Gyrokinetic study of slab universal modes and suppression of the Gradient Drift Coupling (GDC) instability

A local linear gyrokinetic stability analysis of a collisionless, shearless slab geometry in an equilibrium pressure balance with constant $p_0+B_0^2/(8\pi)$. We focus on $k_\parallel=0$ modes, electromagnetic universal (or, entropy) modes driven by density or temperature gradients at small and large plasma $\beta$. These are small-scale non-MHD instabilities with growth rates that typically peak near $k_\perp\rho_i\sim1$ and vanish in the long wavelength limit ($k_\perp\to0$). Analytic analysis indicates that a necessary condition for instability is that at least one of $\eta_e$ or $\eta_i$ be negative, where $\eta_\alpha=L_n/L_{T\alpha}$ is the ratio of the density and temperature gradient scale lengths. That is, the density gradient must point in the opposite direction as the electron or the ion temperature gradient for this slab mode to be unstable [1]. This instability is also explored with GENE, and we discuss its relation to the Gradient Drift Coupling (GDC) instability [2,3], which arises from neglecting the pressure balance equilibrium and was described to have a finite growth rate $\gamma\simeq\sqrt{\beta/[2(1+\beta)]}C_s/|L_p|$, with $C_s^2=p_0/\rho_0$ at $k_\perp\to0$ (long wavelength).

[1] Rogers, B. N., Zhu, B., Francisquez, M., PoP, 25(5), 052115 (2018).
[2] M. Pueschel, P. Terry, D. Told, and F. Jenko, PoP 22, 062105 (2015).
[3] M. Pueschel et. al., PPCF 59, 024006 (2017).