Helicon double layer thruster

hdlt team

The Helicon Double Layer Thruster (HDLT) is being developed in the Space Plasma, Power and Propulsion Group (led by Professor Christine Charles) at the Research School of Physics at The Australian National University in Canberra, Australia.

Dr Christine Charles has invented the world's first Helicon Double Layer Thruster or HDLT. This new propulsion concept has the potential to propel humans to Mars and beyond and greatly decrease the costs of maintaining satellites and spacecraft in their desired orbits.

What is a plasma?

Plasma is a 'fourth state of matter' in which many of the atoms or molecules are ionized. Plasmas have unique properties compared to the other states of matter i.e solids, liquids, and gases. Most plasmas can be thought of at first as extremely hot gases, but their properties are generally quite different.

Some examples of plasmas include the sun, fluorescent light bulbs and other gas-discharge tubes, very hot flames, much of interplanetary, interstellar, and intergalactic space, the earth's ionosphere and parts of the atmosphere around lightning discharges. Plasmas actually make up nearly 99 per cent of the matter in the universe but are extremely rare on Earth. One of the most spectacular examples of a plasma is the aurora. Known as the Aurora Australis or the Southern Lights this plasma forms as a result of interactions between the Earth's ionosphere and the solar wind.

What is a double layer?

Thurster team

Electric double layers are like cliffs of potential (like a riverwaterfall) and can energise charged particles falling through them. They exist in the plasma environment of the earth and the stars and can cause phenomena as diverse as aurorae, luminous draperies in the polar sky, and electromagnetic radiation from rotating neutron stars called pulsars [1].

We have discovered such a double layer in our laboratory plasma systems and measured the energy of the highly supersonic ions it has accelerated. The fascinating part is that the double layer is not triggered by forcing two plasmas (independantly generated by grids with separate potentials, much like a man-made dam) to interact, but self generates under certain parameters, much like the riverbed suddenly falling away to create a waterfall. We are optimising this effect to create a very efficient thruster for interplanetary spacecraft.

1. M.A. Raadu, 'The physics of double layers and their role in astrophysics', Physics reports 178, 25-97 (1989)

How does HDLT work?

HDLT testing

Our laboratory is internationally recognised as the inventor and prime developer of the helicon source, arguably the most efficient plasma source available. Krypton (but eventually Xenon) gas is injected into a tube, called the source, that is open at one end and terminated at the other and is energised by a radio frequency antenna. Solenoids create an expanding magnetic field that is roughly uniform in the source tube but that expands very rapidly out into space until it is only a few gauss 20cm away from the source. The high density plasma formed in this way is restricted from exiting the source by a non-linear plasma effect known as a current free electric double layer that is located near the exit of the source tube.

This double layer can be thought of as a thin standing shock wave across which there exists a strong electric field gradient. It is this electric field that accelerates ions from the source plasma to very high exhaust velocities creating thrust. Because the double layer is purely the result of plasma density, system and magnetic field geometry, no accelerating grids are required. Also, because there is equal flux of electrons and positive ions from the thruster there is no need for a neutraliser. It is in this sense that the HDLT is a "true" plasma thruster as it ejects equal numbers of both positive ions and negative electrons.

Power is required only for the maintenance of plasma and the creation of the magnetic field. In our current bench top prototype, 250W is sufficient to create several milli-Newtons of thrust. In space the solenoids that generate the 250 Gauss of magnetic field this requires we estimate could be cooled to 200K, reducing the resistance in the coils by a factor of 5 and representing a power consumption of a few 10s of Watts. Relative to other existing systems this constitutes quite a power saving and is well with-in the capabilities of solar panels. The 0.5sccm of feed gas represents a mass consumption of 160 mg/hr, so that a typical 5 hour burn would use 0.8g of propellant.


Satellite station keeping

Though the HDLT has not yet flown in space, several current orbiting satellites use ion thrusters in the same fashion as is envisaged for the HDLT. In response to any change in a satellite's orbital plane an North South Station Keeping (NSSK) maneuver is engaged. For a satellite carrying ion thrusters, this is typically achieved by firing the NSSK thrusters twice a day for around 5 hours. As the satellite ascends through the Earth's equatorial plane on its inclined plane, the North thruster is fired for a period centered around this point of passage. Twelve hours later when the satellite this time descends through the equatorial plane, the South thruster is fired in the same manner and for the same duration. EWSK and attitude control is undertaken in a similar fashion.

In the future, it is increasingly likely that we will see plasma propulsion systems such as the HDLT flying on low earth orbit (LEO) satellites. It is envisaged that such thrusters would be used to transfer the satellites from their drop-off altitude to their operational orbit and maintain proper station keeping. At the end of their useful lifetimes, the plasma thrusters would be used to deorbit the satellites in a controlled fashion so that they burn up harmlessly in the atmosphere.

Interplanetary missions

Space ship thruster schematics

Artificial satellite's have enabled many scientific advances, but Humanity's imagination has not been satisfied to dwell on the Earth and its very close orbital neighbourhood. Many satellites have been deployed on interplanetary missions and humans have traveled as far as the moon. Though the moon landing is arguably Humanity's greatest engineering achievement, the moon itself has represented the boundary of practical manned missions into the solar system using conventional chemical rockets. The shear quantity of propellant and incredible time scale required to reach even our closest planetary neighbour, Mars, with traditional space technologies has to this day made the dream of crossing interplanetary space prohibitively costly both in financial terms and in potential risk to would-be human travelers. If chemical propulsion is capable of great thrust (on the order of several Mega-Newtons) it also has very low specific impulse a measure of the propellant burn rate efficiency. Maximum velocity is also restricted by low specific impulse making transit time too long for any practical mission with a human payload. If effective manned missions are to be mounted to breach the distance of interplanetary space, a faster and more efficient mode of transport is required. Plasma propulsion can achieve these goals.

It is for the purpose of interplanetary missions that NASA has sought to collaborate with our laboratory. In particular we are currently active with Astronaut F. Chang-Diaz's VASIMR group in the Advanced Space Propulsion Laboratory at Johnson Space Center, Houston. We envisage that the high performance and low risk design of the HDLT will make it a viable candidate for future manned missions to Mars.

The Plasma Research Laboratory at the Australian National University (Australia), the Cooperative Research Center for Satellite Systems and Auspace are also collaborating with the Propulsion and Aerothermodynamics Division at the European Space Agency (ESTEC Holland) on the development and testing of the HDLT prototype. This joint research program is funded by the DEST innovation access grant in Australia.

Project collaborators

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Charles, Christine profile