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Longer wavelengths require cooler temperatures to detect efficiently. The MIRI detects all the way out to 25 μm and so needs to be cooled to 7 K.

Webb can passively cool down to 40 K with its sun-shield, which is very good because at 40 K the black-body radiation is 3000 times weaker than at 300 K. A cryocooler is necessary to cool MIRI further.

A cryocooler is a heat pump, using energy to pump heat from a cold to hot place. The greater the temperature ratio the harder it is to both pump the heat and keep heat from conducting backwards.

The hot end is at 300 K in terms of electronics and pumps, even though passive cooling can reach much lower temperatures.

Does the helium in the compressor approach 300 K? The working fluid hot end, not the hardware temperature, should be kept cold to reduce the load on the pumps.

If so, why? Is it too difficult to passively cool the working fluid to ~100 K or so while keeping the "noisy" pumps far away from the instruments?

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    $\begingroup$ Could you provide a reference for the "hot end" of the MIRI heat pump being 300K? The MIRI control electronics may be on the "hot side" of JWST, but I believe the MIRI heat pump uses the radiator on the science package on the "back side" of the mirror which puts it on the "cold side" of JWST. However, I can't support this with references. $\endgroup$
    – Woody
    Commented Sep 1, 2022 at 18:49
  • $\begingroup$ its sun-shield, please? $\endgroup$
    – Federico
    Commented Sep 2, 2022 at 6:44
  • $\begingroup$ @Woody: If so that would mean that the mechanical pumps and control electronics are at 300K, but the working fluid never gets anywhere near 300K. $\endgroup$
    – Kevin Kostlan
    Commented Sep 3, 2022 at 0:56

2 Answers 2

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It IS passively cooled, there is no other way to get rid of heat in space (except if you evaporate some coolant, which JWST tries not to do)...

So the cryocooler has a radiator attached to it to radiate the heat away. And 300K is just how "hot" it gets. To make it radiate away the heat more efficiently, you'd have to increase the surface area which means more mass and more space used... Mass and space are limited so it's always a tradeoff that has to be made. And the engineers decided, that 300K on the hot side is the best compromise they could find...

Also, you don't want the cryocooler to dump its heat in the cold parts of the spacecraft. You want to dump heat as far away as possible from the passively cooled parts of the craft. This greatly limits the options you have to place the radiators.

Spaceflight is all about compromises and tradeoffs...

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Answer: the MIRI cryocooler CCA is located in JSWT Region 3 (the hottest part of JWST at 300K) because CCA’s two compressors and their electronics consume a lot of electricity and therefore generate heat.

Compressors also generate unwanted vibration. The JWST design placed compressors as far as possible from the ISIM “science package”, on seperate sides of extensive thermal and vibration insulation.

The CCA cools the helium coolant to 18K as well as compressing the helium.

The 18K compressed helium travels 11 meters, passing through a coil of tubing which is part of the Deployment Tower Assembly DTA in region 2 (between 110-293K).

This compressed helium is ultimately expanded in the JT refrigeration Cold Head which is located adjacent to MIRI in the ISIM instrument module in Region 1 (coldest JWST region at 40K). It is this JT expansion which provides the critical refrigeration from 40K down to 6.2K. http://ircamera.as.arizona.edu/MIRI/miricooler.pdf

enter image description here

The original JWST design used a Dewar flask of helium to cool MIRI. This was located in the ISIM adjacent to MIRI. Due to a limited mass budget, this refrigeration system would only function for 5 years, putting a hard limit on MIRI's service lifetime. The decision was made to switch the design to electrically powered refrigeration since its service life was more open-ended. However, to cope with heat and vibration the compressor and electronics needed to be located far from MIRI. A more conventional application would have located the compressor millimeters, not meters, away.

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  • $\begingroup$ Isn't JWST is limited by station-keeping fuel, anyway? $\endgroup$
    – Stack Exchange Supports Israel
    Commented Sep 2, 2022 at 14:53
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    $\begingroup$ @user253751 ...Service life from fuel use is unpredictable. For instance, the launch was very accurate, so the "fuel budget" for L2 orbital insertion was not "spent", effectively doubling JWST's service life to 20 years. It would have been embarrassing to have refrigeration Helium run out after 5 years by design. With practice, operators may get even better at designing orbital maintenance burns, stretching out service life. $\endgroup$
    – Woody
    Commented Sep 2, 2022 at 15:49
  • $\begingroup$ Oh, that's quite long. For some reason I thought it was only planned to last a few years. $\endgroup$
    – Stack Exchange Supports Israel
    Commented Sep 2, 2022 at 15:50
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    $\begingroup$ @user253751 and others that might not know what the 'dewar flask' cooling is... a flask of liquid helium provides liquid helium to locations needing cooling where the liquid absorbs heat, turns to gas which is then vented to space. Long service life would mean a very large, very heavy amount of helium. It would also mean an absolute hard limit to useable lifetime. $\endgroup$
    – BradV
    Commented Sep 2, 2022 at 19:25
  • $\begingroup$ @BradV just asked in Astronomy SE: Which (if any) space telescope would have worked longer if it hadn't simply run out of helium? $\endgroup$
    – uhoh
    Commented Sep 4, 2022 at 0:18

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