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An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. The term is strictly used to refer to gridded electrostatic ion thrusters, but may often more loosely be applied to all electric propulsion systems that accelerate plasma, since plasma consists of ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and accelerate the ions in the direction of the electric field. Electromagnetic ion thrusters use the Lorentz force to accelerate the ions. In either case, when an ion passes through an electrostatic grid engine, the potential difference of the electric field converts to the ion's kinetic energy. Ion thrusters have an input power spanning 1–7 kilowatts, exhaust velocity 20–50 kilometers per second, thrust 20–250 millinewtons and efficiency 60–80%.〔(Choueiri, Edgar Y. (2009). New dawn of electric rocket. The Ion Drive )〕〔 The Deep Space 1 spacecraft, powered by an ion thruster, changed velocity by 4300 m/s while consuming less than 74 kilograms of xenon. The Dawn spacecraft broke the record, reaching 10,000 m/s.〔〔 Applications include control of the orientation and position of orbiting satellites (some satellites have dozens of low-power ion thrusters) and use as a main propulsion engine for low-mass robotic space vehicles (for example Deep Space 1 and Dawn).〔〔 Ion thrusters are not the most promising type of electrically powered spacecraft propulsion (although in practice they have been more successful than others). The ion drive is comparable to a car that takes two days to accelerate from zero to 60 miles per hour; a real ion engine's technical characteristics, and especially its thrust, are considerably inferior to its literary prototypes.〔〔 Technical capabilities of the ion engine are limited by the space charge created by ions. This limits the thrust density (force per cross-sectional area of the engine).〔 Ion thrusters create small thrust levels (for example the thrust of Deep Space 1's engine approximately equals the weight of one sheet of paper〔) compared to conventional chemical rockets, but achieve very high specific impulse, or propellant mass efficiency, by accelerating their exhaust to high speed. However, ion thrusters carry a fundamental price: the power imparted to the exhaust increases with the square of its velocity while thrust increases linearly. Chemical rockets, on the other hand, can provide high thrust, but are limited in total impulse by the small amount of energy that can be stored chemically in the propellants.〔(Electric Spacecraft Propulsion, Electric versus Chemical Propulsion ), ESA Science & Technology〕 Given the practical weight of suitable power sources, the accelerations given by ion thrusters are frequently less than one thousandth of standard gravity. However, since they operate as electric (or electrostatic) motors, a greater fraction of the input power is converted into kinetic exhaust power than in a chemical rocket. Chemical rockets operate as heat engines, hence Carnot's theorem bounds their possible exhaust velocity. Due to their relatively high power needs, given the specific power of power supplies and the requirement of an environment void of other ionized particles, ion thrust propulsion is currently only practical on spacecraft that have already reached space, and is unable to take vehicles from Earth to space. Spacecraft rely on conventional chemical rockets to initially reach orbit. ==Origins== The first person to publish mention of the idea was Konstantin Tsiolkovsky in 1911.〔 However, the first documented instance where the possibility of electric propulsion was considered is found in Robert H. Goddard's handwritten notebook in an entry dated September 6, 1906.〔Mark Wright, April 6, 1999, science.nasa.gov, (Ion Propulsion 50 years in the making )〕 The first experiments with ion thrusters were carried out by Goddard at Clark University from 1916–1917. The technique was recommended for near-vacuum conditions at high altitude, but thrust was demonstrated with ionized air streams at atmospheric pressure. The idea appeared again in Hermann Oberth's "Wege zur Raumschiffahrt” (Ways to Spaceflight), published in 1923, where he explained his thoughts on the mass savings of electric propulsion, predicted its use in spacecraft propulsion and attitude control, and advocated electrostatic acceleration of charged gases. A working ion thruster was built by Harold R. Kaufman in 1959 at the NASA Glenn Research Center facilities. It was similar to the general design of a gridded electrostatic ion thruster with mercury as its fuel. Suborbital tests of the engine followed during the 1960s and in 1964 the engine was sent into a suborbital flight aboard the Space Electric Rocket Test 1 (SERT 1).〔(NASA Glenn Contributions to Deep Space 1 )〕〔Ronald J. Cybulski, Daniel M. Shellhammer, Robert R. LoveII, Edward J. Domino, and Joseph T. Kotnik, (RESULTS FROM SERT I ION ROCKET FLIGHT TEST ), NASA Technical Note D2718 (1965).〕 It successfully operated for the planned 31 minutes before falling back to Earth.〔(【引用サイトリンク】title=Innovative Engines - Glenn Ion Propulsion Research Tames the Challenges of 21st Century Space Travel )〕 This test was followed by an orbital test, SERT-2, in 1970.〔NASA Glenn, "(SPACE ELECTRIC ROCKET TEST II (SERT II) ) (Accessed July 1, 2010)〕〔(SERT ) page at Astronautix (Accessed July 1, 2010)〕 An alternate form of electric propulsion, the Hall effect thruster was studied independently in the U.S. and the Soviet Union in the 1950s and 1960s. Hall effect thrusters had operated on Soviet satellites since 1972. Until the 1990s they were mainly used for satellite stabilization in North-South and in East-West directions. Some 100–200 engines completed their mission on Soviet and Russian satellites until the late 1990s. Soviet thruster design was introduced to the West in 1992 after a team of electric propulsion specialists, under the support of the Ballistic Missile Defense Organization, visited Soviet laboratories. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ion thruster」の詳細全文を読む スポンサード リンク
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