A new type of a wall-less Hall thruster could revolutionise space travel

New plasma thruster paves way for cheaper space missions

French aerospace engineers have improved the design of the so called Hall thruster, which uses plasma to propel spacecraft, paving the way for cheaper space missions. 

In an experiment described in the latest issue of the journal Applied Physics Letters, researchers from the French National Centre for Scientific Research modified an existing wall-less thruster to improve its performance.

The wall-less design, demonstrated by the same team last year, overcomes some of the major drawbacks of Hall thrusters.

Although extremely fuel-efficient, consuming 100 million times less propellant than chemical rockets, conventional Hall thrusters have much shorter life spans than other technologies.

Currently, Hall thrusters have life spans of around 10,000 operation hours while most space missions require at least five times more.

"The major drawback of Hall thrusters is that the discharge channel wall materials largely determine the discharge properties, and consequently, the performance level and the operational time,” said Julien Vaudolon, the primary researcher in the Electric Propulsion team led by Professor Stéphane Mazouffre in the ICARE-CNRS Laboratory, France, who developed the new device

Currently used mostly for station-keeping and attitude control of geosynchronous communication satellites and space probes, Hall thrusters are electric rocket engines that use an extremely fast stream of plasma to push the spacecraft forward.

Their operating principle relies on the creation of a low-pressure quasi-neutral plasma discharge in a crossed magnetic and electric field configuration. The propellant gas, typically xenon, is ionised by electrons trapped in the magnetic field.

In the conventional Hall thruster configuration, the magnetised discharge is confined to an annular dielectric cavity with the anode at one end, where the gas is injected, and an external cathode injecting electrons. Ionisation of the propellant gas occurs inside the cavity, with ions accelerated by the electric field that stretches from the interior to the exterior of the cavity.

Vaudolon explained that the wall materials play a role in the plasma properties mainly through secondary electron emission, a phenomenon where high-energy ions hit the channel wall surface and induce the emission of secondary electrons. Additionally, the erosion of the discharge cavity walls due to bombardment of high-energy ions shortens the thruster’s lifetime.

“Thus, 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,” Vaudolon said.

However, the performance of the small-scale wall-less prototype developed by the French team last year was extremely low.

This was due to the magnetic field lines crossing the thruster's axis. The team has since optimised the prototype by rotating the magnetic barrier by 90 degrees so that it injects the magnetic field lines in parallel with the axis.

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