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I talk a lot about dilution refrigerators, or “DRs” on here. But okay, what the heck is a DR? They’re the workhorses of our lab, and they’re literally one of the coolest things in our universe. #physics #scicomm #coolforprogress
The internal workings of a DR are super sci-fi like. But a lot of what you see here are cables, wiring for thermometery, and test equipment. At its core, a dilution refrigerator is just that: it makes things cold.
A dilution refrigerator is a closed cycle refrigerator, which means we don’t need to fill up anything with liquid cryogens we then dump afterwards to get to base temperature. It provides continuous cooling, extracting heat constantly.
* We do need to fill the “trap” with liquid nitrogen, which helps capture contaminants from our fridge system. But that’s 
with us because liquid cryogens are pretty fun
with us because liquid cryogens are pretty fun
Dilution refrigerators rely on the mixing of Helium-3 (2 protons, 1 neutron) and Helium-4 (2 protons, 2 neutrons) isotopes. After being precooled, the mixture of isotopes interacts in such a way that, thanks to the magic of statistical mechanics, draws heat out of the system.
When the mixture is mainly 4-He, the enthalpy of the 3-He is larger than when the mixture is mainly 3-He. Energy is required to move 3-He atoms to the dilute state from the concentrated state. This energy is taken from the plate being cooled. @BlueFors_Ltd http://tinyurl.com/y6gg2nx6
We get the (rare and very expensive) 3-He from NIST through the government, and it arrives in this little tube that we have to carefully plumb into the circulation system which already contains the 4-He. Mike did the honors last December: https://twitter.com/michaelniemack/status/1083117959432863745
This all leads to the number 1 rule of DR operation: do not. vent. the mixture.
Doing so would cost us tens of thousands of dollars and it takes ~6 months of lead time to procure more, so our system would be entirely down.
Do not. Vent. The mixture.
Doing so would cost us tens of thousands of dollars and it takes ~6 months of lead time to procure more, so our system would be entirely down.
Do not. Vent. The mixture.
Each plate in the DR gets colder and colder, from top to bottom. The bottom plate is cooled using the helium mixture. We call it the “mixing chamber plate” for this reason. A shield is put on every temperature stage to limit radiative load. The final one is the vacuum shell
It's kind of like a cold onion. The temperatures of the stages and shells go like so: 300 K (room temp.), 40 K (precooled stage), 4 K (precooled stage), 1 K, and finally 100 mK (DR cooled stages). If you took away the shields from between the layers, you couldn't get to base.
The peel of the cold onion is the vacuum shell. We pull vacuum using a scroll pump, then a turbo pump which is basically a little jet engine, and then the precooling begins which further increases the vacuum. We end up with an ultra high vacuum of ~1e-6 mbar at base.
So, to cool a thing down, you bolt it to one of the stages of the DR. If you want it the coldest, you bolt it to the bottom stage. Then it’s a game of isolating things from each other you don’t want conducting heat, and connecting things together really well that you do.
So. How cold are we talking?
90,000 times colder than the lowest temperature recorded on Earth.
18,000 times colder than the nighttime temperature on Pluto.
1000 times colder than the vacuum of interstellar space.
We get down to ~0.011 K, and DRs in gen. can reach ~0.002 K.
90,000 times colder than the lowest temperature recorded on Earth.
18,000 times colder than the nighttime temperature on Pluto.
1000 times colder than the vacuum of interstellar space.
We get down to ~0.011 K, and DRs in gen. can reach ~0.002 K.
All this cooling power can be used for some pretty incredible things.
Dilution refrigerators are used for #QuantumComputing #IBMQ to help keep the qubits stable. https://twitter.com/IBMResearch/status/1174302218561970176
Dilution refrigerators are used for #QuantumComputing #IBMQ to help keep the qubits stable. https://twitter.com/IBMResearch/status/1174302218561970176
Read more about the importance of quantum computers keeping their chill using dilution refrigerators: https://medium.com/the-quantum-authority/ice-ice-baby-why-quantum-computers-have-to-be-cold-3a7f777d9728
A custom "Mack truck" of a dilution refrigerator is used in the #neutrino-less double-beta decay experiment CUORE:
https://newscenter.lbl.gov/2014/10/28/creating-the-coldest-cubic-meter-in-the-universe/
https://newscenter.lbl.gov/2014/10/28/creating-the-coldest-cubic-meter-in-the-universe/
Picking this thread back up to explain how we’re using DRs for cosmology! The reason I started this thread in the first place 
We need really specialized cameras to study the cold (2.7 K) microwave light from the cosmic microwave background. The detectors we use are superconducting and need to be cold... really cold! https://twitter.com/drbriankeating/status/1167219588720816129?s=21
Our detectors work thanks to superconductivity. A small change in their temperature (from CMB light shining on our detector arrays) causes a relatively large change in resistance, that we can then read out to determine how much light we just saw.
To cool down our detectors, we build these really large cryostats: basically big ol’ vacuum buckets with optics and detectors inside them, all coupled to the thermal stages of a DR (or other cryogenic system) https://twitter.com/simonsobs/status/1164581301011140608?s=21
These systems can then either stare right at the sky, like the @SimonsObs SATs (small aperture telescopes), or be used as cameras in bigger telescopes, like the LATR, or @CCATObservatory’s Prime-Cam or ModCam which I recently designed
Different kinds of sub-Kelvin testbeds include He-3 sorption fridges, ADRs, and DRs, which are used in various cosmology and astronomy observatories today. DRs have high cooling power and continuous cooling at low sub-K temperatures.
Our dilution refrigerators are the cold, metal heart of our testing operations and the backbone of our cryogenic cameras. Thanks to them, we're gearing up to learn more about our universe and cosmic origins. Thanks, DRs. We think you're pretty . https://twitter.com/SimonsObs/status/1121827254206717953
. https://twitter.com/SimonsObs/status/1121827254206717953
