The Mid Infrared Instrument aboard the James Webb Telescope
reached its final operating temperature of just 6.15 Kelvin.
That makes MIRI literally the coolest thing about the Webb telescope.
Webb's sunshield does most of the heavy thermal lifting; its five layers reject
so much heat from the Sun, Earth, and Moon, that temperatures fall from 324.
Kelvins on the sunlight side all the way down to 42 Kelvins on the telescope side.
Webb's near-infrared instruments are cooled by another 5
Kelvins thanks to a large radiator panel.
And that takes them down to their operating temperatures of around
37 Kelvins.
So NIRCam, NIRISS and NIRSpec are all cooled completely passively.
There's no need for any coolant pipes, cryostats, or compressors.
But MIRI is in a whole other league.
Its detectors have to be cooled to less than 7 Kelvin.
And that means some sort of active cooling mechanism
such as a cryostat or a cryocooler have to be used.
Now, both devices rely on the principle that when a gas expands, it cools.
You can feel this effect yourself when you use a can of compressed air;
the air inside the can is pressurized difluroethane.
This compound boils at -25 degrees centigrade.
And when you let gas escape, the pressure inside the can drops.
Some of the liquid coolant boils and expands into a gas.
As it expands, it cools and draws heat out of the rest of the can.
If you hold down the trigger long enough, the can gets very cold very quickly.
Now, cryostats and cryocoolers
typically use helium as their coolant because
it's the second-lightest gas in the universe.
The lighter the gas, the faster it expands.
The faster it expands, the faster cools.
Even better, helium has the lowest
boiling point at just 4.22 Kelvin.
That makes it an extremely efficient coolant.
And both machines flow this expanded, cooled helium to draw heat off
the science instruments and cool them down to cryogenic temperatures.
The key difference is what they do with the warmed helium gas afterward.
A cryostat allows the helium to escape.
This ensures that the remaining helium continues to boil and cool.
But just like a can of compressed air,
a cryostate only cools as long as there's a escaping gas.
Once the coolant runs out, the cryostat warms back up.
That's what happened to the Spitzer and Herschel infrared telescopes.
Had Webb gone with a cryostat,
MIRI's lifetime would have been limited by its supply of coolant.
A krill cooler, on the other hand, works much like a refrigerator.
In Mary's case, compressed helium is forced through an expansion valve.
As it escapes the valve, it rapidly expands and cools.
This type of cooling by forcing a gas
through a valve is called the Joule-Thompson effect.
You can feel this effect yourself with a teakettle or a pressure cooker.
The steam leaving the valve is very hot, and you don't want to be touching it.
But as it expands, it cools very rapidly.
And that's why you can hover your hand just a few centimeters above the valve
and not burn yourself In a Joule-Thompson cooler, the expanding
gas flows into a heat exchanger, which pulls heat off the instrument.
As it does so, the gas warms back up and flows back to the compressor.
The compressor squeezes the gas and sends it back out and the cycle repeats.
Now the act of compressing a gas also raises its temperature.
So the hotter the gas is before it cools,
the less cool it gets after it expands.
And that's why a cryocooler like this
passes the feed and return gas
through a special kind of heat exchanger called a "recuperator".
The recuperator pulls additional heat out of both lines
and cools the gas even further.
Now, even this oversimplified description
of a cryocooler is more complex than a cryostat.
The compressor means moving parts,
which are typically a no-no when designing spacecraft.
But a cryocooler is a closed system;
its coolant is reused over and over again and never runs out.
And that means, in principle, MIRI's cooler
should last the full lifetime of Webb's mission.
But MIRI's strict thermal requirements means that its cryocooler
has to be unlike anything we've ever flown before.
Its warm
and cold components have to be kept as far apart from each other as possible.
And the cooler itself has to be cooled by a precooler!
Now, I talked about the MIRI instrument along with NIRCam,
NIRISS and NIRSpec, all in a previous video.
But in that video, I mentioned the cryocooler,
but didn't really have time to nerd out on it.
Well, today we're going to nerd out on it!
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Fundamentally, MIRI's cryocooler is a closed-cycle system, like a refrigerator.
But unlike a refrigerator, the cooler's warm components
need to be as far away from the cold components as possible.
And that's why the Cryocooler Compressor Assembly is mounted in the aft
end of the spacecraft bus underneath the sunshield.
However, compressors mean moving parts and moving parts mean vibration.
Aboard Webb, any jitter from moving machinery
could shake the optics and produce blurred images.
And that's why Webb's cryocooler contains only two moving parts: the compressor
for the Joule-Thompson cooler, and a compressor for the Pulse Tube cooler...
...more on that in a moment.
Each pump contains a pair of pistons that are balanced as close
to just exactly perfect to each other.
And they move exactly opposite each other,
opposite direction, the exact same speed.
The result are a pair of pumps that self-cancel
their own vibrations and operate very, very smoothly.
Another problem is that no single cooler is capable of bringing MIRI's
temperature from 42 Kelvin all the way down to less than 7 Kelvin.
And that's why MIRI's cooler is itself
cooled by a thermoacoustic pulse tube cooler.
Now, if you haven't heard of a pulse tube cooler before,
here's my best description as to how it works.
When the pistons in the compressor move inward, the helium is compressed
and forced through a valve into the pulse tube.
There, it expands and cools, but as it reaches
the other end of the tube, it compresses again and warms up.
This sets up a standing sound wave that oscillates back and forth
as the pistons alternately compress, draw backward and compress again.
Now, that by itself doesn't actually cool anything.
But if heat can be extracted from the pulse tube
with each cycle, the gas in the tube gets colder.
So to do that, a stack of thin plates is placed
between the hot and cold gas at one end of the tube.
Heat from the hot gas is pulled into the membranes
and carried off to the radiators by a heat exchanger.
Then, as the pistons retract, the gas is being pulled out of the tube and cools.
This heat, or really the lack thereof, id then carried away
by a second heat exchanger to a cold head assembly.
With each pulse cycle, the helium gas loses heat.
Now, obviously, Webb's real pulse tube cooler
is much more sophisticated than what I've described here.
But if you really would like to learn more about thermoacoustic pulse tube coolers,
then you really need to check out a pair of videos by NighthawkInLight.
It's hands down the best description I have seen of how this thing works,
and I'll leave a link to those videos in the description of this video.
So what then is the cooler cooling?
Well, it's cooling the helium in the main Joule-Thompson cooler.
Now, to be really clear, each cooler has its own supply of helium.
So think of one batch of helium circulating through the pulse tube cooler,
which is then being used to cool the other
batch of helium cycling through the Joule-Thompson cooler.
The Joule-Thompson - or JT - cooler sends its helium through a heat exchanger.
The heat exchanger is thermally connected to the cold head of the cooler.
This helium flows from the Cryocooler Assembly located in the spacecraft bus
all the way up to the Cold Head Assembly mounted behind MIRI.
But that's something like ten meters away.
So the helium actually flows through a long line
called the Cryocooler Tower Assembly.
It's also known as the Refrigerant Line
Deployable assembly, or RLDA.
The CTA or RLDA or whatever it's called,
are actually mounted outside Web's Deployable tower Assembly.
Now, this is the main tower that separates the telescope from the sun hield.
However, all of this had to be squeezed down
so it could fit into the launch vehicle and then deployed once in space.
And that's why the coolant lines are wound like a slinky or an old telephone cord.
The slinky arrangement allows the tube to stretch as it deploys with the tower.
The Cold Head Assembly is the part that physically touches and cools MIRI.
When the helium coolant reaches the Cold Head Assembly,
it passes through the Joule-Thompson valve.
Now, the Joule-Thompson effect works off of the sudden
pressure change as the helium is forced through the opening.
As it does so, it expands and cools to just 6 Kelvins,
and then it goes through some more tubing
until it reaches the copper plate of the cold head mounted to MIRI.
The helium absorbs the heat from the cold head, returns back
through the Cryocooler Tower Assembly, and then back into the Cryo
Cooler Assembly in the spacecraft bus.
Nothing like this has ever been attempted before.
We've never flown a pre-cooler.
We've never had a cooler whose components were spread so far apart.
And if that weren't hard enough,
the Joule-Thompson cooler couldn't just be switched on all at once.
Because if done the wrong way, MIRI would actually warm back up again.
You see, the Joule-Thompson cooler has a really poor cooling ability,
or "heat lift", at temperatures of around 15 Kelvin.
Not only that, but if the pressure gets too high,
then the helium will actually warm up and that would warm up MIRI.
And that's bad.
To avoid this problem, the cooler was run in a bypass mode.
And this allowed the pre-cooled helium to bring MIRI's temperature
down to just below 15 Kelvin.
Now when the cooler transitions from the bypass mode into the Joule-Thompson mode,
it's providing the least amount of cooling power.
So this transition is referred to as the "pinch point".
If everything goes right during the pinch point, the cooler's performance
improves as the temperature gets lower.
But if the heat loads on the cooler were greater than anticipated,
then the cooler's performance dies and MIRI starts warming.
However, when the cooler was tested on the ground, they found
it was at least twice as efficient as what they needed.
So if MIRI were somehow going to be warmer when the cooler switched
to J-T mode, it would have handled it without a problem.
In any event, there was no big problem here.
They practiced this pinch point transitions
so many times on the ground, and everything went just exactly as expected.
It was just another day of non-drama with the Webb telescope.
So with all the instruments at their final operating temperatures,
the optical team finished aligning all the mirrors across
the entire field of view., and the results are amazing.
Even though these are just engineering images,
their results are spectacular and deserve their own breakdown.
So, I'll see you in that video when we are done here.
My thanks as always to my Patreon supporters for helping to keep Launch
Pad Astronomy going, and I'd like to welcome Bonnie Star Acko-Veldey,
Jake Bracey, Coda, Jack Godwin, and Yufei Wu.
Thank you so much for your support.
And if you'd like to join me on this journey through this incredible
universe of ours, including the occasional Webb update,
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Until next time, stay curious, my friend.