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Ceramic Insulators Protect Unique Gas Plasma Engines
Thruster in use.
 
Windsor, Berkshire, United Kingdom — Advanced satellite technology may also require advanced materials, and when researchers at the Australian National University (ANU) needed a material to construct a custom plasma cavity for a new gas plasma space engine, they sought ceramic materials. They were directed to Morgan Advanced Materials, a leading supplier of custom high-precision ceramic components based on high-purity alumina. The firm developed a prototype of the critical cavity, using a high-purity alumina known as AL300, formed of 97.6 percent pure alumina with the demanding electrical properties needed for the cavity. After developing an in-house process to produce the required components, Morgan achieved first-pass success with a short lead time, delivering prototypes to ANU for space-qualification testing.

Seeking Greener Satellites
For more than 15 years, the Space Plasma, Power, and Propulsion Laboratory at ANU has been conducting research on developing the Helicon Double Layer Thruster (HDLT), a new gas plasma space engine suited for satellites. Gas plasma engines support electric propulsion technology, which is growing in popularity because it employs "greener" propellants rather than more toxic chemicals. Most satellites employ chemical thrusters or chemical rockets, leaving propellant residue at the end of the satellite's operating lifetime. At present, about 200 orbiting satellites use electric propulsion; the plasma engine would be used in a new generation of satellites that use safer propellants.

The project to develop these new gas plasma space engines is funded by the Australian Space Research Program, Astrium, an aerospace manufacturer that is a subsidiary of the European Aeronautic Defence and Space Company (EADS), the Surrey Space Centre, the ANU, and Vipac, a multi-disciplinary technical consultancy specializing in mechanical and systems engineering, testing, and instrumentation. According to Christine Charles, ANU Professor and head of the Space Plasma, Power, and Propulsion Laboratory, the researchers will be conducting space qualification and prototype development at ANU's new Advanced Instrumentation Technology Centre. Professor Charles notes: "In our laboratory, we have a small space simulation chamber where we can conduct tests. In the much larger Advanced Instrumentation Technology Centre we can combine research and development, in cooperation with a number of industrial partners."
Concept of the gas plasma engine being developed for satellite use. <emfiller102b.jpg></emfiller102b.jpg>
 


Rod Boswell, a professor in the Space Plasma, Power, and Propulsion laboratory at ANU, reports that aerospace manufacturers are actively sponsoring the project because they want to be ready to respond to the recent move towards safer propellants. They also need systems for addressing existing and expected requirements to move older satellites to their "graveyard orbit," to lower the probability of collisions with operational spacecraft and minimize generation of space debris. Boswell explains: "These newer, greener space applications are being seriously considered by manufacturers like Astrium-EADS, similar to how designers are continually looking to develop better electric engines for cars."

Plasma Engine Basics
The plasma technology being developed uses a small cup and a closed end into which a gas is injected. Argon is the most commonly used gas, although a Hydrazine simulant, composed of ammonia and nitrous oxide (N2{/}O), has also been used. A metal coil generates a radio-frequency (RF) field to heat the gas and enable ionization. The other end of the cavity is open, and the plasma is emitted through this opening into space. Explains Professor Boswell, "It's essentially a rocket that provides thrust to move satellites around, but the innovative part is that it can use almost any propellant. If you could obtain enough solar power to run the RF system, you could just keep using this forever, so long as you have a propellant."

In the HDLT, the coil generates an RF field that turns the gas into a ball of plasma within the ceramic cavity (supplied by Morgan Advanced Materials). The ball of plasma then expands out of the open end and, by so doing, creates an electric field that accelerates the ions to about 10km/s to provide the thrust.

The simple and robust system contains no moving parts, would be quite inexpensive to produce for use in space, and can operate with any propellant. While not providing enough thrust to lift a rocket from Earth into orbit, once in space it provides sufficient thrust for most situations. Dimensionally, the HDLT can be scaled up or down. An example thruster was made to fit within a 25 x 10 x 10cm box, with the ceramic cavity about the size of a grapefruit.

Plasma Cavity Material
Since it would eventually be attached or mounted to a spacecraft, researchers were seeking custom-made, high-quality components for the HDLT. Among the most critical of these components is the plasma cavity, which must withstand the heat that creates the plasma.

The AN
U researchers tried a variety of materials for the plasma cavity, including Pyrex, and knew they needed some kind of ceramic material because of the thermal requirements. The cavity could not be made from metal because heat from the RF energy must be able to pass through into the plasma; metal would reflect the heat. After some difficulty in sourcing the required material from other Australian ceramics manufacturers, the researchers were recommended to Morgan Advanced Materials by a US company with whom they were familiar.

Morgan Advanced Materials manufactures high-purity alumina components and metalized assemblies for sensing control systems used in such applications as aircraft landing gears, doors, flight control surfaces, thrust reversers actuators, and many other non-aerospace applications. The high-purity alumina material is used in precision components and brazed assemblies, machined to exact tolerances and finishes applied that enable consistent performance to the most demanding requirements.

After receiving the inquiry from the ANU researchers, Zachary Waddle, an engineering manager at Morgan suggested the use of AL300 alumina for the cavity. According to Waddle, the material has long been used in high-voltage and RF applications, and is known for its excellent electrical properties: "I knew the material had been successfully used in the manufacture of components generating plasma and also for high voltage insulation used in new and emerging scanning electron microscopes, so I thought it would be an excellent match for this project."

Waddle began development of the cavity by reviewing the drawings provided by Professor Charles. The concept was relatively simple, with the cavity designed to attach easily to the chassis for all tests to be conducted. Waddle provided design feedback on how to best make the geometry robust and also offered a review of the machined tolerances, to help the researchers achieve the most affordable part.

Morgan Advanced Materials then developed an in-house process to make the parts, which have a thin wall and a long aspect ratio. "We had to be careful to fire to size," said Waddle. He added: "Once fired, we only had to grind in one dimension. We had first-pass success on the parts and were able to meet ANU's short lead time." Morgan Advanced Materials then provided ANU's researchers with several of the finished prototypes to be used for space-qualification testing.

The ANU researchers have been extremely pleased with the results, finding the ceramic cavities to be extremely sturdy, and well within the tolerances they required. According to Professor Charles: "In fact, it turned out to be the best material we have ever tested."

 

Contact: Morgan Advanced Materials plc, Quadrant, 55-57 High Street, Windsor, Berkshire SL4 1LP, United Kingdom +44 (0) 1753 837000 fax: +44 (0) 1753 850872 Web:
http://www.morganadvancedmaterials.com

 
 
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