Physics of Gridless Ion Sources for Space Propulsion
A remarkable property of gridless ion sources such as Hall thrusters or cusp-field thrusters is that a large electric field is generated in a quasineutral plasma as a result of a local drop of electron conductivity induced by a magnetic barrier. Although the basic principles of these thrusters are relatively simple, the underlying physics is complex because plasma transport across the magnetic field may be subject to instabilities and turbulence and can be affected by electron-wall interactions. We present an overview of the present understanding of the physics of gridless ion thrusters, with emphasis on cross-field transport, instabilities and electron-wall interactions, and their consequences on thruster performance. Physics issues of alternative Hall thrusters designs such as magnetically shielded thrusters or double-stage thrusters, and of cusp-field thrusters will be also briefly discussed.