MIRI Cooler System Design Update

The Mid Infrared Instrument (MIRI) for the James Webb Space Telescope (JWST) requires cooling at 6 K for the SiAs focal planes, provided by an active cooler. The four stage cooler consists of: a three-stage pulse tube precooler, Joule Thompson (J-T) circulator and upper stage recuperators, located on the JWST spacecraft bus, and the final stage recuperator and 6 K J-T expander, located at the remote instrument module with a 12 meter round trip line at 18-22 K between the spacecraft and instrument. Since our last report on the cooler design, the JWST program has made design changes to the overall thermal design, including the addition by Goddard Space Flight Center (GSFC) of an actively cooled thermal shield surrounding the MIRI Optical Module to increase the overall thermal efficiency and thereby increase the margin between the cooler lift capability and the expected heat loads. This change shifts a portion of the thermal load from the 6 K cooler stage to the 18-22 K stage. Meeting the increased thermal load at 18-22 K and realizing the benefit from the decreased load at 6K requires operating the cooler at a different set of operating points. In this paper, we report on the required changes to the cooler’s operation and design, and demonstrate that the basic design has the capacity to meet the new requirements.