From Purple Haze Comes Thrust
A novel space propulsion concept called an Helicon Double Layer Thruster (HDLT) is now under study by the Advanced Concepts Team and Laboratoire de Physique et Technologie des Plasmas (LPTP), Ecole Polytechnique, France under the ESA Ariadna academic programme. The HDLT concept was invented in 2002 by Dr. Christine Charles and team working in the Plasma Research Laboratory at Australia National University (ANU) led by Prof. Rod Boswell. Testing of the HDLT developed at ANU will soon be performed in the ESTEC Electric Propulsion Facility under a separate collaboration.
The HDLT is a form of plasma propulsion with the potential for scaling to operation at very high electrical power in order to deliver a high thrust and moderate specific impulse. Such performance could have substantial mass/cost reduction benefits for future space missions with applications to deep space science/exploration, and large Earth orbit-raising manoeuvres.
High-density plasma, created by the ionisation of certain gases using a helical-shaped radiofrequency antenna (or helicon) and confined within a tube by a strong magnetic field (a so called helicon reactor), could be accelerated by an intriguing phenomenon called an 'electric double layer' to produce thrust. Experimental and theoretical work at ANU has confirmed the existence of an electric double layer in the helicon reactor laboratory layout. This naturally-occurring process is evident in solar-terrestrial physics at the magneto-shock region formed by the interaction between the solar wind and Earth's magnetosphere. A sudden drop in the electric field in this shock wave causes the plasma to be accelerated to supersonic speeds. By exploiting this phenomenon in a high-density plasma device such as a helicon reactor, propulsive thrust would be produced without the use of any internal components which could be eroded (as experienced in other high-power electric thrusters). Thus, the HDLT is expected to also benefit from a long operational lifetime.
The study being carried out for ACT by Ecole Polytechnique is independent to the research at ANU, and aims to assess the feasibility and scalability of the concept through theoretical work and to investigate conditions influencing the formation and strength of the electric double layer. The latter is being done by Ecole Polytechnique partly by using an experimental helicon reactor used for plasma research in the LPTP. Important parameters such as power, gas species, magnetic field, gas pressure and thruster geometry within the diffusion chamber will be varied and their effects observed and analysed to gain a greater understanding.
The experimental programme of work has already started at Ecole Polytechnique (see images) and very colourful-looking plasmas have been created in the 2kW power helicon reactor, such as the purple plasma produced from Argon gas seen here. Electric double layers have already been measured using the in-situ plasma probe diagnostics. A number of propellants will be investigated, including hydrogen and electronegative gases (e.g. SF6), to assess their effect on double layer formation/stability, magnetic field configuration and overall performance.
In parallel, the analytical and theoretical work will focus on the prediction of propulsive performance at low power in the laboratory and eventually at full-scale high power operation. The study will be concluded by August 2005 and results published soon after. It is expected that this will help to answer some of the initial fundamental questions surrounding the feasibility and real potential of the HDLT for future missions, though a further theoretical/experimental programme of work will be needed to confirm real performance in the space environment.