Description
Beyond Gyrokinetics: Gradient Driven Ion Cyclotron Instabilities in Thermal Tokamak Plasmas
The stability analysis of gyrokinetic slab ITG modes is well-established and results in temperature and density gradient stability thresholds that depend on the parallel and perpendicular wavenumbers.
In a recent PRL [1], we carried out 6D turbulence simulations with a specially optimized phase space fluid code capable of resolving individual Larmor orbits. In simplified slab geometry, we found significant non-gyrokinetic instabilities, for steep, yet realistic, tokamak edge gradients. These modes, at or above the ion cyclotron frequency, can be identified as unstable ion-Bernstein waves (IBW) or more broadly as ion cyclotron waves (ICW).
Ion cyclotron frequency emission (ICE) has recently been detected in virtually all existing tokamaks, both in the core and edge, where it is often more pronounced in H-mode [2]. Although ICE is usually attributed to energetic particles, our stability analysis shows that ICWs can be driven by gradients in purely thermal plasmas. Strikingly, they emerge at gradient levels and wave-numbers not typically associated with kinetic instabilities.
To understand these phenomena, we have extended the low-gradient ITG analysis to high-frequency non-gyrokinetic modes. Similar to the ITG modes, we have derived instability thresholds. However, unlike gyrokinetic ITGs, the non-gyrokinetic modes require only the presence of a temperature gradient and no large ratio of temperature to density gradients. Whereas gyrokinetic ITGs are suppressed when the density gradient becomes too large relative to the temperature gradient (the 𝜂𝑖 criterion), the IBW growth rate actually increases with the density gradient.
Extending the analysis further, we show that drift-kinetic electrons, magnetic drifts, and magnetic perturbations can strengthen the drive of these instabilities. These results suggest that ion-cyclotron–scale turbulence may occur under far broader conditions than previously assumed, pointing toward an expanded regime of strongly magnetized plasma turbulence.
[1] M. Raeth, K. Hallatschek, "High-Frequency Nongyrokinetic Turbulence at Tokamak Edge Parameters", Phys. Rev. Lett 133, 195101, (2024)
[2] L. Liu, et al., Nucl. Fusion 63, 104004, (2023)