Propellant Utilization in Hall Thrusters

Fuel optimal trajectories for electrically propelled spacecraft require a varying thrust acceleration. To achieve this requirement the electric thruster has to be operated with a variable thrust to power ratio at the maximum available input power. In order to implement this operating mode with a Hall thruster it is necessary to modify both the mass flow rate and the discharge voltage during the flight time. A crucial problem, associated with Hall thruster operation in a variable thrust mode, is the degradation of thruster performance under mass flow rate variations. Experiments with a laboratory Hall thruster have indicated that this degradation is mainly caused by a reduction of the propellant utilization at lower values of the mass flow rate. Modifying the channel geometry, namely, the channel length and channel profile, improvements in the behavior of the propellant utilization at small mass flow rate and consequently an improved thruster performance have been achieved. Results of experimental tests of a Hall thruster at various operating points and geometry are presented.