Ion drives may open up the outer Solar System, but they’re anything but high-thrust. With NASA’s Deep Space 1 mission and the later European Smart 1 moon mission, the idea was to operate for long periods of time with very little kick from the engine. The effect is cumulative, and it works. Japan’s Hayabusa asteroid probe used four ion engines designed to burn throughout its cruise to asteroid Itokawa. 20,000 hours of cumulative operation used up a scant 20 kilograms of propellant, highlighting the efficiency of these engines. NASA has run an NSTAR thruster at the Jet Propulsion Laboratory for over 30,000 continuous hours, almost five years of operation.
Now the European Space Agency has conducted successful tests of a new kind of ion drive, one designed to provide greater thrust than its predecessors. The Helicon Double Layer Thruster (HDLT) uses radio waves to ionise argon gas, creating two layers of plasma between which charged particles can be accelerated in a beam. The HDLT design was conceived by Dr. Christine Charles and team working in the Plasma Research Laboratory at Australia National University. Here’s how an ESA news release describes the process they invented, drawing on an interview with Dr. Pascal Chabert (Ecole Polytechnique, Paris):
To create the double layer, Chabert and colleagues created a hollow tube around which was wound a radio antenna. Argon gas was continuously pumped into the tube and the antenna transmitted helicoidal radio waves of 13 megahertz. This ionised the argon creating a plasma. A diverging magnetic field at the end of the tube then forced the plasma leaving the pipe to expand. This allowed two different plasmas to be formed, upstream within the tube and downstream, and so the double layer was created at their boundary. This accelerated further argon plasma from the tube into a supersonic beam, creating thrust.
How much thrust? At this point, ESA simply notes that at the same level of fuel efficiency as the main thruster on the SMART-1 mission, the new engine would produce “…many times more thrust at higher powers of up to 100 kW…”
The effect is not dissimilar to a natural phenomenon witnessed by many on Earth, as ESA explains:
“Essentially the concept exploits a natural phenomenon we see taking place in space,” says Dr Roger Walker of ESA’s Advanced Concepts Team. “When the solar wind, a ‘plasma’ of electrified gas released by the Sun, hits the magnetic field of the Earth, 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 create the aurora.”
Centauri Dreams‘ take: Now that ESA has confirmed the Australian results, the way is open to begin design studies for larger prototypes. The key to the recent ESA tests was to demonstrate that a double plasma layer could remain stable and thus allow for reliable acceleration of the charged particles for thrust. That it can is heady news and points to new sets of options for missions to the outer planets. We need spacecraft capable of delivering maximum payload without being dwarfed by fuel constraints. Coupling ion efficiency with higher thrust is a welcome and necessary step toward that goal.