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ion propulsion[′ī‚än prə′pəl·shən]
Vehicular propulsion caused by the high-speed discharge of a beam of electrically charged minute particles. These particles, usually positive ions, are generated and accelerated in an electrostatic field produced within an ion thruster attached to a spacecraft. Because positive ions cannot be ejected from the thruster without leaving a substantial negative charge on the thruster and spacecraft, electrons must be ejected at the same rate. Ion propulsion systems are attractive because they expel the ions at very high speeds and, therefore, require much less propellant than other thrusters, such as chemical rockets.
The three principal components of an ion propulsion system are the power-generation and -conditioning subsystem, the propellant storage and feed subsystem, and one or more ion thrusters.
The power source can be a nuclear reactor or a radiant-energy collector. In the former, thermal power is released by fission or fusion reactions. Solar radiation can be used to provide electric power directly through photovoltaic (solar) cells or indirectly through a solar collector-heat exchanger system similar to that for a nuclear system. See Solar cell, Solar energy
If the power-generation system involves a nuclear reactor or a solar-thermal subsystem, thermal-to-electric conversion subsystems are required. Those most highly developed involve thermodynamic conversion cycles based on turbine generators. Although most traditional systems have operated on the Brayton gas cycle or the Rankine vapor cycle, more recent efforts include the Stirling gas-cycle system.
Ion-thruster propellants that have been investigated include argon, xenon, cesium, mercury, and fullerenes such as C60. Although mercury received most of the early attention, xenon is now being used on all space missions because of toxicity concerns with mercury.
Ion or electrostatic thrust devices contain three functional elements: an ionizer that generates the ions; an accelerator providing an electric field for accelerating the ions and forming them into a well-focused beam; and a neutralizer or electron emitter that neutralizes the electrical charge of the exhaust beam of ions after they have been ejected.
The positive ions needed for acceleration are produced in a strong electric field, by contact with a surface having a work function greater than the ionization potential of the propellant, or by electron-bombardment ionization. The last method has received the most attention and appears to be the most promising.
Some of the ions produced are directed toward the ion-accelerating subsystem which typically consists of two plates containing large numbers of aligned hole pairs. The upstream plate and the body of the ionizer are maintained at a positive potential with respect to the space downstream from the thruster, whereas the downstream plate is biased negative at a smaller value. For a high extracted ion current density, the plates should be as close together as possible.
Ion propulsion is characterized by high specific impulse and low thrust. Because high specific impulse means low propellant consumption, ion propulsion is attractive for a wide variety of applications.
One functional category includes the use of ion thrusters on satellites for orbit control (against weak perturbation forces) and for station keeping (position maintaining of satellite in a given orbit). Substantial commercial use of ion thrusters in this application began at the end of the twentieth century. An ion propulsion system can also be used advantageously for changing the satellite's position in a given orbit, especially shifting a satellite to different longitudes over the Earth in an equatorial geostationary orbit.
A major functional application of ion propulsion is interplanetary transfer. Here, thrust has to overcome only very weak solar gravitational forces. Because of this, and the long powered flight times of which ion propulsion is capable, transfer times to Venus or Mars need not be longer than transfer times in comparable flights with high thrust drives capable only of short powered flight. At the very large distances to objects in the outer solar system, ion propulsion would yield shorter transfer times than chemical and most high-thrust nuclear concepts. The National Aeronautics and Space Administration (NASA) Deep Space 1 mission, launched in 1998, used a 30-cm-diameter (12-in.) xenon ion thruster to propel a spacecraft to encounters with the asteroid Braille and the comet Borrelly. See Electrothermal propulsion, Plasma propulsion