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

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.

💡 Research Summary

In this paper the authors investigate how fast ions (FIs) interact with core turbulence in the vicinity of safety‑factor rational surfaces, using global nonlinear gyrokinetic simulations performed with the GENE code. The study focuses on a tokamak‑like plasma configuration that develops ion‑temperature‑gradient (ITG) driven turbulence around the q = 1 rational surface. Two different q‑profiles are employed: a reference profile with the q = 1 surface at r/a ≈ 0.47 and a “shifted” profile with the same rational surface at r/a ≈ 0.74. The plasma consists of electrons, bulk ions and a deuterium fast‑ion species; the latter is varied in temperature (T_f = 1–180 keV) while keeping its density fixed at 6 % of the electron density.

The paper first presents a linear analysis. Without fast ions the dominant unstable modes are ITG modes peaking at toroidal mode number n ≈ 25, rotating in the ion‑diamagnetic direction. Introducing fast ions reduces the linear growth rates by roughly 20 % and slightly shifts the real frequencies. This reduction is attributed primarily to the dilution effect: the presence of fast ions forces the bulk ion and electron densities to adjust in order to satisfy quasineutrality, thereby decreasing the effective drive for the ITG instability. To isolate this effect the authors run a special case where fast ions are treated only as a dilution species (i.e. they contribute to the polarization density but do not participate dynamically). The growth‑rate curves for this case coincide with those obtained when the fast ions are fully kinetic, confirming that dilution dominates the linear stabilization observed for T_f = 40 keV.

A second linear effect explored is the quasi‑resonant interaction between fast ions and the ITG mode. By scanning T_f, the authors find that lower fast‑ion temperatures (T_f ≈ 1–10 keV) increase the growth rates, while higher temperatures suppress them. This trend is explained by the resonance condition ω_r ≈ ω_d, where ω_r is the ITG real frequency and ω_d is the magnetic‑drift frequency of the fast ions (Eq. 2 in the manuscript). When the condition is satisfied, fast ions can exchange energy with the mode, enhancing its drive. Velocity‑space diagnostics show that the fast‑ion heat flux peaks around T_f ≈ 2 keV, confirming the resonance picture.

The nonlinear simulations reveal the central role of E × B shear (zonal) structures generated by turbulence self‑interaction. In the presence of fast ions, the shearing rate γ_E×B is strongly amplified at the rational surface, and the threshold for zonal‑mode destabilization is lowered. Consequently, the total (electrostatic + electromagnetic) heat fluxes of ions, electrons and fast ions are reduced by up to 90 % in a narrow radial window (Δr/a ≈ 0.01) centred on q = 1. The authors attribute this dramatic reduction to the combined effect of (i) dilution‑induced linear stabilization, (ii) enhanced zonal‑flow generation due to the lowered threshold, and (iii) the quasi‑resonant interaction that can either boost or diminish the drive depending on T_f.

Beyond the background turbulence, the paper investigates the impact of an n = m = 1 fishbone mode, which is driven unstable by the fast‑ion population when T_f is increased further (e.g., T_f = 180 keV). Linear scans confirm the existence of a low‑frequency, n = 1 mode resonant with the fast‑ion precession frequency. Nonlinear runs show that the fishbone generates a beat‑driven zonal structure (a secondary shear layer) that can further suppress turbulence, provided the mode does not flatten the bulk temperature and density profiles excessively. When the fishbone amplitude is moderate, the beat‑driven shear adds to the turbulence‑self‑generated shear, leading to an even larger reduction of transport. Conversely, a strongly growing fishbone flattens the profiles, weakening the shear and partially restoring turbulent fluxes.

Overall, the study demonstrates that fast ions influence core turbulence through three intertwined channels: (1) dilution of the thermal species, which weakens the linear ITG drive; (2) quasi‑resonant wave‑particle interaction, which can either enhance or diminish the drive depending on the fast‑ion energy; and (3) modification of the zonal‑flow generation threshold, leading to stronger E × B shear at rational surfaces. The most efficient turbulence suppression arises from the reduction of the zonal‑mode destabilization threshold, which allows shear layers to form more readily and quench turbulent eddies. The additional beat‑driven shear from a fishbone mode can amplify this effect, but only if the mode remains sub‑critical with respect to profile flattening. These findings provide valuable insight for future reactor scenarios where alpha particles and auxiliary heating generate substantial fast‑ion populations, suggesting that careful tailoring of fast‑ion parameters could be used as an active tool to control turbulent transport and improve confinement.