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An ion thruster is a type of spacecraft propulsion that uses beams of ions for propulsion. Ions are accelerated by passing them through highly-charged grids (similar in concept to a vacuum tube). Opposite charged ions are also fired into the ion beam and 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 a very efficient form of propulsion, using very little reaction mass (i.e., the specific impulse, or Isp, is very high). Ion thrusters can deliver performance approximately one order of magnitude greater than traditional liquid fuel rocket engines in terms of fuel efficiency.
A major consideration is the amount of energy required to run the engine, partly to ionize the materials, but most especially to accelerate the ions to the extremely high speeds required to have any effect. Exhaust speeds of 30 km/s are not uncommon (which compares well with the 3-4.5 km/s for chemical rockets) and makes for good propellent usage. However, since energy consumption is proportional to exhaust velocity squared (and the thrust per kg of fuel is only proportional to exhaust speed), the overall thrust obtained from a given amount of energy is inversely proportional to exhaust speed (see also the energy computed from the rocket equation). Additionally, the use of Ions as 'fuel' means that fuel flow rates are measured in micro-grams. Thus, in practice, it turns out that with currently practical energy sources, exhaust speeds multiplied by Ion flow rates, and vehicle masses, only extremely modest accelerations are feasible, typically of order a milligee.
Of all the electric thrusters, ion engines have been the most seriously considered commercially and academically in the West for interplanetary missions. Ion engines are seen as the best solution for these missions as interplanetary trajectories require very high ΔV (the overall change in velocity, taken as a single value) that can be built up over long periods of time (years).
Here, however, the life of the thruster becomes important. Ion drives have to be kept running a large part of the time to allow the milli-gee acceleration to build up into something meaningful. In a 'classical' design of engine, the ions often hit the grids on their way through the engine, which leads to the decay of the grids, and their eventual failure. Smaller grids lower the chance of these accidental collisions, but decrease the amount of charge they can handle, and thus lower the acceleration.
The Hall effect thruster is a type of ion thruster that has been used for decades for station keeping by the Soviet-Union and is now also applied in the West: the European Space Agency's satellite Smart 1 uses it.
 A NASA xenon ion engine test 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. These are Electrostatic_Ion_thrusters and work by a different principle than Hall effect thrusters.
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 grid degradation problem with a combination of other electric or magnetic fields.
Other fuels have been considered for use with ion propulsion. Research has been invested in Fullerene for this purpose, specifically C60 (buckminsterfullerene), due in part to its large electron-impact cross section. This property gives the potential for ion engines with higher efficiency than current Xenon-based designs at Isp values of less than 3,000 s.
JP Aerospace has been working to build an orbital airship, which uses a combination of a balloon and ion thrusters to achieve orbit without any use of conventional rockets, for roughly one dollar per ton per mile of altitude.
Ion thrusters in Fiction - Film creator and director George Lucas seems to have some confidence in ion propulsion: in the Star Wars movies, the technologically sophisticated Empire's TIE Fighters get their name from the TIEs used for propulsion — Twin Ion Engines...
- Arthur C. Clarke's 1949 short story Breaking Strain features a cargo ship with an ion drive powered by "Atomic motors".
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