Conservative Numerical Construction of Vlasov-Maxwell Equilibria and an Exact Cancellation of $E\times B$ Flow by an Electrostatic Pressure-Gradient Flow

Abstract

A class of Vlasov-Maxwell equilibria with pressure gradients is considered under symmetry assumptions that reduce the equilibrium equations to a nonlinear system of ordinary differential equations. Such equilibria provide self-consistent kinetic states with nonuniform thermodynamic profiles and potentially strong equilibrium electric fields, useful for fully kinetic stability studies of gradient-driven instabilities in the presence of equilibrium flows. The reduced equilibrium problem is posed as a nonlinear boundary-value problem for numerical solution, and a discretization is developed that preserves an associated energy invariant exactly at the discrete level. The resulting method provides a systematic means of computing nontrivial equilibria without relying on scale separation, and an alternative to constructing only approximately self-consistent initial states. A key property of the species flows, identified within this model for any distribution function of the constants of motion, is an exact cancellation between the $E\times B$ flow and an electrostatically generated component of the pressure-gradient-driven flow. This follows directly from a structural property of the species force balance that arises from this type of symmetry reduction, and is verified numerically. Additional numerical results demonstrate the conservative properties of the discretization. Representative equilibrium configurations, relevant to future kinetic instability studies, are also presented. An asymptotic approximation for the interior equilibrium, derived in the well-magnetized, quasi-neutral regime, provides an independent check on these results. The present setting is deliberately simple, providing a controlled framework for constructing and analyzing self-consistent kinetic equilibria and a starting point for extensions to more general symmetry-reduced configurations.