Closures for moment-based gyrokinetic models

Moment-based models bridge the gap between fluid and kinetic descriptions of plasmas. While high collisionality naturally produces the fluid limit from the infinite moment hierarchy, systems that are weakly collisional do not have this advantage, leaving the question of how the resolved moments are affected by their coupling to the unresolved moments. This is the classic moment closure problem. What is the “right” closure? How many moments (how much velocity space resolution) are required to resolve the key physics in any given calculation? Existing gyrokinetic algorithms generally rely upon high resolution to sidestep these questions. This approach can be very expensive. Closures, on the other hand, must address complications such as finite Larmor radius effects and nonlinearities, which introduce complicated couplings to unresolved moments. We present recent work that builds upon the parallel phase-mixing closures of Hammett and Perkins [PRL, 1990], in the context of gyrokinetics.