The HEMPT Concept - A Survey on Theoretical Considerations and Experimental Evidences

The High Efficiency Multistage Plasma Thruster HEMPT represents a novel concept for ion thrusters for the thrust range from sub to several 100 mN compared to the well established technologies represented by the Hall Ef fect Thruster HET and the Grid Ion Thruster GIT. The HEMPT concept has been invented by Thales Electron Devices GmbH TEDG with a first patent launched in 1998. Since th en TEDG has successfully developed the HEMPT concept from demonstrator level during the feasibility phase to engineering models in different thrust ranges. Since recently TEDG is int egrating HEMPTs into an ion propulsion system which shall be verified on the SmallGEO platform in course of the Hispasat AG1 mission. Alike GITs and HETs, HEMPTs generate thrust by electrostatic acceleration of propellant ions which generated in a plasma discharge. However, for this the HEMPT employs a specific magnetic field topology which confines the plasma electrons that are ionising the propellant and are focussing the propellant ions. Plasma transport occurs in a multi-stage magnetic cusp structure which grossly inhibits contact to the wal l and thus wall erosion. The electrons confined at the thruster exit pull down the plasma potential and act as a virtual acceleration grid. As a result, the plasma potential inside the discharge c hannel stays practically constant with a sharp potential drop at the thruster exit. Ionisation and acceleration are spatially separated as in case of a GIT. On the other hand the accelerated ions ar e extracted from a quasi-neutral plasma allowing for high ion current densities at the thru ster exit as in case of a HET. This paper reviews the HEMPT technology as unique concept including evidence from both experiments and theory. Unlike as in HETs, electron confinement is dominated by electron mirroring in the cusps rather than E x B drift, and the effective separation of ion generation and acceleration results in GIT like ion beam energy sp ectra.