James Webb's cooling system(Cryocooler)

The cooling device for the Mid-Infrared Instrument(MIRI), is one of the James Webb Space Telescope's four instruments. The MIRI requires a lower operating temperature than Webb's other instruments, the cryocooler accommodates this requirement.

On May 24, MIRI's cooler officially passed its pre-ship review. Its main portion, called the cryocooler compressor assembly, was shipped on May 26 to its next destination: the Northrop Grumman Aerospace Systems facility in Redondo Beach, California. There, the cooler will be united with the body of the Webb spacecraft. The MIRI instrument itself is currently at NASA's Goddard Space Flight Center in Greenbelt, Maryland, where it is part of the integrated telescope and instruments. Eventually, those components will make their way to Northrop Grumman too, where the whole observatory will come together in preparation for its momentous 2018 launch.

MIRI is a joint project of Europe and the United States, with the U.S. portion being managed by JPL. The MIRI cooler was developed by Northrop Grumman, and then later sent to JPL for testing to demonstrate its performance and verify its readiness for spaceflight. The Webb telescope mission itself is managed by NASA Goddard.

MIRI will be the coldest instrument onboard the telescope, operating at beyond-frostbite temperatures of no more than 6.7 degrees above absolute zero, or minus 448 degrees Fahrenheit. Why so cold? MIRI sees what is known as mid-infrared light, which is given off by objects at around room temperature. Desks, people, and the air we breathe, for example, are aglow with mid-infrared light that we can't see with our eyes. Specialized instruments like MIRI are designed to pick up this mid-infrared glow, but they must be chilled to avoid background infrared light that can drown out what astronomers want to see.

Cryocooler
Image Credits: NASA/JPL-Caltech

Other infrared telescopes, such as NASA's Spitzer Space Telescope and Wide-field Infrared Survey Telescope (WISE), used thermos-bottle-like coolers filled with coolants, such as liquid helium and solid hydrogen, to chill their instruments. But those systems can be large and heavier to launch. Their biggest downside is that they have finite lifetimes, warming up when their coolants run out.

MIRI started out with a design like this but was later changed to an active cooling system, which works more like a common refrigerator. The MIRI cooler, also called a cryocooler, can chill the instrument without the need for a consumable coolant.

Engineering Challenge

Being an exquisitely sensitive infrared astronomical observatory, the James Webb Space Telescope's optics and scientific instruments need to be cold to suppress infrared background "noise." Moreover, the detectors inside each scientific instrument, that convert infrared light signals into electrical signals for processing into images, need to be cold to work just right. Typically, the longer the wavelength of infrared light, the colder the detector needs to be to do this conversion while also limiting the generation of random "noise" electrons. 

Passive Cooling

Three of Webb's four scientific instruments "see" both the reddest of visible light as well as near-infrared light such as 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 sun shield.

Active Cooling

The basic principle of active cooling is to compress a gas, then let it expand a process that cools the gas. The same thing happens in refrigerators and air conditioners, which are heat pumps that move heat from a colder place to a warmer place, in reverse of what occurs naturally. A gas or “refrigerant” is compressed by a pump, then allowed to expand where you want the cooling to happen. The process of expansion absorbs heat, and the expanded gas is pumped away and its absorbed heat is dumped away by a radiator. The gas is then recycled and recompressed and the process begins anew. 

However, Webb's fourth scientific instrument, the Mid-infrared Instrument, or MIRI, "sees" mid-infrared (MIR) light at wavelengths from 5 to 28 microns. By necessity, MIRI's detectors are a different formulation (Arsenic-doped Silicon (Si: As)), which needs to be at a temperature of less than 7 kelvin to operate properly. This temperature is not possible on Webb by passive means alone, so Webb carries a "cryocooler" that is dedicated to cooling MIRI's detectors.

The MIRI instrument. MIRI operates at temperatures of no more than 6.7 degrees above absolute zero or minus 448 degrees Fahrenheit.
Credit: NASA/Chris Gunn

Cryocooler Advancements 

Webb's cryocooler has advanced the state of the art in spaceflight cryocoolers of this power and temperature class in two ways. The first one is the precooler uses three stages of pulse-tube cooling vs. heritage systems that have only two stages, and the second one is the separation between the precooler and the JT cooling hardware; typically this separation is centimeters, not several meters.

The Cryocooler Electronics during testing.
Image Credits: NASA/JPL-Caltech

Low Vibration

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 has 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. 

To avoid excess heat and vibrations affecting MIRI, the Webb telescope's designers had to place the majority of the cooler behind the telescope's massive sun shield. Webb's telescope and main instrument module are protected from the heat of the sun by a shade about as big as a tennis court. With the pumping portion of the cooler on the other side of the shield, a pair of refrigerant lines -- one feed line and one return line, each roughly one-sixteenth of an inch in diameter -- are used to connect it to MIRI. In total, the cooling system involves roughly 67 feet (20 meters) worth of thin tubing that snakes delicately throughout the observatory, carrying the recirculating helium coolant.

Life Span

Being a refrigerator and a "closed" system, the cryocooler does not consume coolants like an ice chest full of ice or a big container which is known as dewar of liquid helium does, and so its life is limited only by wear in its moving parts (the pumps) or the longevity of its electronics, all of which should last for many years.

In-Depth

The MIRI cooling system has four stages, chilling gas down successively to lower and lower temperatures. The first three stages make up the majority of the cooler and take place in the cold compressor assembly -- the largest portion of the cooler. That compressor, as well as its controlling electronics, recently passed cold and vibration tests at JPL. Engineers first fitted the compressor and their electronics into a special cold chamber and tested it, then they vibrated the compressor to mimic the effects of a rocket launch, and finally tested it once again in the cold chamber, checking out its full range of performance.

The Webb MIRI cryocooler is basically a sophisticated refrigerator with its pieces distributed throughout the observatory. The primary piece is the Cryocooler Compressor Assembly (CCA). It is a heat pump consisting of a precooler that generates about 1/4 Watt of cooling power at about 14 kelvin (using helium gas as a working fluid), and a high-efficiency pump that circulates refrigerant (also helium gas) cooled by conduction with the precooler, to MIRI. The precooler features a two-cylinder horizontally-opposed pump and cools helium gas using pulse tubes, which exchange heat with a regenerator acoustically. The high-efficiency pump is another two-cylinder horizontally-opposed piston device that circulates a different 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 than 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 the 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.

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 Credits: NASA/JPL-Caltech