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Gridded ion thrusters

History

The apparent simplicity of their operating principle has placed gridded ion thrusters at the focus of most western electric propulsion programs during several decades.
In the US, the investigation of ion bombardment technology was initiated in the early 60's under the guidance of scientist H. R. Kaufman. These studies were complemented by several test flights in the 70's and eventually led to the current generation of US ion thrusters, the XIPS (marketed by Boeing) and the NSTAR engine that propelled the Deep Space I probe.

The British-build UK-10 (a.k.a. T5) and UK-25 (T6) thrusters use a similar technology, although electron confinement is ensured by electromagnets instead of permanent magnets. In Germany, however, a different technology based on a Radio-Frequency ionizer was worked out for the RIT family of ion thruster. Japan also developed its own ionization technology based on microwave sustained electron cyclotron resonance, and inaugurated the first operational use of ion thrusters on a satellite in 1994.

Principle of operation

Schematic view of an ion bombardment thruster

This category of ion thrusters rely on straightforward electrostatic acceleration of ions through a system of grids biased at different potentials.
In ion bombardment engines such as the one represented here, the propellant (most often xenon) is introduced in an ionization chamber where electrons produced by a central cathode are confined by means of applied magnetic fields. While moving along pseudo-cycloidal trajectories, electrons collide with neutrals and generate ions. The first grid (extraction grid) is set at a slightly negative potential in order to filter out electrons while at the same time allowing the extraction of ions. The next grid is set at a very low potential (typically -1000 V) and accelerates the extracted ions.

The German RIT thrusters and the Japanese microwave thrusters follow similar operating principles, except for the ionization chamber where ionization is respectively ensured by RF waves and microwaves.

Typical capabilities and use

Ion thrusters typically deliver specific velocities within the range 25-40 km/s and thrusts below 0.1 N, for an overall efficiency close to 60%. Higher thrusts are theoretically achievable, but the net space charge that builds between the grids due to the absence of electrons makes it difficult to design reasonably compact high thrust engines.

Ion thrusters are routinely used for north-south station-keeping missions on US commercial geostationary satellites since 1997. They have also brilliantly demonstrated their ability to propel space probes as attested by the encounter of NASA Deep Space-1 spacecraft with comet Borrelly in September 2001. Soon after, ion thrusters unexpectedly performed the first electric propulsion aided orbit transfer of a satellite, following the failed orbital injection of ESA's Artemis mission. The year 2003 marked the first use of a microwave ion thruster on a space probe, the Japanese Muses-C spacecraft.

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