The cooling device for the Mid-Infrared Instrument, or MIRI, one of the James Webb Space Telescope's four instruments. The MIRI requires a lower operating temperature than Webb's other instruments, the cryocooler accomodates this requirement. Image: NASA/JPL-Caltech
Being an exquisitely sensitive infrared astronomical observatory, the James Webb Space Telescope's optics and scientific instruments need to be cold to suppress infraredbackground "noise." Moreover, the detectors inside each scientificinstrument, that convert infrared light signals into electrical signals forprocessing into images, need to be cold to work just right. Typically, the longerthe wavelength of infrared light, the colder the detector needs to be todo this conversion while also limiting the generation of random "noise"electrons.
Three of Webb's four scientific instruments "see" both the reddest of visible light as well as near-infrared light (light with wavelengths from 0.6 microns to 5 microns). These instruments have detectors formulated with Mercury-Cadmium-Telluride (HgCdTe), which work ideally for Webb at 37 kelvin. We can get them this cold in space "passively," simply by virtue of Webb's design, which includes a tennis court-sized sunshield.
However, Webb's fourth scientific instrument, theMid-infrared Instrument, or MIRI, "sees" mid-infrared (MIR) light atwavelengths from 5 to 28 microns. By necessity MIRI's detectors are adifferent formulation (Arsenic-doped Silicon (Si:As)), which need to be at atemperature of less than 7 kelvin to operate properly. This temperatureis not possible on Webb by passive means alone, so Webb carries a"cryocooler" that is dedicated to cooling MIRI's detectors.
This image shows the cooling device for the Mid-Infrared Instrument, or MIRI, one of the James Webb Space Telescope's four instruments. This photo was taken after the cryocooler had completed testing, and was taken out of the test chamber in preparation for being placed into its shipping container. Image: NASA/JPL-Caltech
The MIRI instrument. MIRI's operates at temperatures of no more than 6.7 degrees above absolute zero, or minus 448 degrees Fahrenheit. Credit: NASA/Chris Gunn
The Cryocooler Electronics during testing. Image: NASA/JPL-Caltech
Webb's cryocooler has advanced the state of the art in spaceflight cryocoolers of this power and temperature class in two ways:
Moreover, one of the cryocooler's most challenging requirements is low-vibration. Vibration levels need to be very low to preclude jitter (induced shaking) of the optics and resultant blurred images. The pulse tube cooling in the precooler in the CCA and the Joule-Thomson effect cooling in the CHA have no moving parts. The only moving parts in the cryocooler are the two 2-cylinder horizontally opposed piston pumps in the CCA, and by having horizontally-opposed pistons that are finely balanced and tuned and move in virtually perfect opposition, vibration is mostly cancelled-out and thus minimized.
For additional information see the feature article on MIRI and the cryocooler on NASA.gov.
The Cryocooler Compressor Assembly. This photo shows the flight cryocooler installed "upside-down" in a vacuum chamber for testing, before the chamber was closed. Image: NASA/JPL-Caltech
The Webb MIRI cryocooler is basically a sophisticated refrigerator withits pieces distributed throughout the observatory. The primary piece isthe Cryocooler Compressor Assembly (CCA). It is a heat pump consisting ofa precooler that generates about 1/4 Watt of cooling power at about 14kelvin (using helium gas as a working fluid), and a high-efficiency pump thatcirculates refrigerant (also helium gas) cooled by conduction with theprecooler, to MIRI. The precooler features a two-cylinderhorizontally-opposed pump and cools helium gas using pulse tubes, which exchange heat witha regenerator acoustically. The high-efficiency pump is anothertwo-cylinder horizontally-opposed piston device that circulates adifferent batch of helium gas separate from the precooler's helium.
The CCA is located in the heart of the spacecraft bus, on the sun-facing "warm" side of the observatory, and it precools and pumps cold helium gas through plumbing to MIRI, which is roughly 10 meters away in the integrated science instrument module (ISIM). The CCA is controlled by the Cryocooler Control Electronics Assembly (CCEA), which is a collection of electronics boxes mounted in the spacecraft bus inside the port-side equipment panel. The CCA is connected to the ISIM via the Cryocooler Tower Assembly (CTA), which is a pair of gold-plated stainless steel tubes (a feed line and a return line), each about 2 millimeters in diameter, held every foot or so by a series of delicate suspension assemblies (called Refrigerant Line Supports, or RLSs), mounted to the outside of the observatory structure. The CTA connects to the final piece of the cryocooler called the Cryocooler ColdHead Assembly (CHA), which resides in the ISIM. Within the CHA plumbing, inside a gold-plated cylinder roughly the size and shape of a large coffee can, is a small (less than 1 millimeter) orifice that the cooled helium refrigerant passes through, resulting in expansion and final cooling of the helium gas down to about 6 kelvin, care of the Joule-Thomson (JT) effect. This coldest of refrigerated helium gas passes through more 2 millimeter tubing to a palm-sized copper block fastened to the backside of the MIRI detectors. This is where the target heat is exchanged, resulting in cooling of MIRI's detectors to nominally around 6.2 kelvin. The CHA also contains valves that allow helium to bypass the JT restriction when the observatory and MIRI are in cooldown mode (such as shortly after launch during deployment and commissioning). The CCA, CTA and CHA tubing are connected together with pairs of 7/16 inch fittings that on the outside resemble automotive hydraulic brake line connections.