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Description
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], using 6D turbulence simulations with a specially optimized code that resolves the Larmor orbits, we found potentially significant non-gyrokinetic instabilities for steep, but not unrealistic, gradients in the tokamak edge with simplified slab geometry. These instabilities may be classified as unstable ion-Bernstein waves (IBW). To understand these phenomena, we extended the ITG analysis to high-frequency non-gyrokinetic modes. This extension requires modifying the zero-frequency gyrokinetic polarization using the Gordeyev function, as well as providing rigorous estimates of the magnitudes of the infinite sums appearing in the dispersion relation.
Similar to the ITG modes, we derive a threshold criterion for the instability. However, unlike gyrokinetic ITGs, the non-gyrokinetic modes require only the presence of a temperature gradient and not a particularly high ratio of temperature to density gradients. In contrast, gyrokinetic ITGs are suppressed when the density gradient is too large relative to the temperature gradient (the $\eta_i$ criterion). Interestingly, the IBW growth rate tends to increase with the density gradient.
It is likely that including additional physics (e.g., magnetic curvature or interactions with kinetic electrons) will significantly amplify the drive of similar non-gyrokinetic instabilities, potentially leading to an expansion of the field of strongly magnetized plasma turbulence.
[1] M. Raeth, K. Hallatschek, "High-Frequency Nongyrokinetic Turbulence at Tokamak Edge Parameters", Phys. Rev. Lett 133, 195101
