The hot-electron closure of the moment-based gyrokinetic plasma model

Abstract: We derive the hot-electron limit (HEL) closure of the moment hierarchy used to solve the gyrokinetic equations, denoted as gyromoment (GM). By expanding gyroaveraging kernels in the small temperature ratio Ti/Te limit and retaining only essential O(Ti/Te) terms, we obtain a closed system for density, parallel velocity, and parallel and perpendicular temperatures. In the Z-pinch geometry, the GM system with HEL closure is analytically equivalent to the one developed by Ivanov et al. (2022). Numerical benchmarks confirm the closure's accuracy in Z-pinch geometry, reproducing established linear growth rates, nonlinear heat transport, and low-collisionality dynamics. Extension to tokamak-relevant s-alpha geometry, and comparison with gyrokinetic simulations, reveals the HEL closed GM model capabilities and limitations: while transport levels and temporal dynamics are qualitatively preserved even at Ti/Te=1, the absence of higher-order kinetic moments prevents accurate Dimits shift prediction and transport suppression.