Mechanisms of Thrust Production Hollow Cathodes

Paper number

IAC-09.C4.P.15

Author

Mr. Angelo Grubisic, University of Southampton, United Kingdom

Coauthor

Mr. Stephen Gabriel, University of Southampton, United Kingdom

Year

2009

Abstract

This paper presents an explanation for thrust production mechanisms in the T5, T6 and XIPS hollow cathodes based on previous systematic experimental analysis of thrust performance and ion energy measurements. The data shows that the thrust mechanism of hollow cathodes is likely composed of multiple components which in some cases can generate specific impulse of over 1000s. These components include gas dynamic thrust, particularly as a result of intense electron pressure at the cathode exit, ion acceleration due to plasma potential hills and magneto-hydro-dynamic (MHD) forces at high currents and low flow rates arising from the self-induced azimuthal magnetic field within the orifice and resulting cross-field interaction. 


While this initial characterization can only loosely attribute the relative magnitude each mechanism plays it does show clear evidence for each. Measurements and analysis also show that in higher current cathodes such as the T6, MHD thrust scales quadratically with discharge current while losses scale linearly. Since accelerating forces are very low, high specific impulse can only be attained when mass flow rate is substantially reduced. We have shown that this can be achieved by manipulation of anode geometry to aid in electron collection from the discharge thus allowing operation at high discharge currents without the typical transition to a high voltage ‘plume’ mode. Since current levels are relatively small electron in an electromagnetic thruster sense convective losses dominate performance and the device consequently suffers poor inefficiency. Smaller cathodes however, such as the T5, show the necessary performance and efficiency to compete with conventional miniaturized electric propulsion systems at a mission level. These cathodes operate at lower currents and have much more efficient thrust mechanisms. 


It is hoped that this work lays the foundations for the development of dedicated thrusters based on an understanding of the influence of terminal parameters. Consideration is given to application on both small spacecraft as a stand-alone thruster and on larger spacecraft as part of an integrated primary/secondary propulsion system. For small spacecraft this type of thruster may present a means to produce more optimal performance in terms of thrust and specific impulse than conventional microthrusters, while for larger spacecraft which carry primary electric propulsion systems, large mass savings may be possible from reuse of propellant management architecture and power systems in a dual configuration.

Abstract document

IAC-09.C4.P.15.pdf

Manuscript document

(absent)