Known as a Hall thruster, these engines have been operating in space since 1971, and are now routinely flown on communication satellites and space probes to adjust their orbits when needed. These things are awesome, and scientists want to use them to get humans to Mars, except there’s one - rather large - problem: the current lifespan of a Hall thruster is around 10,000 operation hours, and that’s way too short for most space exploration missions, which require upwards of 50,000 hours.
Hall thrusters work just like regular ion thrusters, which blast a stream of charged ions from an anode to a cathode (positively and negatively charged electrodes), where they get neutralised by a beam of electrons. This causes the elections to shoot one way, and the attached rocket to shoot another, propelling it forward.
The difference with Hall thrusters is instead of having a physical cathode, they combine a magnetic field and a trapped electron cloud to create a completely hollow, 'virtual' cathode. A small amount of propellant gas - typically xenon - is injected into the thruster's channel to produce a charged ion stream, and because these ions are too heavy to be caught up in the magnetic field of the virtual cathode, they can zip through unimpeded to be neutralised. This creates a low-pressure plasma discharge, which produces thrust in the opposite direction from that of the ion flow.
All of this is fine and works great, but the part of the Hall thruster that contains the anode, virtual cathode, and electron cloud is what’s holding it back. This container, called the discharge channel wall, is being constantly bombarded with high-energy ions, and this wears it down so much, the whole engine will eventually need to be recalled to have the wall repaired or replaced.
So scientists from the French National Centre for Scientific Research decided to remove the discharge channel wall altogether. "An effective approach to avoid the interaction between the plasma and the discharge channel wall is to move the ionisation and acceleration regions outside the cavity, which is an unconventional design named a Wall-Less Hall Thruster," said lead researcher, Julien Vaudolon.
Applied Physics LettersUnfortunately, as Esther Inglis Arkell explains at Gizmodo, their first prototype (on the left in the image above) was a total failure."The red anode should be lined up on the wall emitting xenon. Instead, it's in the magnetic field, allowing electrons to glom onto it, reducing performance," she says. "The new design (on the right) makes the small change, which allows the anode to keep clear of the field. This one seems to work."
The research has been published in Applied Physics Letters.
Because the engine consumes a whole lot less fuel than conventional chemical rockets, this frees up room in the spacecraft to send large amounts of cargo, or perhaps more people. That means the potential for long-duration, deep space missions, like the ones we’d need to transport humans and regular supplies to Mars.
There’s no word yet on how much this new design could extend the lifespan of the Hall thruster, but if the researchers can get it up around the 50,000-hours mark, it could revolutionise space exploration in the future. They just have to get it done before scientists figure out the "impossible" EM Drive, which can theoretically blast us to Mars in just 70 days. Game on.
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