INTRODUCTION
An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.
An ion thruster ionizes a neutral gas by extracting some electrons out of atoms, creating a cloud of positive ions. These ion thrusters rely mainly on electrostatics as ions are accelerated by the Coulomb force along an electric field. Temporarily stored electrons are finally reinjected by a neutralizer in the cloud of ions after it has passed through the electrostatic grid, so the gas becomes neutral again and can freely disperse in space without any further electrical interaction with the thruster. By contrast, electromagnetic thrusters use the Lorentz force to accelerate all species (free electrons as well as positive and negative ions) in the same direction whatever their electric charge, and are specifically referred to as plasma propulsion engines, where the electric field is not in the direction of the acceleration.
Ion thrusters in operation typically consume 1–7 kW of power, have exhaust velocities around 20–50 km/s, and possess thrusts of 25–250 mN and propulsive efficiency of 65–80%. However, experimental versions have achieved 100 kW (130 hp), and 5 N (1.1 lbf).
The Deep Space 1 spacecraft, powered by an ion thruster, changed velocity by 4.3 km/s (2.7 mi/s) while consuming less than 74 kg (163 lb) of xenon. The Dawn spacecraft broke the record, with a velocity change of 11.5 km/s (7.1 mi/s), though it was only half as efficient, requiring 425 kg (937 lb) of xenon.
Applications include control of the orientation and position of orbiting satellites (some satellites have dozens of low-power ion thrusters) and use as the main propulsion engine for low-mass robotic space vehicles (such as Deep Space 1 and Dawn).
Ion thrust engines are practical only in the vacuum of space and cannot take vehicles through the atmosphere because ion engines do not work in the presence of ions outside the engine; additionally, the engine's little thrust cannot overcome any significant air resistance. Moreover, notwithstanding the fact of an atmosphere (or lack thereof) an ion engine cannot generate sufficient thrust to achieve initial liftoff from any celestial body with significant surface gravity. For these reasons, spacecraft must rely on other methods such as conventional chemical rockets or non-rocket launch technologies to reach their initial orbit.
ORIGIN
The first person who wrote a paper introducing the idea publicly was Konstantin Tsiolkovsky in 1911 The technique was recommended for near-vacuum conditions at high altitudes, but the 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 1929, 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 gasses.
A working ion thruster was built by Harold R. Kaufman in 1959 at the NASA Glenn Research Center facilities. It was similar to a gridded electrostatic ion thruster and used mercury for propellant. Suborbital tests were conducted during the 1960s and in 1964, the engine was sent into a suborbital flight aboard the Space Electric Rocket Test-1 (SERT-1) It successfully operated for the planned 31 minutes before falling to Earth This test was followed by an orbital test, SERT-2, in 1970.
An alternate form of electric propulsion, the Hall effect thruster, was studied independently in the United States and the Soviet Union in the 1950s and 1960s. Hall effect thrusters operated on Soviet satellites from 1972 until the late 1990s, mainly used for satellite stabilization in north-south and in east-west directions. Some 100–200 engines completed missions on Soviet and Russian satellites. 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.
WORKING PRINCIPLE
on thrusters use beams of ions (electrically charged atoms or molecules) to create thrust in accordance with momentum conservation. The method of accelerating the ions varies, but all designs take advantage of the charge/mass ratio of the ions. This ratio means that relatively small potential differences can create high exhaust velocities. This reduces the amount of reaction mass or propellant required but increases the amount of specific power required compared to chemical rockets. Ion thrusters are therefore able to achieve high specific impulses. The drawback of the low thrust is low acceleration because the mass of the electric power unit directly correlates with the amount of power. This low thrust makes ion thrusters unsuited for launching spacecraft into orbit, but effective for in-space propulsion over longer periods of time.
Ion thrusters are categorized as either electrostatic or electromagnetic. The main difference is the method for accelerating the ions.
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 move the ions.
Electric power for ion thrusters is usually provided by solar panels. However, for sufficiently large distances from the sun, nuclear power may be used. In each case, the power supply mass is proportional to the peak power that can be supplied, and both provide, for this application, almost no limit to the energy.
