Low power scaling of hall thrusters

Some problems concerning the operation of Hall thrusters at low power levels are discussed. Different approaches to obtain a sufficient propellant utilization at reduced mass flow rates are examined. A simplified analysis shows that the straightforward approach of scaling down the channel will result in increased power losses and a reduced overall thruster efficiency unless the length of the "acceleration region" is scaled down with the channel width and, at the same time, the magnetic field strength has to increase as the inverse of it. However, the implementation of these requirements faces difficulties due to volume constraints on the magnetic circuit and/or saturation in its iron parts. An alternative approach to overcome the propellant utilization problem, by extending the channel length, is also discussed. Applying this approach with a Soreq built Hall thruster, efficiencies of about 40% were obtained in the power range of 200-400 Watts.