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Hall thrusters

History

The history of Hall thrusters dates back to the early 1960's when developments of Hall effect propulsion technology were concomitantly initiated in the USSR and in the US. The modern design of these engines essentially inherits, however, from the extensive Soviet research effort from which also stems most of the current theoretical understanding of these engines. The Soviet research program is itself much indebted to the work of Russian scientist A. I. Morozov who pioneered the most popular Hall thruster variant in current use, often referred to as the Stationary Plasma Thruster (SPT).

Despite the great complexity of the governing physical processes of these devices, their technology is now relatively mature and enjoys an outstanding flight experience with over 100 units sent on the board of Russian spacecrafts since the 1970's. The Russian success has motivated in the past ten years a rapidly growing number of research and development efforts worldwide, notably in the US, in Japan and in Europe where the first Western Hall thrusters have flown in 2003 on the SMART-1 spacecraft.

Principle of operation

Schematic view of a Hall thruster

The operation of Hall type plasma sources rely on the confining effect of a magnetic field on electrons to achieve propellant ionization. Taking benefit from the fact that the cyclotron radius of electrons is much smaller than that of ions, a magnetic field B of appropriate strength is applied which acts selectively on electrons without affecting significantly the motion of ions over the extend of the device. The confinement of electrons is further enhanced by using an annular channel which closes their azimuthal drift in the E × B direction.

A relatively small fraction of the electron stream emitted from the cathode enters the channel and is partially trapped by the radial magnetic field. The diffusion of electrons towards anode is thus strongly impeded, and is mostly ensured by collisions with neutrals and walls or by instabilities of the azimuthal electric field. These electrons ionize in turn the propelant (xenon in general) and the resulting ions are accelerated through the high magnetic field region located near the exhaust of the channel. Expelled ions are neutralized by the remaining electrons emitted by the cathode.

Typical capabilities and use

The typical specific velocities delivered by Hall thrusters range from 15 to 25 km/s for an efficiency nearing 50%. The developed thrust is generally of the order of 0.1 N, though thrusts as high as a few Newtons have been demonstrated by a NASA prototype.

These operating parameters make them especially suitable for missions such as north-south station-keeping (NSSK) of geostationary satellites, satellite orbit transfers or space probe propulsion. Hall thrusters have been in wide use for NSSK missions in Russia since the 70's. They are now scheduled to fly on US and European satellites, and already propel the European SMART-1 lunar probe.

Photograph of a SPT-100 in the Pivoine facility
SPT-100 type Hall thruster inside the Pivoine facility (France)
Artistic view of Smart-I
Artist's impression of the SMART-I lunar probe, propelled by the PPS-1350 thruster.

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