It's not just engineering, it's art - the James Webb Space Telescope's infrared detector cooling system

The James Webb Space Telescope in front of the galaxy cluster SMACS 0723, the first galaxy field revealed by the telescope. Early results from the James Webb Space Telescope have revealed surprisingly large early galaxies, challenging current cosmic models. Credit: NASA, ESA, CSA, STScI
James Webb Space Telescope. Credit: NASA, ESA, CSA, STScI

Last July the world celebrated with great excitement two years since the first image taken by the James Webb Space Telescope (James Webb Space Telescope – JWST) [1]. In honor of the event, 4 whole and super interesting chapters were dedicated to this topic on the Tel Aviv University podcast channel, and among other things were also published Here on the site. In each episode a different researcher appeared who told how with the help of JWST his field of research is expected to receive a serious promotion. It is about a variety of topics - the search for life outside the earth, a deeper understanding of dark matter, the study of neutron stars, black holes and more.

The raw information reaches the researchers in the following way: the radiation is absorbed by the main mirror of the telescope (the largest mirror ever sent into space, 6.5 meters in diameter, in the figure below you can see the comparison with the mirror of the Hubble telescope and a human figure) and is then transmitted to four detectors that translate it into images in the light spectrum Infrared in different wavelengths.  

Role of each detector based on measured wavelength, from [1]
Comparing the size of the main mirrors of the Hubble Space Telescope - on the left and the Webb Space Telescope and both with the size of a person. From the James Webb Space Telescope website

The Hubble Space Telescope after completing functional tests, from [17]

Each detector is designed according to the role given to it:

  • NIRCam detector  - An infrared camera whose job is to pick up light from distant stars and galaxies, from the beginning of the universe [3]. 
  • NIRSpec detector - An infrared camera that works in the same frequency range as NIRCam, where the uniqueness of NIRSpec is the ability to scan up to 100 objects simultaneously [4].
  • MIRI - An infrared detector that works in a different wavelength range than NIRSpec and NIRCam, with which smaller objects, such as comets and extrasolar planets, are searched for and studied [5]. 
  • FGS-NIRISS - An infrared camera, whose role is operational, through which the trajectory of the telescope is controlled [6].  

In the figure below you can see a technical summary of the work areas of the detectors and their detection tasks.

The scheme of the four-level cryogenic refrigerator, from [9]

The nature of the work of an infrared detector requires deep cooling (cryogenic) or in other words to get a normal image without noise, the detector needs to be constantly at very low temperatures [2]. Depending on the wavelengths of the radiation that each detector is supposed to "see", its working temperature is determined and it can range from a few degrees above absolute zero (-273.15⁰C) to several tens of degrees above absolute zero.

As we understand, cryogenic cooling of the detectors, especially in space, becomes a difficult engineering problem. So for the three detectors NIRSpec, NIRCam and FGS-NIRISS, the business is relatively simple: their working temperature is -235⁰C and they are cooled passively (by attachment to the surface). Altogether in a very cold space and by different methods of heat management the engineers were able to plan a cycle A thermal that stabilizes the temperature of these detectors at the required value.

As for the fourth detector, MIRI (as for the name, it could certainly be that an Israeli engineer worked there who wanted to make a present for his wife...) the matter is more complicated. A detector that scans a wide spectrum of frequencies must be cooled to a temperature of only 6K, -267⁰C. Such a cold temperature cannot be obtained by passive means only (passive means in the end it is attached to a colder surface in some way) and therefore the engineers had to design a dedicated refrigerator for this task.

Well, This work will review the detector cryogenic refrigerator MIRI, its main parts and we will also touch briefly on the tests they performed on the cryogenic system before launch.

OK, so how do you organize the cooling of something, in our case a camera that weighs a few tens of grams, from room temperature, about 25⁰C to a temperature of -267⁰C (a little lower than the temperature of an industrial MRI machine), only that the camera is 1.5 million kilometers above the earth and should work continuously At least 10 years without the ability to repair and maintain?

The structure of the refrigerator

After the characterization of the MIRI detector was completed in the late 90s, the engineers began to think about the dedicated refrigerator for the mission. First, the leading cooling concept was a solid hydrogen reservoir that was suspended about a meter above the optical assemblies of MIRI that actually require the cold temperature, then by a process of mirroring (sublimation, a process in which a substance in a solid phase passes into the gaseous phase without passing through the liquid phase on the way), the gaseous hydrogen was obtained which was flows in the pipe connecting the reservoir to the MIRI and that is how the cooling was obtained. This concept was developed until about 2005 and then they realized that it causes serious weight problems. Therefore, it was decided to move to the development of another cooling concept - a mechanical refrigerator, a concept that was initially abandoned due to the claims of technological immaturity of the leading components [7]. 

If you are asking yourself "What, it took them 5 years (!) to realize that the refrigerator they are planning is too heavy?", then you are absolutely right. A team of engineers who wrote about the first years in the development of the refrigerator [7], with impressive honesty admits this and enumerates a host of reasons why at least most of them were not technical, but organizational and managerial. So yes, it is nice to read that delays in important projects due to bureaucracy and politics do not only happen in Israel.

The development of the mechanical refrigerator, which at that time was already at a medium maturity level (they did not have to develop everything from scratch) continued fully until the successful demonstration in 2008 [8]. what fun

Well, the refrigerator for the MIRI detector is a refrigerator with helium cooling gas, which has four levels of cooling, each level lowers the temperature to a lower value, with the first three levels of cooling achieved using a cooling method Pulse Tube And the last degree of cooling, the one that lowers the helium temperature from 18K to 6K is of the type Joule-Thomson (JT). A scheme of the refrigerator is shown in the following figure.

The cryogenic refrigerator before being mounted on the satellite, from a NASA website, credit NASA/JPL-Caltech [10]
The cryogenic refrigerator before being mounted on the satellite, from the NASA website, credit NASA/JPL-Caltech
[10]

Before we dive in and go into detail about the mentioned methods, let's dig a little deeper about the presented agreement. So what we have here: Helium gas comes out of the JT compressor when it is at room temperature, that is relatively hot and passes through heat exchangers until it cools down to 18K. Finally, after passing through the JT component, which we will soon explain what it is, it reaches the target temperature, 6 degrees Kelvin, cools the detector and returns to the compressor, where it is compressed and returns to the cold.

Both compressors, also of Pulse Tube and also of Joule-Thomson They each receive their electricity from a separate electronic system, where the electronic system is also responsible for controlling and collecting real-time data from the refrigerator.

We will now briefly tell about the cooling methods that are applied here.

Joule-Thomson

This method is named after the two great researchers James Prescott Joule and William Thomson (Lord Kelvin) who discovered together in 1852 an interesting effect: when a compressed gas expands to a lower pressure, it cools [11]. How much the gas cools, whether it is a few degrees or tens of degrees, depends on several parameters, such as the type of gas and the pressure difference.

What is it, that simple? Yes, that simple.

Now we understand that the strange "JT" component in the refrigerator schematic is simply a nozzle, a narrowing in a tube or a hole. In the figure below we see a schematic description of such a device, called in the professional language "converging-diverging nozzle". Such a device is better to use, we will get a drop in temperature and the gas velocity will be maximum. 

Section of a standard nostril, from [12]
Section of a standard nostril, from [12]

Because of the simplicity and high reliability, cryogenic cooling systems based on the Joule Thomson effect are applied in quite a few technological fields, the main one being the cooling of infrared detectors in tactical missiles [13].

Let's also note that in order to cool, there is no need for moving parts (compared to, for example, the air conditioners in our homes) that create vibrations and shocks, therefore this method is often preferred in systems with delicate optics such as the space telescope.

Also, we now understand why the refrigerator engineers used helium gas: it is the only substance in the universe that is in the gaseous phase in such a wide range of temperatures - helium is a gas at room temperature and it only becomes a solid at a temperature of about 1.5K above absolute zero, so it is easy to flow him.

At the same time, the method also has disadvantages, the main one being - the cooling effect is relatively weak, in addition to the uniqueness of helium - if it spreads at room temperature, it will actually heat up. In order to get the desired cooling effect at the end, the helium in the Joule-Thomson cooling line must be cooled to a sufficiently low temperature. That's why another cooling cycle is integrated here, Pulse Tube.

Pulse Tube

In this cooling system developed in the 80s of the last century [14], the properties of helium are utilized as a real gas in a cycle of compression and expansion under different conditions that allow for gradual cooling of the gas. This method is more efficient than the previous method, so they decided to use it for the pre-cooling of the Joule Thomson stage. Using several levels of cooling allows working at lower pressures, and so even though the splitting of the refrigerator into the different levels lowers the overall reliability of the system, each level is relatively simple and therefore reliable.  

To conclude this chapter, we will say that gradual cooling is the accepted method for obtaining such a deep cooling. When we read on the science and technology news sites "scientists managed to cool and reach a temperature of a billion degrees Kelvin by reversing the rotation of the electron", then we will remember that before the games with the electrons the scientists used the cooling methods described above to get down to the temperatures where these games with electrons begin to manifest.

The refrigerator tests before launch  

As mentioned, one of the most difficult requirements for any assembly in the James Webb Space Telescope is reliability and lack of maintenance. Everything has to work perfectly, from the first time and throughout the life of the product. If you think this is a purely imaginary problem, then we will remember James Webb's father, the Hubble Space Telescope.

The Hubble, a technological marvel in its own right, was semi-disabled for its first three years due to a small manufacturing defect in its main mirror [15]. It took quite a bit of engineering brilliance as well as a lot of luck (for example, the structure on which the mirror was placed during its tests was not disassembled and raised dust in the warehouse of the company that produced it. This greatly helped the engineers to understand what exactly happened there) to solve the problem from a design point of view, and then a special space mission was launched for Apply the solution to the telescope itself.

This whole story cost 50 million dollars [16], although if you ask me, if they had turned to my handyman, Niv, then he would have fixed them just as well for 400, a roof of 600 shekels, as long as it was in cash (who was a house committee understand what I'm talking about)…

Okay, so we get it, James Webb has to be right from the start. We will also remember that compared to the Hubble which was launched to an altitude of only about 500 kilometers, JWST was launched to an altitude of 1.5 million kilometers. Getting there if necessary will be much more complex to the point of being impossible.

Well, how do we test a space product? Basically, the test phases are divided into two main phases in the life of the product: the launch phase and the space work phase.

The launch phase, is a phase where usually the product should not work, it just has to survive, as this phase is hard and brutal, accompanied by strong vibrations, shocks and acoustic noise. The characterization of the power of the launch is usually provided by the manufacturer of the launcher: for example, when you want to be launched by the Space X launcher, you receive from Elon Musk a thick book with all the definitions under which conditions the satellite must withstand and not disintegrate during launch. Accordingly, the MIRI refrigerator was also tested and confirmed to meet the launch conditions.

The phase of working in space is a completely different phase. At this stage the satellite, of course, needs to work, on the other hand there are no vibrations and blows, life is relatively peaceful and calm. The only expected environmental change is only in temperature - the satellite rotates, changes its position relative to the sun and therefore its temperature changes.

We will return to the MIRI cryogenic refrigerator and James Webb. The refrigerator first underwent various tests only as a refrigerator [7] and then was mounted on the satellite. Finally, in 2017, the concluding experiment took place [17], in which the complete satellite stayed for 100 days in a vacuum chamber, a huge facility that is able to simulate the unruly conditions in space: very low temperature and complete lack of gravity (deep vacuum).

Out of the 100 days, about 35 days the satellite was exposed to full operational conditions, during which comprehensive tests were carried out for all its systems and detectors during their work, including the cryogenic refrigerator. In the figure below, the image of the satellite appears after the end of the experiment, you can definitely get an impression of the size of the facility.

The sharp-eyed will recognize the satisfied smiles of the staff..

The James Webb Space Telescope after completing functional tests, from [17]

Hope it turned out well and you enjoyed it.

Like all my articles, this article is also dedicated to our beloved daughter, the late Michal.

miss you

Sources

More of the topic in Hayadan:

6 תגובות

  1. It is easier to understand the JT effect from everyday experience - releasing air under pressure from a bicycle wheel for example

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