Magnetic Field Effects on Secondary Electron Emission in Hall Thrusters

Abstract

Secondary electrons emitted at the dielectric walls of a Hall thruster are accelerated by the sheaths and enter as monoenergetic beams into the main plasma where they are magnetized. This work is focused on the influence of the magnetic field on secondary beams in the low-collisionality limit. First, it is shown that a magnetic field oblique to the walls may have an important trapping effect on the beam. Second, effects of curvature and grad-B drifts, and of magnetic mirror are evaluated on individual particles for magnetic field topologies of Hall thrusters. I. Introduction The operation and lifetime of a Hall thruster are strongly affected by the interaction of the bulk plasma with the dielectric walls. A key factor to explain this interaction is the secondary electron emission from the channel walls, since it adjusts the potential in the sheath and enhances the electron conductivity across the magnetic field. The current models of plasma-wall interaction 1-4 define a parameter of effective secondary electron emission, and introduce a partial trapping effect on secondary electron beams inside the plasma channel. However, it is an unsolved issue how much trapped the secondary electrons are. The secondary electrons progress through the plasma bulk subjected to the electromagnetic field, the collisional processes acting upon them, and the plasma instabilities. This work analyzes the influence of the magnetic field topology on secondary beams in the low-collisionality limit. We show that a trapping effect on the secondary beam is possible for curved and straight oblique magnetic fields. However, whether the beam remains trapped within the bulk of the plasma or recollected by one of the walls is hard to predict because of the small electron Larmor radius. We also study the asymmetric effects introduced by the electric and magnetic fields, such as magnetic mirror, and the particles drifts due to the gradient and curvature of the magnetic field. The rest of this paper is organized as follows. In Sec. II we present the fundamentals of the model. In Secs. III and IV we show the results for straight oblique and curved magnetic fields, respectively. Conclusions are presented in Sec. V. II. Formulation of the model A. Geometry and electromagnetic field.