AIRS Cryogenic and Cooling Systems
The focal plane assembly operates at 58 K for high sensitivity and is packaged in a permanent vacuum dewar which mates directly to the 155 K grating spectrometer. This complex thermal mechanical arrangement in combination with the large number of leads placed great demands on the dewar design. The dewar outer shell is constructed primarily of stainless steel and uses a glass inner bore to accommodate the 58 K/155 K thermal transition. Low loss heat transfer from the focal plane to the pulse tube cryocooler is provided through a single crystal sapphire rod connected to the dewar end well. Attachment of the rod to the cryocooler uses a flexible coupling.
Low vibration, long life focal plane operation near 58 K is critical to the success of AIRS, and the rapid advance of pulse tube cryocoolers has proven to be a key enabling technology in this area. The AIRS focal plane cryocooler, developed under contract with TRW (now NGST), is a fully redundant pulse tube refrigerator with each redundant assembly consisting of an actively balanced compressor, separate pulse tube coldhead, and independent control electronics.
The pulse tube cryocooler is a variation of the Stirling cycle refrigerator, where the moving displacer in the traditional coldhead assembly is replaced by a passive combination orifice and gas reservoir to bring about the proper phase relationship between pressure and mass flow rates. The result is an accessible cold spot for FPA cooling without the penalties of coldhead vibration, electromagnetic interference, and displacer reliability inherent in the standard approach. AIRS was the first NASA instrument to use pulse tube coolers.
Radiators and Earth Shield Assembly
The AIRS radiator assembly (not shown) is a two-stage flat panel design. The first stage is an open-back beryllium structure with a painted black aluminum hexel radiating surface. It extends around the spectrometer and operates in the 171-190 K temperature range. The second stage radiator provides extended cooling to 145-160K. The second stage radiator is an aluminum honeycomb and face sheet structure with a black painted aluminum hexel radiating surface.
The AIRS Earth Shield Assembly provides shielding of the cold radiator surfaces to earth radiation. The basic design is a honeycomb base with aluminum face sheets and core. The side panels are beryllium and fold inward toward the middle in the closed position. The door was activated once after launch and will remain open throughout the mission.
Decontamination heaters on the optical bench provide enough energy to the spectrometer to outgas condensed water in the event icing occurs after opening the Earth Shield.