Limits on the efficiency of several electric thruster configurations

Limits on the efficiency of several thruster configurations are discussed. The efficiency of the Pulsed Plasma Thruster is reduced when part of the magnetic field energy that is converted into particle energy does not become directed kinetic energy but rather thermal energy. The partitioning of the power when the propellant exhibits slug, snowplow or specular-reflection acceleration is analyzed. It is suggested how to distribute the propellant mass along the accelerating channel so as to efficiently use this thermal energy. Steady acceleration to supersonic velocities is examined in two configurations: the Magneto-Plasma Dynamics (MPD) thruster and the Hall thruster. Limits on the efficiency of the MPD thruster in a nondiverging geometry are derived. The efficiency in the self-field acceleration is higher than in the azimuthal applied-field acceleration. These limits can be overcome in a converging–diverging geometry, analogous to the Laval nozzle, in which the efficiency can approach unity. The acceleration efficiency in the Hall thruster becomes unity for an infinite magnetic Reynolds number even in a nondiverging geometry. The efficiency can be enhanced by the pressure that results from electron heating or by employing segmented emitting electrodes.