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

Abstract

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