Plasma Thruster Development: Magnetoplasmadynamic Propulsion, Status and Basic Problems.

This report provides an overview of the present state and the problem areas of magnetoplasmadynamic MPD space propulsion devices and systems potentially suitable for orbit raising and maneuvering of large space structures, i.e., thrust and power levels of presently most promising steady or quasisteady pulsed thruster types, self field, axial applied field arcjets and Hall ion thrusters are reviewed in terms of performance trends, present understanding, uncertainties and ground testing problems, with the conclusion that there is as yet no reliable basis for choosing one type for final development. Typical efficiency vs. specific impulse curves for all MPD thrusters show the dominant role of the propellant on the thrust to input power ratio as for ion engines, and the necessity of achieving relatively high specific impulse values for each propellant for acceptable efficiencies. The specific impulse or onset limit of self-field thrusters using a variety of propellants is discussed, together with predicted trends from several theoretical models. Considering system complexity and development cost as well as performance, the advantages and disadvantages if pulsed quasisteady vs. continuous thruster operation and the propellant selection criteria are reviewed. Electrode erosion, especially on cathodes, losses and limits imposed by radiation cooling are emphasized as critical problem areas for larger MPD thrusters. Besides the electrode attachments, the unresolved basis theoretical problems of efficient plasma acceleration and of the midstream flow discharge stability are identified, and proposed approaches towards gaining additional understanding are outlined.