Neutral flow interaction with a magnetic dipole plasma. I. Theory and scaling

Abstract

The interaction between a high-speed neutral gas flow and dipole magnetized plasma is investigated theoretically to examine how mass, momentum, and energy are transferred to the plasma. Single-particle trajectory analysis reveals the existence of an ion-trapping region within which ions are born into closed orbits around the magnet, thus adding mass and energy to the plasma. Ion deceleration through trapping or deflection is analyzed to quantify momentum transfer to the magnetic field. The drag force and transfer rate of mass and energy from the flow to the plasma are found to scale with two dimensionless parameters: (1) a characteristic ion Larmor radius normalized by the magnet radius and (2) a characteristic neutral reaction rate normalized by the neutral gas rate of transit. The energy transfer rate is maximized at a specific reaction rate, above which increased reactivity rapidly decreases energy capture as the interaction moves away from the ion-trapping region. As the electron energy confinement time increases, there is a transition from a mode in which seed plasma is required to sustain the interaction to a high-density mode that is sustained primarily by mass and energy from the neutral flow. Two distinct flow-sustained regimes are identified that depend on the ratio of the effective ionization energy to kinetic energy of the neutral gas particles. One of the two regimes corresponds to the well-known critical ionization velocity phenomenon. The other, in which charge exchange collisions are the dominant energy transfer mechanism, has not been previously identified as a separate physical regime.