Naked Science Forum

General Science => General Science => Topic started by: Joshua on 03/08/2008 10:18:08

Title: Why does the LHC need to operate at close to absolute zero?
Post by: Joshua on 03/08/2008 10:18:08

Joshua asked the Naked Scientists:

I read that the LHC operates at a temperature of less than 2 degrees Kelvin.

Why does it need to be so cold? Is it to eliminate noise? Does it have
something to do with superconductors? Am I missing something completely?

What do you think?

Title: Why does the LHC need to operate at close to absolute zero?
Post by: DoctorBeaver on 03/08/2008 10:49:30

I believe it's mainly for the huge magnets that are used to keep the protons on course. The magnets use enormous amounts of electricity and need to offer as little electrical resistance as possible. Keeping them at such a low temperature means there is almost no resistance.

Here is some interesting data (http://www.hep.ucl.ac.uk/undergrad-projects/3rdyear/PPguide/cool.htm) from University College London.

Title: Why does the LHC need to operate at close to absolute zero?
Post by: daveshorts on 03/08/2008 15:34:55

I would have thought tat the magnets have to be cold but not that cold - most low temperature superconductors worth their salt will work at 4.2K (liquid helium at 1atm). However if you cool them below this temperature they will stay a superconductor at higher magnetic fields so they actually run at about 1.9K to squeeze out that extra performance

The detectors are also cooled down to a similar temperature to reduce noise

Title: Why does the LHC need to operate at close to absolute zero?
Post by: DoctorBeaver on 03/08/2008 16:02:57

Dave - I only stated what was said on the Discovery Science channel. That page I posted the link to states the same:-

"LHC superconducting magnets will sit in a 1.9 K bath of superfluid helium at atmospheric pressure"

From what I have ascertained, the magnets use a lot more power than the detectors so it stands to reason that they use more supercooling fluid.

Title: Why does the LHC need to operate at close to absolute zero?
Post by: chrisdsn on 05/08/2008 05:44:08

I'm not an expert on experimental design, but I would guess the reason is
that they are producing dramatically large magnetic fields:
~8.36 Tesla which is quite close to the expected ~10 Tesla breakdown
point for this kind of super-conducting magnet. Previous magnets of the
same type (for the Tevatron) did work at ~4.2 K but only produced ~4 Tesla.

Also, I think they are specifically using helium in it's
superfluid state (which doesn't set in until ~2.2 K) because of it's
particularly friendly heat transfer properties (superfluids are weird),
so they may be cooling down to more than the magnets need just to
exploit this property.