Enabling ultra-high vacuum for quantum optical instruments in space

Clouds of ultra-cold atoms or a Bose-Einstein Condensate (BEC), perhaps borne in microgravity, can be used as test objects for high-precision matter wave interferometers. Quantum optical instruments based on this technology have the potential to validate scientific theories, such as the unification of the standard model and general relativity, or to measure inertial forces for advanced navigation of vehicles. The point-like nature of a BEC and the unlimited time of free-fall in space increase the achievable accuracy of matter wave interferometers. Any disturbance of the ultracold atoms can trigger the interferometer to lose precision. Following, Ultra-High Vacuum (UHV) is fundamental to achieve a BEC and perform high-precision matter wave interferometry. The hits with the background gas in the vacuum system result in impulse changes and thereby a loss rate, which in turn causes loss of interferometer contrast. In the scope of this thesis, various key
technologies for ultra-cold atom UHV systems in space are investigated, simulated, and tested. The advantages and disadvantages of pump systems and their usage in cold atom instruments are shown. A focus is set on the lifetime verification for filaments of a Titanium Sublimation Pump (TiSub) and the optimization of gas species dependent pumping. Numerical analysis of the filament degradation is used to configure a TiSub for long-term missions. Usually, the limiting factor for pump rates is, in the regime of molecular flow, the transmission probability and conduction of appliances. Simulation work with a Test Particle Monte-Carlo (TPMC) method is performed to gather detailed information on transmission probabilities for in-vacuum components. Several geometries and their dimensions were modified and analyzed. The optimal parameters for meshes, sintered filters, atom chip holders, and a Differential Pumping Stage (DPS) were found to maximize the effective pump rate. Experimental setups were developed to verify and validate important parameters of UHV systems. An outgassing rate measurement testbed is built, characterized, and used for the measurement
of assemblies and materials. Here, challenges of the measurement setup are tackled and elaborated. Material specific outgassing rates were measured and are in good agreement to literature values. First assembly measurements showed a significant influence of heat and driving currents on the outgassing rate. Three laboratory setups were developed, to investigate on thermal, dynamic mechanical, and corrosive properties for all-metal seals of UHV windows. Results show the leakage rate of Indium (In), Lead (Pb), Gold (Au), and glue sealed windows. For leak tight connections, the leakage rate was investigated for various material combinations and surface qualities under representative dynamic mechanical and thermal loads — with no significant increase for In, Pb, and glue as seal. With another test setup ovens for alkali metal evaporation were characterized by their partial pressure and temperature behaviour. Results showed clearly that a valve is perfect to enable an immediate alkali metal partial pressure shut down. On the other hand, thermal ovens with cracked ampule and no valve were not suitable to completely shut down the alkali metal pressure, and in addition, needed longer activation times. In experiments using cracked ampules, the long times to establish a high partial pressure were probably caused by the disadvantageous heat transport and transmission of the alkali metal through the cracked glass. The measurements for filled, valve-regulated, and cracked ovens point out that there is no systematic root cause for the delay. The laboratory setups and simulation results were used to develop the International Space Station (ISS) payload Bose-Einstein-Condensate and Cold Atom Laboratory (BECCAL) and especially the UHV system for this space instrument. An overview of BECCAL and the UHV system in detail is given. Here, the ISS as platform, the goals of BECCAL, and the payload setup are shown. Finally, the pressure distribution of the UHV system were analyzed and simulated. With new methods and numerical simulations, investigations of pressure differences in the vacuum system are performed and thus help to verify scientific data and validate requirements. The analysis proves that the requirement of < 1 × 10−8 Pa (< 1 × 10−10 hPa) at the location of ultra-cold atoms can be achieved. In a prototype setup, the validity of the analysis is shown.
The results of this thesis lay the groundwork for the development and optimization of future UHV systems in space dedicated for quantum optical instruments.