This barbaric acronym refers to the second phase of activity of the Working Group PaCSXI, standing for “Passive Cryogenic SIM/X-IFU Interface”. This working group was set up after the I-PRR (Instrument Preliminary Requirement Review) in order to implement one of its recommendation: studying an alternative configuration of the instrument. It would reduce the risk related to microvibrations as well as reduce the complexity of the instrument by considering the number of active coolers and compressors as the main quantitative figure of merit. Among others, one driving idea behind this recommendation was to modify the SIM/X-IFU configuration and interfaces to make a more extensive use of the radiative cooling offered by the cold space, and implement two stages of passive cooling for the Instrument (170K, 80K), in order to reduce the active cooling needs in the hottest stages.
After a first phase during the spring of 2019, the joint ESA/CNES/CEA working group presented early July a new SIM/X-IFU cryochain configuration complying with these requirements, including 6 active coolers instead of 9, for a total of 15 compressors instead of 22. This new configuration, called CryoSIM and implying the switching of the two payload positions, showed no obvious design show-stopper at that stage. It was considered beneficial for microvibrations management and mechanical design, and neutral in mass with regard to the baseline.
However, in addition to some unavoidable losses in phase-A design maturity, in particular with the introduction of new 3K-JT coolers, and possible impacts on the project schedule, this configuration would have induced major changes on verification activities, logic and resources, both at Instrument and SIM level, due to the cryogenic temperature of the Dewar outer vessel in operations.
Consequently, the configuration was not considered sufficiently beneficial in terms of microvibrations to justify the added complexity in integration and testing. This solution will not be further analysed. Nevertheless, the work done by the working group has highlighted some interesting ideas for the mechanical and thermal improvement of X-IFU cryochain, which could be worth studying in a “non-CryoSIM” configuration, i.e. not implying a cryogenic temperature for the Dewar external skin.
In conclusion, for the second phase of the study, the working group was charged with studying the following ideas in the context of the SIM/X-IFU baseline:
• Accommodation of the coolers’ cold heads into separate cryostats, which would give more flexibility for microvibration management, and potentially for AIT/AIV activities;
• Implementation of a single 3K-JT coolers’ stage instead of 2K-JT + 4K-JT, which would reduce the number of kinds of coolers, the number of coolers themselves, and possibly the number of compressors. This could alleviate the development and integration processes, and reduce the cryochain complexity, at least from an industrial point of view;
• Various mechanical and thermal optimisations, aiming at removing one of the five PT15K coolers.
These different topics are currently under assessment within the X-IFU and SIM teams, with conclusions to be submitted before the Mission Formulation Review board mid-November.
CNES Project Team