Study of the impact of fast ions on core turbulence at rational surfaces via global gyrokinetic simulations

TL;DR

A global gyrokinetic study using GENE demonstrates that fast ions can suppress core turbulence around rational $q$ surfaces through a dilution mechanism and a self-generated $E\times B$ shear layer that forms via eddy self-interaction, especially near $q=1$. The suppression is strongest when fast-ion effects raise the local shear and reduce turbulent transport by up to ~90%, but is balanced by a quasi-resonant channel that can destabilize drift waves near resonance. When a high-$T_f$ fishbone driven by EPs is present, beat-driven interactions between the fishbone and turbulence generate additional zonal structures that further modulate transport, though excessive profile flattening can negate the benefit. Overall, the work links fast-particle physics and rational-surface geometry to turbulence control, offering insights for achieving enhanced confinement in tokamaks with substantial fast-particle populations.

Abstract

In this work, the interplay between fast ions and safety factor rational surfaces is studied in a turbulent plasma via global nonlinear gyrokinetic simulations. Initially, the fast particles-induced enhancement of shearing structures from turbulence self-interaction is analyzed. Our study takes into account the competition between this mechanism and other fast ions effects, i.e. thermal profiles dilution and quasi-resonant interaction. We find the fast ions-induced reduction of destabilization threshold for the zonal modes to be a very efficient way to suppress turbulence. Indeed, it leads to the formation of regions where turbulent transport is reduced by 90\% of its original value. Furthermore, an $n=m=1$ fishbone is driven unstable inside the plasma and its interaction with turbulence is studied. We find the beat-driven zonal structure generate by this mode to further reduce turbulence when its presence does not drastically flatten the thermal profiles.

Figures (20)

• Figure 1: (a) Density, temperature and (b) normalized density gradients radial profiles for each plasma species. (c) Safety factor and magnetic shear profiles for the "reference" and "shifted" rational surface (identified by the $\:\tilde{(...)}\:$ superscript) setups. In (a), the fast ions density is rescaled by a factor 5 in order to facilitate visualization.
• Figure 2: Normalized (a) growth rate and (b) frequency radial average for three scenarios. One without fast particles, one with $T_f=40$ and one with $T_f=40$ but fast ions set as a dilution species. In the setup without fast particles, $n_i$ and $\omega_{n_i}$ are raised to 1.06 and 0.75, respectively.
• Figure 3: Normalized (a) growth rate and (b) frequency radial average for some of the scenarios considered. In particular, one with $T_f=1$ and one with $T_f=10$ are added with respect to the previous figure.
• Figure 4: (a) Electrostatic heat flux for the fast ions minority at $T_f=10$, $n=25$, $z=0$ in velocity space. Resonance curves are plotted from Eq.(\ref{['Eq2']}) using $k_y=0.31$ and $\omega_r=0.18$, corresponding to $n=25$ in figure \ref{['figure2bis']}. (b) Normalized growth rates from local runs at $r/a=0.5$ for the $n=10$, 25, 40 modes at different $T_f$ values. Dashed lines represent $\gamma$ values obtained setting fast ions as a dilution species.
• Figure 5: Turbulent total (electrostatic + electromagnetic) heat fluxes for (a) ions, (b) electrons and (c) fast ions species for different $T_f$ values. All the profiles are averaged over a time interval in which turbulence is saturated. The shaded region represents an interval of amplitude $r/a=0.01$ around the $q=1$ position.