An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. An ion thruster creates a cloud of positive ions from a neutral gas by ionizing it to extract some electrons from its atoms. The ions are then accelerated using electricity to create thrust. Ion thrusters are categorized as either electrostatic or electromagnetic.
Electrostatic thruster ions are accelerated by the Coulomb force along the electric field direction. Temporarily stored electrons are 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 thruster ions are accelerated by 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. Reprint:
Ion thrusters in operation typically consume 1–7 kW of power, have exhaust velocities around 20–50 km/s (12–30 mi/s, Isp 2000–5000s), and possess thrusts of 25–250 mN and a propulsive efficiency of 65–80%; experimental ion thrusters have achieved , .
The 1998 Deep Space 1 spacecraft changed velocity by with its ion thruster, and consumed of xenon. The 2007 Dawn spacecraft achieved velocity change of , though with less efficiency, having consumed of xenon.
Applications include control of the orientation and position of orbiting satellites (some satellites have dozens of low-power ion thrusters), use as a main propulsion engine for low-mass robotic space vehicles (such as Deep Space 1 and Dawn), and serving as propulsion thrusters for crewed spacecraft and space stations (e.g. Tiangong).
Ion thrust engines are generally practical only in the vacuum of space as the engine's minuscule thrust cannot overcome any significant air resistance without radical design changes, as may be found in the 'Atmosphere Breathing Electric Propulsion' concept. The Massachusetts Institute of Technology (MIT) has created designs that are able to fly for short distances and at low speeds at ground level, using ultra-light materials and low drag aerofoils. An ion engine cannot usually 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.
Origins
Robert H. Goddard first suggested the use of electric propulsion in 1906 and later expanded the idea in a document for the Smithsonian titled Report Concerning Further Development, having presented two early patents in 1920 (US1363037, Method Of And Means For Producing Electrified Jets Of Gas) and 1931 (US1809115, Apparatus For Producing Ions). From 1910 to 1928 he carried out intermittent work using electrical devices in experiments aimed at determining the feasibility of ion propulsion for space travel. The Ion Collector together with the Ion Propulsion Test Tube are preserved in the National Air and Space Museum. In 1925–1926 he had two of his students working with him on this subject, producing independent results. 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 (1929; Ways to Spaceflight), 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.
The concept of an ion thruster spacecraft was first popularized in a science fiction story by American Jack Williamson in 1947.Williamson, Jack (March 1947), "The Equalizer"; Astounding Science Fiction. It subsequently became a staple of genre science fiction stories.
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, and 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.
On the 12 October 1964 Voskhod 1 carried out tests with ion thrusters that had been attached to the exterior of the spacecraft.
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.
General working principle Ion 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 of 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 accelerate the ions in the direction perpendicular to the electric field.
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.
Electric thrusters tend to produce low thrust, which results in low acceleration. Defining , the standard gravitational acceleration of Earth, and noting that , this can be analyzed. An NSTAR thruster producing a thrust force of 92 mN will accelerate a satellite with a mass of 1ton by 0.092N / 1000 kg = 9.2m/s (or 9.38g). However, this acceleration can be sustained for months or years at a time, in contrast to the very short burns of chemical rockets.
Where: F is the thrust force in N, η is the efficiency P is the electrical power used by the thruster in W, and Isp is the specific impulse in seconds.
The ion thruster is not the most promising type of electrically powered spacecraft propulsion, but it is the most successful in practice to date. An ion drive would require two days to accelerate a car to highway speed in vacuum. The technical characteristics, especially thrust, are considerably inferior to the prototypes described in literature. Technical capabilities are limited by the space charge created by ions, which limits the thrust densitythe force per cross-sectional area of the engine. Ion thrusters create small thrust levels compared to conventional chemical rockets (the thrust of Deep Space 1 being approximately equal to the weight of one sheet of paper, for example) but achieve high specific impulsealso known as propellant mass efficiencyby accelerating the exhaust to high speed. The power imparted to the exhaust increases with the square of exhaust velocity while thrust increase is linear. Conversely, chemical rockets provide high thrust, but are limited in total impulse by the small amount of energy that can be stored chemically in the propellants. Given the practical weight of suitable power sources, the acceleration from an ion thruster is frequently less than one-thousandth of standard gravity. However, since they operate as electric (or electrostatic) motors, they convert a greater fraction of input power into kinetic exhaust power. Chemical rockets operate as heat engines, and Carnot's theorem limits the exhaust velocity.
Electrostatic thrusters Gridded electrostatic ion thrusters
Gridded electrostatic ion thrusters development started in the 1960s and, since then, they have been used for commercial satellite propulsion and scientific missions.J. S. Sovey, V. K. Rawlin, and M. J. Patterson, "Ion Propulsion Development Projects in U. S.: Space Electric Rocket Test 1 to Deep Space 1", Journal of Propulsion and Power, Vol. 17, No. 3, May–June 2001, pp. 517–526. Their main feature is that the propellant ionization process is physically separated from the ion acceleration process.
The ionization process takes place in the discharge chamber, where by bombarding the propellant with energetic electrons, as the energy transferred ejects valence electrons from the propellant gas's atoms. These electrons can be provided by a hot cathode filament and accelerated through the potential difference towards an anode. Alternatively, the electrons can be accelerated by an oscillating induced electric field created by an alternating electromagnet, which results in a self-sustaining discharge without a cathode (radio frequency ion thruster).
The positively charged ions are extracted by a system consisting of 2 or 3 multi-aperture grids. After entering the grid system near the plasma sheath, the ions are accelerated by the potential difference between the first grid and second grid (called the screen grid and the accelerator grid, respectively) to the final ion energy of (typically) 1–2 keV, which generates thrust.
Ion thrusters emit a beam of positively charged ions. To keep the spacecraft from accumulating a charge, another cathode is placed near the engine to emit electrons into the ion beam, leaving the propellant electrically neutral. This prevents the beam of ions from being attracted (and returning) to the spacecraft, which would cancel the thrust.
Gridded electrostatic ion thruster research (past/present): NASA Solar Technology Application Readiness (NSTAR), 2.3 kW, used on two successful missions NASA's Evolutionary Xenon Thruster (NEXT), 6.9 kW, flight qualification hardware built; used on DART mission Nuclear Electric Xenon Ion System (NEXIS) High Power Electric Propulsion (HiPEP), 25 kW, test example built and run briefly on the ground EADS Radio-frequency Ion Thruster (RIT) Dual-Stage 4-Grid (DS4G)
Hall-effect thrusters
Hall-effect thrusters accelerate ions by means of an electric potential between a cylindrical anode and a negatively charged plasma that forms the cathode. The bulk of the propellant (typically xenon) is introduced near the anode, where it ionizes and flows toward the cathode; ions accelerate towards and through it, picking up electrons as they leave to neutralize the beam and leave the thruster at high velocity.