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{{short description|Spacecraft engine that generates thrust by generating a jet of ions}}{{About|a kind of reaction engine|the air propulsion concept|ionocraft}}{{Use American English|date=November 2020}}{{use dmy dates |date=December 2020}}File:Ion Engine Test Firing - GPN-2000-000482.jpg|thumb|290px|right|The 2.3{{nbsp}}kW NSTAR ion thruster developed by NASA for the Deep Space 1 spacecraft during a hot fire test at the Jet Propulsion LaboratoryJet Propulsion Laboratory(File:NEXIS thruster working.jpg|thumb|290px|right|NEXIS ion engine test (2005))(File:Xenon ion engine prototype.png|thumb|A prototype of a xenon ion engine being tested at NASA's Jet Propulsion Laboratory (2005))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.BOOK, Jahn, Robert G., Physics of Electric Propulsion, 1968, 1st, McGraw Hill Book Company, 978-0070322448, Reprint: BOOK, Jahn, Robert G., Physics of Electric Propulsion, 2006, Dover Publications, 978-0486450407, BOOK, Jahn, Robert G., Choueiri, Edgar Y., Encyclopedia of Physical Science and Technology, 2003, 3rd, 5, Academic Press, Electric Propulsion,weblinkweblink 2022-10-10, live, 125–141, 978-0122274107, Ion thrusters in operation typically consume 1–7 kW of power, have exhaust velocities around 20–50 km/s (Isp 2000–5000{{nbsp}}s), and possess thrusts of 25–250 mN and a propulsive efficiency 65–80%WEB,weblinkweblink 2022-10-10, live, Choueiri, Edgar Y., (2009) New dawn of electric rocket The Ion Drive, though experimental versions have achieved {{cvt|100|kW}}, {{cvt|5|N}}.WEB,weblink NASA's new ion thruster breaks records, could take humans to Mars, futurism.com, The Deep Space 1 spacecraft, powered by an ion thruster, changed velocity by {{cvt|4.3|km/s}} while consuming less than {{cvt|74|kg}} of xenon. The Dawn spacecraft broke the record, with a velocity change of {{cvt|11.5|km/s}}, though it was only half as efficient, requiring {{cvt|425|kg}} of xenon.WEB,weblink The Human Exploration of Mars, Jim, Haldenwang, Jim's Science Page, 3 May 2019, 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).WEB, 保淑 (Baoshu), å¼  (Zhang), 配置4台霍尔电推进发动机 "天宫"掀起太空动力变革 [Hall-effect thruster for Tiangong set off space drive revolution ],weblink 中国新闻网, 2021-07-18,weblink" title="web.archive.org/web/20210706020905weblink">weblink 2021-07-06, 2021-06-21, Chinese, 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. 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

(File:SERT-1 spacecraft.jpg|thumb|290px|right|SERT-1 spacecraft)The first person who wrote a paper introducing the idea publicly was Konstantin Tsiolkovsky in 1911.WEB, Ion Propulsion — Over 50 Years in the Making,weblink Science@NASA, dead,weblink" title="web.archive.org/web/20100327120759weblink">weblink 2010-03-27, 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),JOURNAL, Wolf, K., 1931-12-01, Wege zur Raumschiffahrt, Monatshefte für Mathematik und Physik, de, 38, 1, A58, 10.1007/BF01700815, 115467575, 1436-5081, free, 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.WEB,weblinkweblink 2022-10-10, live, A Critical History of Electric Propulsion: The First 50 Years (1906–1956), 2016-10-18, E. Y., Choueiri, 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).WEB,weblink Contributions to Deep Space 1, 14 April 2015, NASA, {{PD-notice}}WEB, Ronald J., Cybulski, Daniel M., Shellhammer, Robert R., Lovell, Edward J., Domino, Joseph T., Kotnik,weblinkweblink 2022-10-09, live, Results from SERT I Ion Rocket Flight Test, NASA-TN-D-2718, NASA, 1965, {{PD-notice}} It successfully operated for the planned 31 minutes before falling to Earth.WEB,weblink Innovative Engines - Glenn Ion Propulsion Research Tames the Challenges of 21st Century Space Travel, 2007-11-19, dead,weblink" title="web.archive.org/web/20070915023928weblink">weblink 2007-09-15, {{PD-notice}} This test was followed by an orbital test, SERT-2, in 1970.WEB, NASA Glenn Research Center,weblink Space Electric Rocket Test II (SERT II),weblink" title="web.archive.org/web/20110927004353weblink">weblink 2011-09-27, dead, 1 July 2010, {{PD-notice}}SERT {{webarchive |url=https://web.archive.org/web/20101025005136weblink |date=2010-10-25}} page at Astronautix (Accessed on 1 July 2010)On the 12 October 1964 Voskhod 1 carried out tests with ion thrusters that had been attached to the exterior of the spacecraft.BOOK, Siddiqi, Asif A, 2000, Challenge To Apollo: The Soviet Union and The Space Race, 1945–1974,weblink NASA, 423, 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.WEB,weblink Native Electric Propulsion Engines Today, Novosti Kosmonavtiki, 1999! Thruster! Propellant! data-sort-type=number | Input power (kW)! Specific impulse (s)! Thrust (mN)! Thruster mass (kg)! Notes

archive-url=https://web.archive.org/web/20110606033558weblink

language=ru, 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 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 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.WEB, Ion Propulsion: Farther, Faster, Cheaper,weblink NASA, 4 February 2022, 11 November 2020,weblink dead, Electric thrusters tend to produce low thrust, which results in low acceleration. Defining 1g = 9.81; mathrm{m/s^2}, the standard gravitational acceleration of Earth, and noting that F = ma implies a = F/m, this can be analyzed. An NSTAR thruster producing a thrust force of 92 mN will accelerate a satellite with a mass of 1{{nbsp}}ton by 0.092{{nbsp}}N / 1000 kg = 9.2{{e|−5}}{{nbsp}}m/s{{sup|2}} (or 9.38{{e|−6}}{{nbsp}}g). However, this acceleration can be sustained for months or years at a time, in contrast to the very short burns of chemical rockets.F = 2 frac{eta P}{g I_text{sp}}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.JOURNAL, Choueiri, Edgar Y., 2009, New dawn of electric rocket, Scientific American, 300, 2, 58–65, 10.1038/scientificamerican0209-58, 19186707, 2009SciAm.300b..58C, 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. This limits the thrust density (force per cross-sectional area of the engine). Ion thrusters create small thrust levels (the thrust of Deep Space 1 is approximately equal to the weight of one sheet of paper) compared to conventional chemical rockets, but achieve high specific impulse, or propellant mass efficiency, by 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.Electric Spacecraft Propulsion, Electric versus Chemical Propulsion, ESA Science & Technology 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 (File:Ion engine.svg|thumb|290px|A diagram of how a gridded electrostatic ion engine (multipole magnetic cusp type) works)Gridded electrostatic ion thrusters development started in the 1960sJOURNAL, Mazouffre, 2016, Electric propulsion for satellites and spacecraft: Established technologies and novel approaches,weblink Plasma Sources Science and Technology, 25, 3, 033002, 10.1088/0963-0252/25/3/033002, 2016PSST...25c3002M, 41287361, July 29, 2021, and, since then, it has been used for commercial satellite propulsionWEB, 601 Satellite Historical Snapshot,weblink Boing, 2021-07-26, WEB,weblink Electric Propulsion at Aerospace {{!, The Aerospace Corporation|website=www.aerospace.org|access-date=2016-04-10|archive-date=20 April 2016|archive-url=https://web.archive.org/web/20160420102803weblink|url-status=dead}}WEB,weblink XIPS (xenon-ion propulsion system), www.daviddarling.info, 2016-04-10, 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.WEB,weblink Space Electric Rocket Test, 2010-07-01,weblink" title="web.archive.org/web/20110927004353weblink">weblink 2011-09-27, dead, Their main feature is that the propellant ionization process is physically separated from the ion acceleration process.JOURNAL, SANGREGORIO, Miguel, XIE, Kan, 2017, Ion engine grids: Function, main parameters, issues, configurations, geometries, materials and fabrication methods, Chinese Journal of Aeronautics, 31, 8, 1635–1649, 10.1016/j.cja.2018.06.005, free, 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)PRESS RELEASE, ESA and ANU make space propulsion breakthrough, ESA, 2006-01-11,weblink 2007-06-29, WEB,weblink ANU and ESA make space propulsion breakthrough, 2007-06-30, ANU Space Plasma, Power & Propulsion Group (SP3), 2006-12-06, The Australian National University,weblink" title="web.archive.org/web/20070627103001weblink">weblink 2007-06-27, Hall-effect thrusters (File:Wfm hall thruster.svg|thumb|290px|Schematic of a Hall-effect thruster)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.The anode is at one end of a cylindrical tube. In the center is a spike that is wound to produce a radial magnetic field between it and the surrounding tube. The ions are largely unaffected by the magnetic field, since they are too massive. However, the electrons produced near the end of the spike to create the cathode are trapped by the magnetic field and held in place by their attraction to the anode. Some of the electrons spiral down towards the anode, circulating around the spike in a Hall current. When they reach the anode they impact the uncharged propellant and cause it to be ionized, before finally reaching the anode and completing the circuit.WEB,weblink Advanced Hall Electric Propulsion for Future In-Space Transportation, 2007-11-21, Oleson, S. R., Sankovic, J. M., dead,weblink" title="web.archive.org/web/20040122155512weblink">weblink 2004-01-22, {{PD-notice}} Field-emission electric propulsion Field-emission electric propulsion (FEEP) thrusters may use caesium or indium propellants. The design comprises a small propellant reservoir that stores the liquid metal, a narrow tube or a system of parallel plates that the liquid flows through and an accelerator (a ring or an elongated aperture in a metallic plate) about a millimeter past the tube end. Caesium and indium are used due to their high atomic weights, low ionization potentials and low melting points. Once the liquid metal reaches the end of the tube, an electric field applied between the emitter and the accelerator causes the liquid surface to deform into a series of protruding cusps, or Taylor cones. At a sufficiently high applied voltage, positive ions are extracted from the tips of the cones.WEB,weblink FEEP - Field-Emission Electric Propulsion, 2012-04-27, dead,weblink" title="web.archive.org/web/20120118051025weblink">weblink 2012-01-18, WEB,weblink Experimental Performance of Field Emission Microthrusters, Marcuccio, S., etal, 2012-04-27, dead,weblink" title="web.archive.org/web/20130520151812weblink">weblink 2013-05-20, WEB,weblink In-FEEP Thruster Ion Beam Neutralization with Thermionic and Field Emission Cathodes, liquid state and wicked up the needle shank to the tip where high electric fields deform the liquid and extract ions and accelerate them up to 130 km/s through 10 kV, 2007-11-21, Colleen, Marrese-Reading, Jay, Polk, Juergen, Mueller, Al, Owens, dead,weblink" title="web.archive.org/web/20061013162109weblink">weblink 2006-10-13, {{PD-notice}} The electric field created by the emitter and the accelerator then accelerates the ions. An external source of electrons neutralizes the positively charged ion stream to prevent charging of the spacecraft. Electromagnetic thrusters {{Self-contradictory|section=about=electromagnetic thrusters|article=Electrically powered spacecraft propulsion|date=April 2018}} Pulsed inductive thrusters Pulsed inductive thrusters (PITs) use pulses instead of continuous thrust and have the ability to run on power levels on the order of megawatts (MW). PITs consist of a large coil encircling a cone shaped tube that emits the propellant gas. Ammonia is the gas most commonly used. For each pulse, a large charge builds up in a group of capacitors behind the coil and is then released. This creates a current that moves circularly in the direction of jθ. The current then creates a magnetic field in the outward radial direction (Br), which then creates a current in the gas that has just been released in the opposite direction of the original current. This opposite current ionizes the ammonia. The positively charged ions are accelerated away from the engine due to the electric field jθ crossing the magnetic field Br, due to the Lorentz force.WEB,weblink Pulsed Inductive Thruster (PIT): Modeling and Validation Using the MACH2 Code, 2007-11-21, Pavlos G., Mikellides, dead,weblink" title="web.archive.org/web/20061010033732weblink">weblink 2006-10-10, {{PD-notice}} Magnetoplasmadynamic thruster Magnetoplasmadynamic (MPD) thrusters and lithium Lorentz force accelerator (LiLFA) thrusters use roughly the same idea. The LiLFA thruster builds on the MPD thruster. Hydrogen, argon, ammonia and nitrogen can be used as propellant. In a certain configuration, the ambient gas in low Earth orbit (LEO) can be used as a propellant. The gas enters the main chamber where it is ionized into plasma by the electric field between the anode and the cathode. This plasma then conducts electricity between the anode and the cathode, closing the circuit. This new current creates a magnetic field around the cathode, which crosses with the electric field, thereby accelerating the plasma due to the Lorentz force.The LiLFA thruster uses the same general idea as the MPD thruster, though with two main differences. First, the LiLFA uses lithium vapor, which can be stored as a solid. The other difference is that the single cathode is replaced by multiple, smaller cathode rods packed into a hollow cathode tube. MPD cathodes are easily corroded due to constant contact with the plasma. In the LiLFA thruster, the lithium vapor is injected into the hollow cathode and is not ionized to its plasma form/corrode the cathode rods until it exits the tube. The plasma is then accelerated using the same Lorentz force.JOURNAL,weblinkweblink 2022-10-10, live, A Survey of Propulsion Options for Cargo and Piloted Missions to Mars, 2016-10-18, K., Sankaran, L., Cassady, A.D., Kodys, E.Y., Choueiri, Annals of the New York Academy of Sciences, 2004, 1017, 1, 450–467, 10.1196/annals.1311.027, 15220162, 2004NYASA1017..450S, 1405279, WEB,weblink High Power MPD Thruster Development at the NASA Glenn Research Center, 2007-11-21, Michael R., LaPointe, Pavlos G., Mikellides, dead,weblink" title="web.archive.org/web/20061011063710weblink">weblink October 11, 2006, {{PD-notice}}WEB,weblink Utilization of Ambient Gas as a Propellant for Low Earth Orbit Electric Propulsion, May 22, 1999, Buford Ray, Conley, dead,weblink" title="web.archive.org/web/20110629174257weblink">weblink June 29, 2011, In 2013, Russian company the Chemical Automatics Design Bureau successfully conducted a bench test of their MPD engine for long-distance space travel.WEB,weblink "Ð’ Воронеже создали двигатель для Марса" в блоге "Перспективные разработки, НИОКРы, изобретения" - Сделано у нас, 17 December 2013, Сделано у нас, Electrodeless plasma thrusters Electrodeless plasma thrusters have two unique features: the removal of the anode and cathode electrodes and the ability to throttle the engine. The removal of the electrodes eliminates erosion, which limits lifetime on other ion engines. Neutral gas is first ionized by electromagnetic waves and then transferred to another chamber where it is accelerated by an oscillating electric and magnetic field, also known as the ponderomotive force. This separation of the ionization and acceleration stages allows throttling of propellant flow, which then changes the thrust magnitude and specific impulse values.WEB,weblink Development of a High Power Electrodeless Thruster, 2007-11-21, Gregory D., Emsellem,weblink" title="web.archive.org/web/20080515145645weblink">weblink 2008-05-15, dead, Helicon double layer thrusters A helicon double layer thruster is a type of plasma thruster that ejects high velocity ionized gas to provide thrust. In this design, gas is injected into a tubular chamber (the source tube) with one open end. Radio frequency AC power (at 13.56 MHz in the prototype design) is coupled into a specially shaped antenna wrapped around the chamber. The electromagnetic wave emitted by the antenna causes the gas to break down and form a plasma. The antenna then excites a helicon wave in the plasma, which further heats it. The device has a roughly constant magnetic field in the source tube (supplied by solenoids in the prototype), but the magnetic field diverges and rapidly decreases in magnitude away from the source region and might be thought of as a kind of magnetic nozzle. In operation, a sharp boundary separates the high density plasma inside the source region and the low density plasma in the exhaust, which is associated with a sharp change in electrical potential. Plasma properties change rapidly across this boundary, which is known as a current-free electric double layer. The electrical potential is much higher inside the source region than in the exhaust and this serves both to confine most of the electrons and to accelerate the ions away from the source region. Enough electrons escape the source region to ensure that the plasma in the exhaust is neutral overall. Variable Specific Impulse Magnetoplasma Rocket (VASIMR) The proposed Variable Specific Impulse Magnetoplasma Rocket (VASIMR) functions by using radio waves to ionize a propellant into a plasma, and then using a magnetic field to accelerate the plasma out of the back of the rocket engine to generate thrust. The VASIMR is currently being developed by Ad Astra Rocket Company, headquartered in Houston, Texas, with help from Canada-based Nautel, producing the 200 kW RF generators for ionizing propellant. Some of the components and "plasma shoots" experiments are tested in a laboratory settled in Liberia, Costa Rica. This project is led by former NASA astronaut Franklin Chang-Díaz (CRC-USA). A 200 kW VASIMR test engine was in discussion to be fitted in the exterior of the International Space Station, as part of the plan to test the VASIMR in space; however, plans for this test onboard ISS were canceled in 2015 by NASA, with a free flying VASIMR test being discussed by Ad Astra instead. An envisioned 200 MW engine could reduce the duration of flight from Earth to Jupiter or Saturn from six years to fourteen months, and Mars from 7 months to 39 days.WEB, Zyga, Lisa, 2009, Plasma Rocket Could Travel to Mars in 39 Days,weblink Phys.org, Microwave electrothermal thrusters {{multiple image | align = right | header = Microwave electrothermal thruster | caption_align = center | header_align = center | total_width = 500 | image1 = MET Sketch 1.jpg | width1 = 1221 | height1 = 629 | alt1 = Thruster components | caption1 = Thruster components | image2 = MET Sketch 2.jpg | width2 = 1233 | height2 = 1297 | alt2 = Discharge Chamber | caption2 = Discharge chamber }} Under a research grant from the NASA Lewis Research Center during the 1980s and 1990s, Martin C. Hawley and Jes Asmussen led a team of engineers in developing a microwave electrothermal thruster (MET).NEWS, Less Fuel, More Thrust: New Engines are Being Designed for Deep Space, The Arugus-Press, Owosso, Michigan, 10, 128, 48, 26 February 1982, In the discharge chamber, microwave (MW) energy flows into the center containing a high level of ions (I), causing neutral species in the gaseous propellant to ionize. Excited species flow out (FES) through the low ion region (II) to a neutral region (III) where the ions complete their recombination, replaced with the flow of neutral species (FNS) towards the center. Meanwhile, energy is lost to the chamber walls through heat conduction and convection (HCC), along with radiation (Rad). The remaining energy absorbed into the gaseous propellant is converted into thrust. Radioisotope thruster A theoretical propulsion system has been proposed, based on alpha particles ({{chem|He|2+}} or {{chem|4|2|He|2+}} indicating a helium ion with a +2 charge) emitted from a radioisotope uni-directionally through a hole in its chamber. A neutralising electron gun would produce a tiny amount of thrust with high specific impulse in the order of millions of seconds due to the high relativistic speed of alpha particles.JOURNAL, Revisiting alpha decay-based near-light-speed particle propulsion, Applied Radiation and Isotopes, 114, 14–18, 10.1016/j.apradiso.2016.04.005, 2016, Zhang, Wenwu, Liu, Zhen, Yang, Yang, Du, Shiyu, 27161512, free, 2016AppRI.114...14Z, A variant of this uses a graphite-based grid with a static DC high voltage to increase thrust as graphite has high transparency to alpha particles if it is also irradiated with short wave UV light at the correct wavelength from a solid-state emitter. It also permits lower energy and longer half-life sources which would be advantageous for a space application. Helium backfill has also been suggested as a way to increase electron mean free path. Comparisons {| class"wikitable sortable"|+ Test data of some ion thrusters

NASA Solar Technology Application Readiness>NSTAR| Xenon| 2.3

1700}}–{{val

ARCHIVE-URL=HTTPS://WEB.ARCHIVE.ORG/WEB/19990222082331/HTTP://ECCENTRIC.MAE.CORNELL.EDU/BOYDGROUP/JBALA/IONPROPULSION.HTML, 1999-02-22, | 92 max.

TITLE= PERFORMANCE OF THE NSTAR ION PROPULSION SYSTEM ON THE DEEP SPACE ONE MISSION.

PAGES= 965

URL= HTTPS://TRS.JPL.NASA.GOV/BITSTREAM/HANDLE/2014/12165/01-0061.PDF

ARCHIVE-DATE=2022-10-09

ACCESS-DATE=2021-09-16,

Deep Space 1 and Dawn (spacecraft)>Dawn space probes.

PPS-1350 Hall effect >

1660}}

90

5.3|

NEXT (ion thruster)>NEXTSHIGA>FIRST=DAVID

URL=HTTPS://WWW.NEWSCIENTIST.COM/ARTICLE/DN12709-NEXTGENERATION-ION-ENGINE-SETS-NEW-THRUST-RECORD.HTML

NEWSPAPER=NEWSCIENTIST, 2007-09-28, | Xenon

FIRST=DAVID

ACCESS-DATE=JUNE 26, 2013,

4190}}HTTPS://NTRS.NASA.GOV/ARCHIVE/NASA/CASI.NTRS.NASA.GOV/20080047732_2008047267.PDF >ARCHIVE-URL=HTTPS://GHOSTARCHIVE.ORG/ARCHIVE/20221009/HTTPS://NTRS.NASA.GOV/ARCHIVE/NASA/CASI.NTRS.NASA.GOV/20080047732_2008047267.PDF

URL-STATUS=LIVE

FIRST1=GEORGE R.

FIRST2=MICHAEL J.

FIRST3=SCOTT W.

first=Daniel A.

contribution=NASA's Evolutionary Xenon Thruster (NEXT) Project Qualifi cation Propellant Throughput Milestone: Performance, Erosion, and Thruster Service Life Prediction After 450 kg

publisher=NASA - Glenn Research Center

date=3–7 May 2010

archive-url=https://ghostarchive.org/archive/20221009weblink

url-status=live|access-date=2014-03-08}} {{PD-notice}}|236 max.||

Starlink#v1.0 (operational)>Starlink Gen1 Hall effect

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