Electric Micro Propulsion

Abstract

There is a trend in the miniaturisation of satellites. They not only become smaller, but lighter and more powerful as well. A large growth in the nano-satellite sector is present in the form of CubeSats. These milk carton size satellites use commercial of the shelf components in order to keep the costs low and to allow for technology demonstrations for future larger space missions. In order to increase their current mission lifetime from 1 - 2 years, electric micro
propulsion systems are being developed. These systems are able to provide sufficient velocity increments in order to maintain the CubeSat within the required orbit.

Electric micro propulsion systems make use of Coulomb forces to accelerate charged particles. The method is to inject a propellant gas such as xenon into a cylindrical discharge chamber. The propellant is ionised by means of electron bombardment. The electrons that are required in order to do so are pulled from the cathode into the thruster system. Inside the discharge chamber they experience an electromagnetic field, where the resulting Lorentz force makes them gyrate in the projection plane that is perpendicular to the magnetic field lines. As the propellant is inserted in the discharge chamber, a neutral gas pressure builds up. Next, the electrons that are emitted from the cathode are colliding with the neutral gas particles, which causes them to be ionised. These charged particles get accelerated out of the discharge chamber by the electrostatic field in order to create the required thrust. The goal is to develop an affordable, low complexity and efficient cathode for a high performancemicro propulsion system that can be used on small satellites. In the future, the whole system can be miniaturised in order to realise an electric thruster system that can be implemented on CubeSats in order to expand their mission lifetime.

The current graphite needle tip cathode can be successfully used as an electron source for propellant ionisation and plasma plume neutralisation in combination with the xenon fed engineering model electric thruster. The thermionic LaB6 cathode is capable of achieving an emission current up to 64 mA with 56 Watts of input power. This is realised by using graphite needle tips on the end of molybdenum posts that heat the LaB6 emitter pellet. The micro High
Efficiency Multistage Plasma Thruster is operated with a mass flow of 2.0 sccm. It uses an anode voltage and anode current of 700 V and 63 mA respectively. The cathode current is equal to 5 mA. Furthermore, the nominal power to
thrust ratio is equal to 25.5 W/mN and the specific impulse that is achieved equals 930 s, dependent on the applied settings. The divergence efficiency ranges from 80 - 90 % and the total system efficiency is currently equal to 80 %.
This total efficiency needs to be improved in the subsequent iterations of the design.