Neutral flow interaction with a magnetic dipole plasma II. Global model

Abstract

A simulation study is presented for a collisional, low-Temperature dipole plasma encountered by rarefied, hypersonic neutral gas flow. A global model is developed and averaged over a toroidal control volume to describe the mass and energy exchange between the neutral stream and plasma. Simulations over a large range of magnet and freestream parameters reveal three distinct physical regimes that have significant bearing on the magnitude of plasma/flow interaction. The transitions between these regimes exhibit characteristics of resistive critical ionization, whereby the relative kinetic energy between plasma and neutral gas collisionally heats electrons, driving rapid and complete ionization of the gas. Two regime transitions are observed here with sudden exponential increases in plasma density occurring at velocity thresholds that depend on several energy loss mechanisms. The higher-velocity transition is a classical presentation of critical ionization where flow neutrals are ionized directly by plasma electrons. The other is a unique case in which charge exchange between ions and flow neutrals supplies both the particles and energy required to initiate critical ionization. This transition is distinct from any critical ionization effect reported in literature and indicates the existence of a lower critical velocity governed by collisional and diffusive effects as opposed to ionization energy losses only. Drag force on the magnetic field is considered by examining absorption and deflection of ionized flow by the dipole. The critical ionization thresholds increase the force on the magnet by up to two orders of magnitude compared to aerodynamic drag on an equivalently sized flow impediment.