Fluid simulation of axisymmetric mirror in orthogonal, field-aligned coordinates

Axisymmetric mirrors have regained academic and commercial interest in recent years, owing to their intrinsic engineering simplicity and recent advances in mirror physics. In this work, we present an explicit construction of a field-aligned and orthogonal coordinate system for finite-beta axisymmetric mirrors, which facilitates both theoretical analysis and numerical simulations. As realized in early studies [1], the necessary condition for the existence of such a coordinate system is the vanishing of plasma current along magnetic field in the equilibrium, which is generally satisfied for axisymmetric mirror configurations. We implement this coordinate system in the multifluid plasma simulation framework Hermes-3 [2], enabling more efficient and accurate simulations. Using the new coordinate system, we demonstrate fluid simulations of interchange instabilities in axisymmetric mirrors.