A group of French physicists has optimized a type of ion thruster to significantly extend its lifetime: the new development made it sturdy enough to be able withstand a long trip into deep space. Such a thruster would require 100 million times less fuel than common thrusters that use chemical reactions to propel a spacecraft forward.
Ion thrusters accelerate ions – positively charged atoms – using electrical fields. The ions are accelerated from the anode (positive electrode) to the cathode (negative electrode). Along their path, the ions are hit by a beam of electrons which make them electrically neutral. Consequently, this means they won’t interact with the cathode but simply continue on their path out of the engine. The continuous stream of atoms into space propels the spacecraft forward.
The specific model they worked on is a Hall thruster; it is very similar to other ion engines, but it has a negatively charged plasma (usually a cloud of electrons) cathode trapped in a magnetic field. Expelled ionized gas is attracted by the negatively charged plasma and accelerated. As the ions pass through the cathode, they become neutral and they continue unaffected by electric and magnetic fields moving forward at high speed, propelling the craft in the opposite direction.
Over 240 Hall thrusters have flown to space since 1971 with a 100% success rate, mostly used for satellites in geosynchronous orbit. They can accelerate the exhaust gas to a speed between 10 to 80 kilometers per second (6 to 50 miles per second). The average lifespan of a Hall thruster is 10,000 working hours as the ion flow degrades the walls of the engine significantly. To guarantee safety for a round trip to Mars, it would require at least three times as much, if not more.
As the engine’s wall was the part most affected by deterioration, the researchers at the French National Center for Scientific Research simply removed it. The first prototype didn’t work – the anode was within the magnetic field, producing interactions with the electron cloud and reducing thrust performance (pictured below, on the left).
(Left) Basic configuration of a wall-less Hall thruster: the anode is simply moved at the channel exit plane. The magnetic field lines intercept the anode. (Right) Optimized wall-less design: the magnetic field lines are parallel to the anode, by Vaudolon et al.
In the latest model, the scientists moved the anode to be outside the magnetic field line, resulting in an optimized wall-less Hall thruster that doesn’t suffer from reduced thrust or degradation.
The team is very hopeful following this breakthrough. “Despite decades of research, the physics of Hall thrusters is still far from being understood, and the device characterization methods still rely on trials and testing, leading to expensive efforts,” Julien Vaudolon, lead author of the study, said in a statement. “The major difficulty in developing predictive simulations lies in modeling the interaction between plasma and wall. The wall-less design would be an effective solution, potentially making future predictive simulations feasible and reliable.”
The study was published in Applied Physics Letters