Nonlinear co-existence of beta-induced Alfvén eigenmodes and beta-induced Alfvén-acoustic eigenmodes

Abstract

The nonlinear coexistence of b-induced Alfv en eigenmode (BAE) and b-induced Alfv en-acoustic eigenmode (BAAE) is found in simulations using the gyrokinetic toroidal code, which provides a new mechanism responsible for BAAE excitation in tokamaks. Here, the normalized pressure b is the ratio between plasma thermal pressure and magnetic pressure. The nonlinear simulation results show that the BAAE branch emerges after the BAE branch is saturated. The mode structure's evolution shows that existence of BAAE will change the original BAE mode structure. The perturbed distribution functions in the velocity phase space show that a new resonance region manifesting the wave-particle resonance in the BAAE branch appears during the nonlinear coexistence stage.

Gyrokinetic simulation of low-frequency Alfvénic modes in DIII-D tokamak

“…In this work, for physics identification and understanding of the BAE and LFM observed in the experiments, we perform first-principle linear simulation study of the DIII-D shot #178631 using global gyrokinetic toroidal code (GTC) [19], which has been extensively verified and validated for AE simulations [20][21][22] and applied for physics study of low-frequency AEs [23][24][25][26][27][28][29]. In particular, earlier GTC simulations of a tokamak with concentric cross-section find that BAAE and BAE can be simultaneously excited with comparable linear growth rates even though damping rate of BAAE is much larger than BAE in the absence of EPs [27].…”

Fine structure zonal flow excitation by beta-induced Alfvén eigenmode

“…It is observed in HL-2A tokamak that the BAEs can also be driven unstable by energetic electrons (EE) generated in both Ohmic and electron cyclotron resonance heating (ECRH) plasmas [28,29]; and it is found that the condition for this EE-driven BAE (eBAE) destabilization is related to not only the population, but also the pitch angle and energy of EEs. It is shown, in both gyrokinetic simulations using HL-2A parameters [30,31], as well as gyrokinetic analytical theory based on generalized fishbone like dispersion relation [32][33][34], that eBAE can be driven unstable by the precessional resonance of trapped EEs. Nonlinear generation of zonal field by BAE, on the other hand, is investigated using both gyrokinetic simulation [35] as well as gyrokinetic theory [25].…”

Verification of Energetic-Particle-Induced Geodesic Acoustic Mode in Gyrokinetic Particle Simulations

“…The GTC has been successfully applied to study GAMs, [10,15] turbulence, [24,43,44] and MHD instabilities induced by energetic particles such as fast-ion-driven toroidal Alfvén eigenmodes (TAEs), [45,46] as well as energetic-electron-driven 𝛽induced Alfvén eigenmodes (e-BAE). [47,48] In our simulations, the thermal ions are described by nonlinear gyrokinetic equations. [49] The thermal electrons are described by an electrostatic version of the fluid-kinetic hybrid model, [50−52] in which the electrons' response is expanded into a lowest-order adiabatic element and a high-order non-adiabatic kinetic perturbation element.…”