Achieving very low temperatures

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Achieving very low temperatures

Postby bjgriffin4 » Tue Jan 20, 2009 9:24 pm UTC

After a long wikipedia run, i came accross the concept of superfluidity. This got me wondering how temperatures this low (like 2 K) are achieved and maintained. Espeshially for some expieriments where it will need to be kept at this temperature for a long time.

Googling tends to only bring up vaguely mentioned methods that doesnt really explain anyhting in any detail.

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Re: Achieving very low temperatures

Postby Mat » Tue Jan 20, 2009 10:11 pm UTC

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Re: Achieving very low temperatures

Postby Rentsy » Tue Jan 20, 2009 11:42 pm UTC
That does a really good job of explaining it. U of C at B are, after all, the first to have done it.

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Re: Achieving very low temperatures

Postby danpilon54 » Wed Jan 21, 2009 2:11 am UTC

Often in experiments liquid helium is used, but that is around 4K. This of course does not explain how the liquid helium was made. Kind of a useless response... Sorry.
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Re: Achieving very low temperatures

Postby The-Rabid-Monkey » Wed Jan 21, 2009 2:24 am UTC

I'm pretty sure they also can use pressure to achieve low temperatures.
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Re: Achieving very low temperatures

Postby frezik » Wed Jan 21, 2009 3:08 am UTC

Mat wrote:Lasers!
and also evaporative cooling

You can't get down to 2K with evaporative cooling without having the surrounding air being 2K in the first place:

Wiki wrote:When the temperatures are the same, no net evaporation of water in air occurs, thus there is no cooling effect.

But it is possible to transfer energy in a way conceptually similar to Maxwell's Demon by putting work into it. In other words, you can fight entropy in a localized area as long as entropy is increased in the universe as a whole. This is what an Air Conditioner does, and you can get down to 2K with one if you're willing to put enough energy into it.
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Re: Achieving very low temperatures

Postby Vieto » Wed Jan 21, 2009 4:09 am UTC

use a very precise laser, and fire *just* enough photons to bring the atom to a near halt, lowering it's temp.

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Re: Achieving very low temperatures

Postby JWalker » Wed Jan 21, 2009 5:01 am UTC

For temperatures around 2K liquid helium is usually used, until helium becomes too expensive. (If you want to know how this is done just ask). Normally, liquid helium is ~4K, But to achieve temperatures lower than this, one way of doing it is to collect liquid helium in a pot and pump the pressure down. This causes the liquid to rapidly evaporate and results in an overall decrease in temperature. If you have ever used a propane grill or lantern, you may have noticed that as you are pulling gas from the tank, it cools down quite noticeably. This is the same phenomenon. Using this method, temperatures around 0.9K can be achieved. To go lower than this fancier methods are needed, but these sort of helium refrigerators are the workhorse of most cryogenic experiments.

Its worth noting that although the link to the University of Colorado website talks about Bose-Einstein condensation, this has nothing to do with superfluidity (which was NOT first achieved at UC at B), and typically requires lower temperatures.

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Re: Achieving very low temperatures

Postby asad137 » Wed Jan 21, 2009 6:04 am UTC

Evaporative and laser cooling only work for things like gases. If you want to cool something useful (like a chunk of some material, a detector, what have you) you have to do something else.

You can use a mechanical closed-cycle refrigerator to get to liquid helium temperatures and below (For example, I've used a pulse-tube cryocooler whose no-load base temperature is around 2.5 K)

As JWalker says, you can get to temperatures of about 1K using pumped helium-4. You can do something similar with Helium-3 and get to around 200-300mK, but want to do it in a clever way so as not to lose the helium-3 (it's very expensive).

Once you get down that low, you have some options for getting it colder. If you only need to go to ~50mK or so, you can use an adiabatic demagnetization refrigerator, which works by using the latent heat of magnetic phase transition of a special salt 'pill'.

If you want to get to ~10mK or so, you use a Helium-3/Helium-4 dilution refrigerator.

If you want to get something colder than that...well, I don't know how you'd do it.

And, of course, you can't just have something sitting in a room-temperature environment that's that cold. It has to be in vacuum, and there have to be stages at various temperatures to reduce the heat loads on subsequent colder stages. For example, the cryogenic system I'm working with right now (a liquid helium cryostat) has seven different stages (not including room temperature):

- Liquid-nitrogen-vapor-cooled shield (~200K)
- liquid nitrogen tank (77K)
- liquid-helium-vapor-cooled shield (~30K)
- liquid helium tank (4.2K)
- closed-cycle pumped He4 (~1K)
- closed-cycle pumped He3 (400mK)
- another closed-cycle pumped He3 (250 mK)


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