Description
Gyrokinetic theory typically assumes a local Maxwellian equilibrium, an approximation that has proven successful in describing microinstabilities and turbulence in the cores of conventional large–aspect-ratio tokamak plasmas. However, strong plasma gradients are known to drive substantial neoclassical currents. In leading-order gyrokinetic theory, the Maxwellian is independent of the sign of the particle parallel velocity, thereby neglecting neoclassical physics. These missing effects are expected to modify microstability and turbulent transport in the cores of spherical tokamaks and in H-mode pedestal plasmas, where electromagnetic instabilities often dominate turbulent transport. An extended gyrokinetic theory that retains the non-Maxwellian equilibrium and captures neoclassical physics is presented in Dudkovskaia $\textit{et al.}$ (2023a).
Here, we couple the $\delta \! f$-gyrokinetic code $\texttt{stella}$ (Barnes $\textit{et al.}$, 2019) to the drift-kinetic solver $\texttt{NEO}$ (Belli and Candy, 2008) to compute an extended equilibrium that self-consistently incorporates neoclassical corrections within finite-$\beta$ electromagnetic gyrokinetic theory. The corresponding gyrokinetic–Maxwell system is modified accordingly in the local limit, assuming a weak poloidal magnetic field. The resulting framework, $\texttt{NEO-stella}$, is benchmarked against $\texttt{NEO-gs2}$ (Dudkovskaia $\textit{et al.}$, 2023b) in both electrostatic and electromagnetic regimes, with kinetic ions and electrons, for circular and strongly shaped Miller equilibria. We find that linear growth rates can deviate from conventional predictions in the presence of strong plasma gradients. Such deviations are observed for electromagnetic drift waves and may be particularly relevant for high-$\beta$ spherical tokamak core plasmas. In addition, neoclassical corrections are found to become important at large values of $\rho_\star = \rho/L$, with potential implications for gyrokinetic simulations of burning plasmas containing energetic $\alpha$-particles. A natural extension to this work is first-of-their-kind nonlinear simulations in the presence of neoclassical corrections, with the aim of exploring their impact on the transition to regimes of enhanced heat flux.