Technical Ceramics in Hall Effect Thrusters
Technical ceramics used for lining ion engine acceleration channels are very vulnerable to spluttering
Temperatures within the acceleration channel of a Hall Effect thruster reach between 300 and 540 degrees C. Technical ceramics such as boron nitride are used to line these channels, as they are able to withstand the high temperatures without failure or chemical changes. However, their vulnerability to spluttering is a significant factor in limiting the thrusters’ operational lifetime.
Modern satellites and spacecraft used in deep space exploration use two types of propulsion: chemical and electrical. Chemical propulsion is needed to provide a fast and large enough thrust for a rocket to escape from Earth’s gravitational pull. Once in orbit, the spacecraft can be controlled with precision through electrical propulsion using ion engines.
There are two types of ion engine in use today on spacecraft: electrostatic, also called gridded ion engines, and electromagnetic, known as Hall Effect thrusters. Both types of engines use electricity generated by solar radiation or from radioisotope decay heat to ionise a gas, usually xenon, and accelerate it through a discharge chamber. When the plasma escapes from the exhaust of the spacecraft, it creates a thrust. This thrust is minute compared with that from a chemical propellant, but once the spacecraft is in orbit, the ion engine is ten times more efficient than a chemical engine and can propel the spacecraft to higher speeds, albeit over an extended period of time.
The main applications for Hall thrusters have been for station-keeping for commercial satellites by counteracting external forces such as the Earth’s gravitational drag and ensuring that the satellite remains in its designated orbit. One of the most notable examples of the use of a Hall Effect thruster was in the European Space Agency’s Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) that was used to map the Earth’s gravitational field. Launched in March 2009, it returned to Earth in November 2013 after running out of fuel.
However, although the acceleration chamber’s lining of technical ceramics is capable of withstanding the heat of the ionised plasma once the ion engines are switched on, the ceramics are very vulnerable to spluttering. This is a form of erosion of internal components from the impact of charged particles.
As a result, Hall thrusters have operational lifetimes of around 6,000 to 7,000 hours compared with lifetimes of around 17,000 to 20,000 hours for gridded ion engines.
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