Challenges of Cryopumping EP-Propellants in DLR’s Electric Propulsion Test Facility

Abstract

The German Aerospace Center DLR operates in Goettingen the test facility STG-ET for electric space propulsion. This facility has been especially designed for electric propulsion testing with a large graphite target that reduces the impact of sputtering, and ion beam diagnostics for monitoring thruster operation. In the early times of electric space propulsion development mercury or caesium were typical propellants due to their easy cryopumping in vacuum chambers, simple handling and low ionization energies. Thruster erosion, metal deposition on chamber walls and on other devices inside chambers, and major health and safety issues forced a switch of propellant to safer noble gases. Electric space propulsion is on the rise over the last few decades, and the most common propellant for mN electric thrusters actually is the noble gas xenon. Therefore, at the STG-ET cryopumps with high pumping speed for xenon are installed. Xenon is the favorite propellant, but this gas is rare and expensive. The rapid market growth triggered activities for studying alternative propellants. This paper describes the pumping system of STG-ET and pumping speed measurements for xenon, krypton and argon. Another aspect of operation and pumping speed measurement is the usage of precise pressure transducers. These devices are often treated as robust and accurate, but hands-on experience often draws a different picture. We found a rather short lifetime if nominal accuracy is expected. Furthermore, one must be aware of gas composition dependence of measurements.