An ion thruster is a type of spacecraft propulsion that uses beams of ions for propulsion. Ions accelerated by passing them through highly-charged grids (similar in concept to a vacuum tube). Opposite charge ion beams are fired into the ion beams accelerated through the grid as they leave the thruster. This keeps the spacecraft and the thruster beams neutral electrically. The acceleration received from the thruster is very efficient. Ion thrusters can deliver performance several orders of magnitude greater than traditional liquid fuel rocket engines.
Ion thrusters have two major problems however. One is that it is difficult to ionize materials, meaning that the total amount of mass they can accelerate tends to be very small. This, in turn, means that ion thrusters have very low thrusts, typically only a few Newtons. Another problem is that the ions often hit the grids on their way through the engine, which leads to the decay of the grids, and their eventual breaking. Smaller grids lower the chance of these accidental collisions, but decrease the amount of charge they can handle, and thus lower the acceleration.
Of all the electric thrusters, ion engines have been the most seriously considered commercially and academically. Ion engines are best used for missions requiring very high ΔV (the overall change in velocity, taken as a single value), interplanetary missions, for example. This is because the more performance required of the propulsion system, the faster a high efficiency system like an ion engine will pay off.
NASA has developed an ion engine called NSTAR for use in their interplanetary missions. This engine was tested in the highly successful space probe Deep Space 1. Hughes has developed the XIPS (Xenon Ion Propulsion System) for performing stationkeeping on geosynchronous satellites.
In 2003 NASA ground-tested a new version of their ion engine called High Power Electric Propulsion, or HiPEP. The HiPEP engine differs from earlier ion engines because the xenon ions are produced using a combination of microwaves and spinning magnets. Previously the electrons required were provided by a cathode. Using microwaves significantly reduces the wear and tear on the engine by avoiding any contact between the speeding ions and the electron source.
Most other electric spacecraft engine designs are based on the same principles, but attempt to avoid the problems with grids with a combination of other electric or magnetic fields.
See also: Spacecraft propulsion