Editing Ion thruster (section)

== Electrostatic thrusters ==
=== Gridded electrostatic ion thrusters ===
{{main|Gridded ion thruster}}
[[File:Ion engine.svg|thumb|upright=1.8|A diagram of how a gridded electrostatic ion engine (multipole magnetic cusp type) works]]

[[Gridded ion thruster|Gridded electrostatic ion thrusters]] development started in the 1960s<ref>{{cite journal |last1= Mazouffre|date= 2016|title= Electric propulsion for satellites and spacecraft: Established technologies and novel approaches|url= https://www.scopus.com/record/display.uri?eid=2-s2.0-84973355693&origin=inward&txGid=d2acc43c5b6bd3518f0fda0be9a7a74e|journal= Plasma Sources Science and Technology|volume= 25|issue= 3|page= 033002|doi= 10.1088/0963-0252/25/3/033002|bibcode= 2016PSST...25c3002M|s2cid= 41287361|access-date=July 29, 2021}}</ref> and, since then, they have been used for commercial satellite propulsion<ref>{{cite web|author1=|title=601 Satellite Historical Snapshot|url=https://www.boeing.com/history/products/601-satellite.page|website=Boing|date=|access-date=2021-07-26}}</ref><ref>{{Cite web|url=http://www.aerospace.org/crosslinkmag/fall-2014/electric-propulsion-at-aerospace/|title=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/20160420102803/http://www.aerospace.org/crosslinkmag/fall-2014/electric-propulsion-at-aerospace/|url-status=dead}}</ref><ref>{{Cite web|url=http://www.daviddarling.info/encyclopedia/X/XIPS.html|title=XIPS (xenon-ion propulsion system)|website=www.daviddarling.info|access-date=2016-04-10}}</ref> and scientific missions.<ref name="Sovey">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.</ref><ref>{{Cite web |url=http://www.grc.nasa.gov/WWW/ion/past/70s/sert2.htm |title=Space Electric Rocket Test |access-date=2010-07-01 |archive-url=https://web.archive.org/web/20110927004353/http://www.grc.nasa.gov/WWW/ion/past/70s/sert2.htm |archive-date=2011-09-27 |url-status=dead }}</ref> Their main feature is that the propellant [[ionization]] process is physically separated from the ion acceleration process.<ref>{{cite journal|last1=SANGREGORIO|first1=Miguel|last2=XIE|first2=Kan|date=2017|title=Ion engine grids: Function, main parameters, issues, configurations, geometries, materials and fabrication methods|journal=Chinese Journal of Aeronautics|volume=31|issue=8|pages=1635–1649 |doi=10.1016/j.cja.2018.06.005|doi-access=free}}</ref>

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]] [[electrical filament|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&nbsp;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.<ref name="Glenn"/>

Gridded electrostatic ion thruster research (past/present):
* [[NASA Solar Technology Application Readiness]] (NSTAR), 2.3&nbsp;kW, used on two successful missions
* NASA's Evolutionary Xenon Thruster ([[NEXT (ion thruster)|NEXT]]), 6.9&nbsp;kW, flight qualification hardware built; used on [[DART mission]]
* Nuclear Electric Xenon Ion System (NEXIS)
* High Power Electric Propulsion ([[HiPEP]]), 25&nbsp;kW, test example built and run briefly on the ground
* EADS Radio-frequency Ion Thruster (RIT)
* [[Dual-Stage 4-Grid]] (DS4G)<ref>{{cite press release|title=ESA and ANU make space propulsion breakthrough|publisher=ESA|date=2006-01-11|url=http://www.esa.int/esaCP/SEMOSTG23IE_index_0.html|access-date=2007-06-29}}</ref><ref>{{cite web |author=Australian National University Space Plasma, Power & Propulsion Group |date=2006-12-06 |title=ANU and ESA make space propulsion breakthrough |url=http://prl.anu.edu.au/SP3/research/SAFEandDS4G/webstory |archive-url=https://web.archive.org/web/20070627103001/http://prl.anu.edu.au/SP3/research/SAFEandDS4G/webstory |archive-date=2007-06-27 |access-date=2007-06-30 |publisher=The Australian National University}}</ref>

=== Hall-effect thrusters ===
{{Main|Hall-effect thruster}}
[[File:Wfm hall thruster.svg|thumb|upright=1.8|Schematic of a Hall-effect thruster]]

[[Hall-effect thruster]]s 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 effect|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.<ref name="Oleson">{{cite web|url=http://gltrs.grc.nasa.gov/reports/2001/TM-2001-210676.pdf|title=Advanced Hall Electric Propulsion for Future In-Space Transportation|access-date=2007-11-21|last1=Oleson|first1=S. R.|last2=Sankovic|first2=J. M.|url-status=dead|archive-url=https://web.archive.org/web/20040122155512/http://gltrs.grc.nasa.gov/reports/2001/TM-2001-210676.pdf|archive-date=2004-01-22}} {{PD-notice}}</ref>

=== Field-emission electric propulsion ===
{{main|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 cone]]s''. At a sufficiently high applied voltage, positive ions are extracted from the tips of the cones.<ref>{{cite web |title=FEEP – Field-Emission Electric Propulsion |url=http://www.alta-space.com/index.php?page=feep |url-status=dead |archive-url=https://web.archive.org/web/20120118051025/http://www.alta-space.com/index.php?page=feep |archive-date=2012-01-18 |access-date=2012-04-27}}</ref><ref name="JPP98">{{cite web|url=http://www.alta-space.com/uploads/file/publications/feep/Marcuccio-JPP14_5_1998.pdf|title=Experimental Performance of Field Emission Microthrusters|author=Marcuccio, S.|display-authors=etal|access-date=2012-04-27|url-status=dead|archive-url=https://web.archive.org/web/20130520151812/http://www.alta-space.com/uploads/file/publications/feep/Marcuccio-JPP14_5_1998.pdf|archive-date=2013-05-20}}</ref><ref name="Nasa">{{cite web|url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/11649/1/02-0194.pdf|title=In-FEEP Thruster Ion Beam Neutralization with Thermionic and Field Emission Cathodes|quote=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|access-date=2007-11-21|first1=Colleen|last1=Marrese-Reading|first2=Jay|last2=Polk|first3=Juergen|last3=Mueller|first4=Al|last4=Owens|url-status=dead |archive-url=https://web.archive.org/web/20061013162109/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/11649/1/02-0194.pdf|archive-date=2006-10-13}} {{PD-notice}}</ref> 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.