The PRobe far Infrared Mission for Astrophysics (PRIMA) is a mission concept for a NASA cryogenic observatory in an Earth-Sun L2 halo orbit, designed to reveal how abundant elements are built up in galaxies over cosmic time. To achieve the target sensitivity in the wavelength range of 25 to at least 200 microns, its wideband spectrometer and multiband spectrophotometric imager/polarimeter need to operate at 0.1 K, and its primary mirror and relay optics need to be at 4.5 K. To meet these cooling needs, the PRIMA study team is designing a cryogen-free cooling system with a combination of passive radiators and thermal shields, a hybrid mechanical cooler and a continuous Adiabatic Demagnetization Refrigerator system. The PRIMA mechanical cooler is slightly modified JWST Mid-Infrared Instrument (MIRI) cooler that uses 3He as the working fluid in its JT stage. This paper first discusses the overall thermal requirements and the system's thermal architecture and then describes the main thermal subsystems in the cryocooling chain. Finally, the paper presents compressor and 3He Joule-Thomson (JT) effect test results to validate the predicted performance of PRIMA’s 4.5 K mechanical cryocooler.
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