Recent advances in the physics of Hall thrusters

In comparison with chemical engines, electric propulsion (EP) systems offer significant advantages for commercial and scientific space missions in terms of payload mass, launch cost and mission duration due to a fast propellant ejection speed that warrants a large propellant mass saving. For more than two decades, ion thrusters have been used mainly for north–south station keeping of geosynchronous communication satellites and for space probe journeys towards celestial bodies like asteroids and dwarf planets. With the increase in electrical power available onboard spacecraft, new possibilities are emerging like orbit topping and orbit transfer maneuvers of large satellites, so making astronautics fall into the all-electric satellite era. Besides, progress in the miniaturization of existing EP devices and development of new technologies and concepts will open the way for novel applications like micro-satellite constellations, nano-satellite formation flying and active spacecraft end-of-life deorbiting.

Among all the different EP devices, two technologies, namely the gridded ion engine (GIE) and the Hall thruster (HT), are at the present time mature with a long flight heritage. GIEs are characterized by a purely electrostatic acceleration of ions, the latter being extracted from a plasma through a set of high-voltage grids. In contrast, HTs are gridless engines. They provide thrust by acceleration of ions in a low pressure discharge in a crossed electric and magnetic field configuration. GIEs are characterized by a high ion ejection velocity and a relatively low thrust level. On the contrary, HTs provide a relatively large thrust with a moderate specific impulse. Due to their large thrust to-power ratio and large thrust density, Hall thrusters represent an attractive electric propulsion technology for satellites maneuvers as well as for several of the foreseen applications for which thrust is a critical parameter.

Despite 40 years of HT development and investigation, many physical mechanisms that govern the behavior and the performance of such a magnetized discharge are nevertheless ill-understood and not well quantified. Among others one can cite electron diffusion across the transverse magnetic field, plasma–wall interactions, plasma instabilities as well as geometrical and size effects. This lack of knowledge is clearly a limiting factor for various aspects of the technology: optimization of existing thrusters, derivation of scaling laws, utilization of alternative propellants and development of new architectures. In this contribution, I will review some of the advances regarding the physics of the Hall thrusters obtained in the last years through experimental works performed at the ICARE laboratory in Orléans, France. As we shall see, results show the possibility to extend the HT lifetime and to broaden the operating envelope without degradation of the performance level. Moreover, miniaturization of HTs down to sizes suitable for micro-spacecraft now appears feasible.