEGA with kinetic treatment of both thermal and energetic ions. Even in the presence of drift-kinetic electrons the EGAM transfers most of its energy to the thermal ions, electron dynamics still clearly decreases the mode level in the considered AUG discharge, and in such a way reduces the energy flow from the EPs to the thermal ions
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