ESA's helicon double layer thruster. Image: LPTP, Ecole Polytechnique.
A helicon thruster is a device that works in a similar way to a pulsed-plasma thruster but with one important difference: a traveling electromagnetic wave interacts with a current sheet to maintain a strong force on a plasma moving along an axis. This circumvents the pulsed-plasma thruster's problem of the force falling off as the current loop gets larger. The traveling wave can be created in a variety of ways, and a helical coil is often used.
Details of operation
High-density plasma is produced by the use of a helical radio frequency antenna to ionize a neutral gas such as argon, krypton, xenon, helium, or hydrogen, in a tube closed at one end. The helical antenna excites the gas, stripping it of electrons and generating highly energetic ions. Solenoid coils surrounding the tube create an magnetic field inside to confine the plasma within the tube and reach a high ion particle density.
The plasma can be heated to very high temperature by another antenna downstream (operating at the ion-cyclotron resonance frequency) and expanded through a nozzle into vacuum, as in the case of the VASIMR thruster.1 Alternatively plasma can be accelerated to supersonic speeds by being forced through an electric double layer – a magneto-shock region with sudden drop in potential – created by a rapidly expanding magnetic field very close to the open end of the tube. The principle of this helicon double layer thruster is similar to that behind a natural phenomenon in space. When the solar wind from the Sun hits the Earth's magnetic field, it creates a boundary consisting of two plasma layers. Each layer has differing electrical properties, and this can accelerate some particles of the solar wind across the boundary, causing them to collide with the Earth's atmosphere and give rise to the aurora. Other methods such as a "magnetic nozzle" may also be used to accelerate the plasma in the source to high exhaust velocities, thus producing a moderate thrust and a high specific impulse.
Helicon double layer thruster breakthrough
In 2003, researchers at the Australian National University in Canberra created a plasma double layer in the laboratory.2 Then, in December 2005, the European Space Agency announced that its initial testing of a helicon double layer thruster, using argon gas as a source of ions, had been successful. The ESA team not only replicated the Australian findings but also showed that the double layer can remain stable enough to accelerate ions reliably. Having proven the principle, ESA is now proceed with simulations and perhaps bigger prototypes.
Helicon-type thruster concepts are scalable to high power (kW to MW) operations, which means that they can potentially attain much higher thrust levels (of the order of one newton) and even higher specific impulses (of the order of 10,000 s or more), with the additional advantage of requiring no high-current cathode, acceleration grids, or neutralizer that presently limit the operating lifetime in other electric thrusters. However, the scaling to high power is a challenging task since non-linear interactions between plasma flow and magnetic and electric fields at higher energies are difficult to predict and small-scale instabilities arising may cause a reduction in thrust efficiency. Research is now focussing on detailed simulation modelling and experimental programs to optimize thruster geometry, radio frequency system and applied magnetic field for high-energy plasma generation efficiency, confinement, and acceleration.
Despite the challenges to be overcome, in principle the potential for helicon-type
thrusters operating at high power levels to produce a high continuous thrust
and high, variable specific impulse (compared to some other types of electric
propulsion) make them an attractive choice for propelling large spacecraft
requiring high delta-V and acceptable transfer times. These missions include
human or cargo missions to Mars,3 and robotic missions with large science payloads to orbit planets in the
outer solar system,4 or even to deflect near-Earth asteroids.5
1. J.P. Squire, F.R.C. Diaz, T.W. Glover, V.T. Jacobson, D.G. Chavers,
R.D. Bengtson, E.A. Bering, R.W. Boswell, R.H. Goulding And M. Light,
" Progress in experimental research of the VASIMR engine ". Fusion Science
and Technology 43 , 111-117 (2003).
2. C. Charles, R.W. Boswell, " Laboratory evidence of a supersonic ion beam generated by a current-free "helicon" double-layer ". Phys. Plasmas 11 , 1706-1714 2004.
3. Future Power System for Space Exploration, S54 study, ESA publication, February 2002.
4. Report of the NASA Science Definition Team for the Jupiter Icy Moons Orbiter (JIMO), http://ossim.hq.nasa.gov/jimo/JIMO_SDT_REPORT.pdf
5. Williams, B.G., Durda, D.D., Scheeres, D., The B612 Mission Design, 1 st AIAA Planetary Defense Conference, Orange County, California, 2004.