Fig. 1: Micronewton thruster assembled on a thrust-measuring device in a vacuum chamber, together with further diagnostics (cooperation laboratory, Airbus DS, Friedrichshafen).
Propulsion systems with very low thrust performance are needed in the study of micronewton thrust to create an interference-free environment for satellites embarking on future scientific missions. They are needed because the extremely high-resolution instruments that will be used for future missions, such as the inertial sensors on a satellite and highly accurate distance measuring between satellites, will only work reliably in a low-noise environment.
A suitable thruster concept is the High Efficiency Multistage Plasma Thruster (HEMPT) by Thales Electron Devices. This already works with millinewtons and shows promising properties with regard to the system's complexity and durability, as well as the possible applications in the micronewton area.
The principle behind the HEMP thruster – A neutraliser on the exterior of the thruster creates free electrons, which, due to their negative charge, are attracted towards an anode, accelerating quickly. Xenon gas is admitted into the anode itself, which ionises the free electrons on their way to the anode. The positively charged ions (plasma) that result from this process are electrostatically repelled from the anode. The recoil of the increasing ion current then triggers the engine thrust. Annular permanent magnets, which form the outer boundary of the thruster, create a magnetic field, which forces the electrons towards the anode and into an orbit. This allows more xenon atoms to be ionised, thereby increasing efficiency. The magnetic field also shields the plasma from the chamber wall, which increases the durability of the thruster. Without this, the plasma would react with the chamber wall, which would gradually destroy it. There would then be no advantage of a thruster that does not suffer such wear and tear and has a high level of efficiency.
In a joint project between Airbus DA (Friedrichshafen), the DLR Institute of Space Systems in Bremen, the Centre of Applied Space Technology and Microgravity (ZARM) at Bremen University and the Christian Albrechts Universität zu Kiel, a micronewton HEMP thruster was constructed and characterised during an experiment, which took place at the cooperation laboratory at Airbus DS. In order to verify the thrust in the micronewton range, a high-precision optically selected thrust-measuring device was developed, which works within the requirements of the LISA mission (Laser Interferometer Space Antenna).
Fig.2: Left: Prototype of the HEMP thruster in operation (cooperation laboratory, Airbus DS, Friedrichshafen). Right: Thruster simulation (DLR Bremen)
The micronewton development is supported by simulations. The variations in micro-thrust are too small to carry out plasma diagnostics from inside. Using computer simulations, the effects that various magnetic field configurations have on plasma can be studied. The knowledge acquired should lead to an optimal design for a downscaled micronewton HEMPT.