View source for Helicon double-layer thruster

{{Short description|Prototype spacecraft propulsion engine}}
The '''helicon double-layer thruster''' is a prototype [[Spacecraft electric propulsion|electric spacecraft propulsion]]. It was created by Australian scientist [[Christine Charles]], based on a technology invented by [[Rod Boswell|Professor Rod Boswell]], both of the [[Australian National University]].

The design has been verified by the [[ESA]], which is participating in its development.

==Theory of operation==
A helicon double-layer thruster (HDLT) is a type of [[plasma thruster]], which ejects [[ionized]] gas at high velocity to provide [[thrust]] to a [[spacecraft]]. In this thruster design, gas is injected into a tubular chamber (the ''source tube'') with one open end. [[Radio frequency]] AC power (at [[ISM band|13.56&nbsp;MHz]] in the prototype design) is coupled into a specially shaped [[Antenna (radio)|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 (physics)|helicon]] wave in the plasma, which further heats the plasma.

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, there is a sharp boundary between the dense plasma inside the source region, and the less dense plasma in the exhaust, which is associated with a sharp change in electrical potential. The plasma properties change rapidly across this boundary, which is known as a ''current-free electric [[Double layer (plasma)|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.  
Like most ion propulsion devices, the HDLT is a low-thrust, high–specific-impulse (high-{{math|''I''<sub>sp</sub>}}) thruster.

A prototype 15&nbsp;cm diameter thruster, operated in low-magnetic-field mode, underwent initial thrust testing in 2010, however, a more complete testing method would be necessary to properly calculate the total thrust.<ref>{{cite journal | title=Thrust measurements in a low-magnetic field high-density mode in the helicon double layer thruster | author=J Ling | author2=M D West | author3=T Lafleur | author4=C Charles | author5=R W Boswell | journal=Journal of Physics D: Applied Physics | year=2010 | volume=43 | issue=30 | publisher=IOP Publishing | doi=10.1088/0022-3727/43/30/305203| bibcode=2010JPhD...43D5203L }}</ref>  In 2014, the final thruster prototype was undergoing tests at the space simulation facility dubbed "Wombat XL" located at the Australian National University (ANU) [[Mount Stromlo Observatory]].<ref name=ResearchCareer>{{cite web | url=http://www.researchcareer.com.au/archived-news/testing-ground-set-for-plasma-jar-to-the-stars | title=Testing ground set for plasma jar to the stars | work=ResearchCareer | date=March 11, 2014 | access-date=July 19, 2016}}</ref><ref>{{cite web | url=http://stories.scienceinpublic.com.au/2014/wombat-puts-electric-rocket-through-its-paces/ | title=Wombat puts electric rocket through its paces | work=Stories of Australian Science | date=May 16, 2014 | access-date=July 19, 2016}}</ref>

The HDLT has two main advantages over most other [[ion thruster]] designs. First, it creates an accelerating electric field without inserting unreliable components like high-voltage grids into the plasma (the only plasma-facing component is the robust plasma vessel); secondly, a ''neutralizer'' is not needed, since there are equal numbers of electrons and (singly charged) positive ions emitted. So, with neither moving mechanical parts nor susceptibility to erosion, Charles explains, 'As long as you provide the power and the propellant you can go forever.'<ref name=ResearchCareer/>

==Applications==
The primary application for this thruster design is intended for satellite station-keeping, long-term [[Low Earth orbit|LEO]]-to-[[Geosynchronous orbit|GEO]] orbit transfers and deep-space applications.  While a typical design could provide a 50-year life span,{{citation needed|date=October 2011}} or a saving of {{convert|1000|lb}} of launch weight for large [[satellites]], this type of thruster could also significantly reduce the length of interplanetary space trips.<ref>{{cite web | url=http://prl.anu.edu.au/SP3/research/HDLT/applications.php | title=HDLT Applications | publisher=Plasma Research Laboratory | access-date=July 19, 2016 | archive-url=https://web.archive.org/web/20110302100330/http://prl.anu.edu.au/SP3/research/HDLT/applications.php | archive-date=March 2, 2011}}</ref>  For example, a trip to Mars could be shortened to three months instead of the eight to nine months with conventional chemical rockets.<ref>{{cite web | url=https://gizmodo.com/5917866/australia-is-building-a-pee-powered-ion-thruster | title=Australia Is Building a Pee-Powered Ion Thruster | work=Gizmodo | date=June 13, 2012 | access-date=July 19, 2016 | author=Tarantola, Andrew}}</ref><ref>{{cite web | url=http://www.astronomycafe.net/qadir/q2811.html | title=How long would a trip to Mars take?  | access-date=July 19, 2016}}</ref>{{failed verification|date=July 2017}}

==See also==
* [[Variable Specific Impulse Magnetoplasma Rocket]] (VASIMR)

==References==
{{reflist}}

==Sources==
* [http://www.physorg.com/news9535.html Plasma thruster tested for Mars mission]
* [https://gizmodo.com/5917866/australia-is-building-a-pee-powered-ion-thruster Australia Building Ion Thruster (2012)]
* [http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2009index/IEPC-2009-236a.pdf ESA Propulsion Lab] {{Webarchive|url=https://web.archive.org/web/20160303224447/http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2009index/IEPC-2009-236a.pdf |date=2016-03-03 }}
* [http://www.researchcareer.com.au/news/testing-ground-set-for-plasma-jar-to-the-stars Plasma Jar to the Stars (2014)]

{{spacecraft propulsion}}
{{emerging technologies|topics=yes|space=yes}}

[[Category:Spacecraft propulsion]]
[[Category:Spacecraft components]]