Helicon Double Layer Thruster
Encyclopedia
The Helicon Double Layer Thruster is a prototype spacecraft propulsion engine. It was created by Australian scientist Dr. Christine Charles, based on a technology invented by Professor Rod Boswell, both of the Australian National University
Australian National University
The Australian National University is a teaching and research university located in the Australian capital, Canberra.As of 2009, the ANU employs 3,945 administrative staff who teach approximately 10,000 undergraduates, and 7,500 postgraduate students...

.

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 high velocity ionized gas to provide thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

 to a spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

. In this thruster design, gas is injected into a tubular chamber (the source tube) with one open end. Radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 AC power (at 13.56 MHz in the prototype design) is coupled into a specially shaped antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

 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
Helicon (physics)
A helicon is a low frequency electromagnetic wave that can exist in plasmas in the presence of a magnetic field. The first helicons observed were atmospheric whistlers, but they also exist in solid conductors or any other electromagnetic plasma....

 wave in the plasma, which further heats the plasma. The device has a roughly constant magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 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
Nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....

. In operation, there is a sharp boundary between 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. The plasma properties change rapidly across this boundary, which is known as a current-free electric double layer
Double layer (plasma)
A double layer is a structure in a plasma and consists of two parallel layers with opposite electrical charge. The sheets of charge cause a strong electric field and a correspondingly sharp change in voltage across the double layer. Ions and electrons which enter the double layer are accelerated,...

. 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 (Isp) thruster. A prototype 15 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 http://iopscience.iop.org/0022-3727/43/30/305203.

The Helicon Double Layer Thruster has two main advantages over most other ion thruster
Ion thruster
An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and...

 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 isn't needed, since there are equal numbers of electrons and (singly charged) positive ions emitted.

Applications

The primary application for this thruster design is intended for satellite station keeping, long term LEO
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

 to GEO
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...

 orbit transfers and deep space applications. While a typical design could provide a 50 year life span, or a saving of 1000 lb (≈454 kg) of launch weight for large satellites. This type of thruster could also significantly reduce the length of interplanetary space trips.http://prl.anu.edu.au/SP3/research/HDLT/applications.php For example, a trip to Mars could be shortened to 3 months instead of the 8 to 9 months with conventional chemical rockets. http://www.astronomycafe.net/qadir/q2811.html In the near term, the next target for HDLT application is to maintain station-keeping for a satellite to be launched around 2013 in collaboration with EADS-Astrium http://news.anu.edu.au/?p=1148.

Sources

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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