Physics and instabilities of low temperature ExB plasmas for spacecraft propulsion and other applications

In low temperature ExB plasmas, an external magnetic field is applied perpendicular to the discharge current or applied electric field to increase the residence time of electrons in the discharge and allow ionization and plasma sustainment at low pressures in small devices. Such ExB configurations are used in various applications such as Hall thrusters for satellite propulsion, magnetron discharges for plasma processing and negative ion sources for neutral beam injection in fusion. The plasmas in these devices are partially magnetized, i.e. electrons are strongly magnetized while ions are not, and are subject to various micro and macro-instabilities that are very distinctive from instabilities in fusion plasmas, and are often triggered by the large difference in electron and ion drift velocities in the ExB direction. The possibility of maintaining a large electric field in the quasineutral plasma of Hall thrusters in spite anomalous electron transport, or the presence of strong double layers associated with azimuthal rotation of plasma structures (“rotating spokes”) in magnetron discharges and Hall thrusters are examples of the very challenging and exciting physics of ExB devices. In this tutorial presentation we will describe some important aspect of this physics, and recent advances, based on simulations, theory, and experiments, in our understanding of the different types of instabilities (electron cyclotron drift instability, Simon Hoh instability, ionization waves rotating in the ExB direction, …) present in these plasmas and of their consequences on the device operations.