Development of the Passive Phase Separator for the Herschel Cryostat

Abstract

The ‘Herschel Space Observatory’ is the fourth cornerstone mission in the European Space Agency (ESA) science program. Herschel will be placed in an operational orbit around the Earth‐Sun L2 point by an Ariane 5 in early 2007 to perform measurements in the wavelength range from 60 to 670 micrometers during its operational lifetime of 3.5 years. The cold environment for the instruments is provided by the evaporation of superfluid helium. The thermo‐mechanical effect in a porous plug is used for the in‐orbit separation of liquid and gaseous phase under zero gravity conditions.The required dynamic range of the Passive Phase Separator (PPS) is defined by the expected post‐launch mass flow rate and bath temperature variations between 1.5 and 20 mg/s and 1.5 and 2.1 K, respectively. Measurement results concerning mass flow rate, bath temperature, temperature difference across the porous plug and pressure drop were gained in the development phase of the porous plug.As a side effect, an unexpected behavior of the temperature difference over the porous plug near the liquid break‐through temperature was observed.The measured characteristics of the selected flight plug are evaluated in the context of system level thermal and pressure drop analysis.