Superfluid Helium and its use at CERN

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Hi,

Wikipedia says CERN uses 100 gallons of superfluid helium 4 to cool its superconducting magnets. Why use superfluid helium 4 (2K) as apposed to regular liquid helium (4.2K). As far as I can see from Internet sources, the helium serves to keep the magnets in order to keep them in a superconducting state. Surely it is cheaper to keep the superconducters at 4.2K using liquid helium as apposed to 2K using superfluid helium 4? What extra advantage do you have using superfluid helium? Does the superfluid state actually help further with the superconductivity of the magnets or is it purely to do with the temperature? I.e if helium 4 was not a superfluid at 2K would that make any difference in its use with superconductors?

Also, does anyone have links to any sources of superfluid theory? I can't find much at all - only on Wikipedia which is all over the place. I only get the general gist that at the lamda temperature the helium 4 acts as a boson and all the atoms are in a single quantum state/wave function but that's about all I can find.

Thanks
 

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  • #2
Vanadium 50
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Why use superfluid helium 4 (2K) as apposed to regular liquid helium (4.2K).
There's your answer. It's colder.
 
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There's your answer. It's colder.
But if they are in a superconducting state at 4K, what is the advantage of bringing them to 2K?
 
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Vanadium 50
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The colder the wire, the more current it can carry before going normal.
 
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The colder the wire, the more current it can carry before going normal.
Thanks. So in the superconducting state although resistance is zero there is a maximum current you can send through it beyond which it won't be superconducting and the lower the temperature the bigger this can be? Is that what you mean?
 
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ZapperZ
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Thanks. So in the superconducting state although resistance is zero there is a maximum current you can send through it beyond which it won't be superconducting and the lower the temperature the bigger this can be? Is that what you mean?
There is something called the critical current density. Above this, the magnetic field generated by the current itself can cause superconductivity to quench.

http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/scbc2.html

Zz.
 
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There is something called the critical current density. Above this, the magnetic field generated by the current itself can cause superconductivity to quench.

http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/scbc2.html

Zz.
Thanks just what I was looking for. How do they contain superfluid helium 4 at CERN because it says that it leaks through microscopic pores in plugs that would ordinarily contain liquid helium? And also they flow up the edges of containers.

Would you be able to answer a related question. Whenever you hear lecturers online discuss fermions and the uncertainty principle they make reference that if electrons were not fermions then they would all collapse into the ground state and there would be no chemistry. From reading the superconductivity info on Wikipedia bosons can be in the same quantum state but they only do this when forced to do so e.g. 2 Kelvin. It says at room temperature that almost every helium atom is in a different state. Is it therefore accurate for them to make this statement? If we pretend for a second that electrons were bosons then surely they wouldn't all collapse into the ground state inside an atom or would they?
 
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it leaks through microscopic pores in plugs
Clearly you don't want any microscopic pores then!
 
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ZapperZ
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Thanks just what I was looking for. How do they contain superfluid helium 4 at CERN because it says that it leaks through microscopic pores in plugs that would ordinarily contain liquid helium? And also they flow up the edges of containers.

Would you be able to answer a related question. Whenever you hear lecturers online discuss fermions and the uncertainty principle they make reference that if electrons were not fermions then they would all collapse into the ground state and there would be no chemistry. From reading the superconductivity info on Wikipedia bosons can be in the same quantum state but they only do this when forced to do so e.g. 2 Kelvin. It says at room temperature that almost every helium atom is in a different state. Is it therefore accurate for them to make this statement? If we pretend for a second that electrons were bosons then surely they wouldn't all collapse into the ground state inside an atom or would they?
The difference between the two is that at room temperature, a gas atom CAN and is usually in its ground state, while helium is a gas!

Secondly, "ground state" energy of liquid helium is the quantum ground state of the entire fluid, not the ground state of one liquid atom. This is the BE ground state. The atom may already it is ground state at a significantly higher temperature.

Superconductivity and superfluidity are many-body phenomena. You can't simply apply the physics of individual atom to the entire conglomerate.

Zz.
 
  • #11
Henryk
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Wikipedia says CERN uses 100 gallons of superfluid helium 4 to cool its superconducting magnets
Are you sure about that info?
Superconducting magnets do use liquid helium for cooling but not superfluid and there is a very good reason for not using superfluid helium - it creeps up along the walls of containers to a region of higher temperature then evaporates. So using superfluid helium would require much larger amount of liquid helium than just plain liquid helium and the critical current of most superconductors are not that much different between 4 and 2 Kelvin.
Plus there is a danger that all the superfluid would leak through even most minute hole into the vacuum space and that would invite a serious problem.
 
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Vanadium 50
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The 100 gallons is definitely wrong - maybe they meant tons. But the helium is definitely superfluid.
 
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ZapperZ
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I just checked, yup, it is superfluid helium, the reason is higher current can be supported. The second reason, superfluid helium has very high thermal conductivity.
And the amount is not 100 gallons, it is 120 tons!!
Check the CERN publication:
http://cds.cern.ch/record/2255762/files/CERN-Brochure-2017-002-Eng.pdf
Actually, they have to use liquid helium that is a mixture of non-superfluid and superfluid. The most obvious reason for this is that you can't pump on purely superfluid helium. It will never flow because there's nothing to drag a volume of it due to it having zero viscosity. With some fraction of non-superfluid He in it, that fluid can "push" on the superfluid volume.

Zz.
 
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f95toli
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Superconducting magnets do use liquid helium for cooling but not superfluid and there is a very good reason for not using superfluid helium - it creeps up along the walls of containers to a region of higher temperature then evaporates. So using superfluid helium would require much larger amount of liquid helium than just plain liquid helium and the critical current of most superconductors are not that much different between 4 and 2 Kelvin.
it is not actually THAT difficult to deal with superfluid helium. Normal stainless steel vessels can contain it quite well, the fact that it has a potential to leak through pores etc is rarely a problem in real life.
It is also possible to design away some of the issues related to the film creeping up the wall; if all else fails one can install a "film burner" which is essentially just a heated wire some distance above the surface, this wire is held at just above 4K meaning when the film reaches the wire it heat up, goes normal and fall back into the bath. This used to be standard equipment on old pumped Helium 4 cryostats (which is why I know about it) but is rarely used these days (and I have no idea if it is used at CERN). Note that there are plenty of 2K cryocoolers on the market; this is very well understood technology.

Also, at a place like CERN they will try to get the most they can out of their equipment and the increase in critical current density between 4K and 2K can be significant. Moreover, the RF losses drop even more dramatically (it can be x10 for e.g. NbN at microwave frequencies) which would obviously be significant for the RF cavities.
 
  • #16
Henryk
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Ok, Ok,
My initial comment came from my personal experience working in low temperature lab. We did studies as a function of temperature and varied temperature by pumping on the liquid helium. It did work fine down to lambda point, below that, it was practically impossible to lower temperature any further.
I do understand why CERN keeps superconducting magnets below 2 K, the increase of the critical current is significant. Plus, flux creep must be reduced even more substantially.
I wonder why they don't switch to helium 3. Granted, way more expensive but doesn't go superfluid and easier to lower temperatures.
 
  • #17
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I wonder why they don't switch to helium 3.
You mean apart from the fact that the LHC needs 10x as much helium as the world has helium-3?

How about the fact that the heat capacity - what you want for cooling - of helium jumps at 2K because of the lambda point. It's heat capacity there is roughly twice that of water (peaking at three times) at room temperature.

but doesn't go superfluid
You persist in thinking that the CERN cryo engineers are a bunch of ignorant stumblebums who can't handle superfluidity. History suggests otherwise. This is not a problem.
 
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f95toli
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I wonder why they don't switch to helium 3. Granted, way more expensive but doesn't go superfluid and easier to lower temperatures.
Another reason is that the price of helium-3 is about $3000 per litre of GAS. Hence, 120 tons of He-3 would be something like 200-300 billion dollars.
Not that this is available; annually the world only uses a few tens of kg of He-3.
 

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