Gyrokinetic investigation of Alfvén instabilities in the presence of turbulence

Abstract

The nonlinear dynamics of beta-induced Alfvén eigenmodes (BAEs) driven by energetic particles (EPs) in the presence of ion-temperature-gradient turbulence is investigated, by means of selfconsistent global gyrokinetic simulations and analytical theory. A tokamak magnetic equilibrium with large aspect ratio and reversed shear is considered. A previous study of this configuration has shown that the electron species plays an important role in determining the nonlinear saturation level of a BAE in the absence of turbulence (Biancalani et al 2020 J. Plasma Phys.). Here, we extend the study to a turbulent plasma. The EPs are found modify the heat fluxes by introducing energy at the large spatial scales, mainly at the toroidal mode number of the dominant BAE and its harmonics. In this regime, BAEs are found to carry a strong electron heat flux. The feed-back of the global relaxation of the temperature profiles induced by the BAE, and on the turbulence dynamics, is also discussed.

Nonlinear electromagnetic interplay between fast ions and ion-temperature-gradient plasma turbulence

“…If the electron plasma beta is increased further, the time-averaged value of the electrostatic and electromagnetic thermal fluxes is strongly increased. These (flux-tube) findings are consistent with the results of Citrin et al (2015) and recent radially global analyses (Biancalani et al 2020).…”

“…An interaction between AEs and zonal structures in the presence of turbulence has also been observed in different scenarios (see e.g. Biancalani et al (2018Biancalani et al ( , 2020), and possible mechanisms have been discussed in Zonca et al (2015). Although AEs are also known to saturate because of the energetic-particle density flattening, the saturation level of AEs is considered here as dominated by wave-wave interaction, consistently with the forced-driven excitation of zonal flows (Chen & Zonca 2012;Qiu, Chen & Zonca 2016).…”

“…If this condition is not fulfilled, i.e. the EPMs are linearly unstable, a significant turbulence destabilization is observed (Biancalani et al 2020) (see figure 4). Typically, these modes become the dominant instabilities when the total plasma pressure and its radial gradient exceeds a particular threshold.…”

“…The smaller this difference, the closer the sub-marginally stable fast-particle-driven TAE is to the linear stability threshold, leading to a more effective cross-scale coupling and to a stronger turbulence suppression. However, if α surpasses α c , a strong turbulence destabilization is observed, which is attributed to linearly unstable energetic-particle-driven modes (Biancalani et al 2020) (see figure 4). These predictions are consistent with the results presented in figure 12(a), where the main-ion turbulent fluxes are shown for different values of β e and magnetic shear.…”

“…These modes strongly increase particle/heat fluxes (see e.g. Pueschel, Kammerer & Jenko (2008), Romanelli et al (2010), Pueschel & Jenko (2010), Biancalani et al (2020), and references therein), thus causing degradation of the overall plasma confinement. Therefore, an essential task is to identify (i) the main parameter dependencies affecting this nonlinear threshold, to keep electromagnetic instabilities driven by supra-thermal particles sub-marginal, and (ii) the parameters leading to the most effective enhancement of the fast-particle turbulence suppression.…”