Influence of Upstream Field Structure on Primary Electron Loss for a Permanent Magnet Cusp

Abstract

An improved understanding of the electron loss behavior for permanent magnet cusps is needed to enable the design of efficient micro-scale plasma devices. Such devices can be used for a variety of applications, including high performance micro-thrusters that are attractive for primary propulsion for microsatellites, secondary propulsion for larger spacecraft, and formation flying. Conventional plasma loss theory for magnetic cusps generally relates the loss area to an analytical expression related to the magnetic field strength at the loss surface. In contrast, this study examines the importance of the upstream magnetic field structure on the loss behavior of high energy electrons. For the experimental effort, an electron gun is used to inject monoenergetic electrons towards a cusp confined discharge, while precision electron loss measurements are made at the face of a single permanent magnet point cusp. Measurements are taken at facility base pressure and with xenon background gas. A Monte-Carlo model is used to provide detailed examination of the electron confinement and loss behavior. Comparison of the experimental and computational results shows that the primary electron loss behavior is strongly influenced by the upstream magnetic field structure and is not simply dictated by the field strength very near the cusp collection surface. NomenclatureB B Block B Cylind H m L m m M q r = = = = = = = = = magnetic field magnetic field induced by a block magnet magnetic field induced by a cylindrical magnet height of a block magnet length of a magnet mass of an electron magnetization charge location of interestradius of curvature Larmor radius location of the magnet center radius of a magnet curvature drift velocity grad-B drift velocity parallel velocity perpendicular velocity width of a block magnet gradient of the magnetic field