Northrop Grumman MIRI Cryocooler

The 6K cooler for the JWST MIRI instrument is derived directly from the TRW ACTDP cryocooler, illustrated above (87). It utilizes a multistage pulse tube refrigerator to precool a J-T loop powered by a linear-motion Oxford-style compressor. The J-T loop provides remote cooling of the 6 K and 18 K loads and isolates the loads from any compressor-generated vibration and EMI. The multistage refrigerator is based on leveraging existing Northrop Grumman (NGAS) flight-quality pulse tube compressors and drive electronics and developmental J-T cold-end technology; these are configured and adapted to meet the specific needs of the JWST/MIRI mission requirements (99). Since the original ACTDP concept design, the actual JWST/MIRI cooler flight requirements have substantially increased due to increased thermal loads. Presently (2011), the cooling lift requirement on the MIRI cooler is on the order of 55mW at 6K plus 232 mW at 18K, for a maximum input power of 475 watts during cooldown and 400 watts during steady-state operation (100,106). This contrasts with the original ACTDP load of 20 mW cooling at 6 K plus 150 mW at 18K, for an input power <200 watts input at end of life. The majority of this load growth has come from an increased temperature of the ISIM instrument module and the piping assembly radiation load. The cryocooler compressors have remained mostly unchanged since they were selected for the ACTDP design; however, the 3-stage pulse tube has matured a great deal in terms of design and fabrication details (112). Another key area that has matured in the flight design is the drive electronics and electronic switch (111); the MIRI flight configuration utilizes a single non-redundant compressor assembly, but has switch-selectable block redundant electronics.