Cooling of last sector started! (About quench training)

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Tau
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Cooling of last sector started! (About quench training)

Post by Tau » Wed Apr 28, 2021 6:20 am

Come folks, where are you?
The excitement is slowly building up.
As is clearly visible on https://op-webtools.web.cern.ch/vistar/ ... p?usr=LHC2, the last sector S67 started cooling!

By the way the sudden peaks in all sectors where the temperature goes up to about 5K are "quenches" because they're "training" the magnets.

That is a story on itself, and a little bit scary.
In short: if the magnetic field of a magnet is too strong in a certain location in the magnet (however tiny), the superconduction stops there. This means that that particular piece of the magnet heats up (because resistance produces heat). That means that the area around that point also stops being superconductive. Before you know it, there is a runaway incident where the magnet suddenly heats up, boiling the helium cooling fluid. This is called a "quench". If you were in the tunnel, you would hear a very scary bang!

Fortunately, the LHC is designed to recover from this: for example, there is a giant diode inside each magnet to divert the current so that the other magnets in the series are not influenced: all very complicated. This mechanism failed in the notorious incident in 2008 https://home.cern/news/press-release/ce ... c-incident.

But the point is, once a magnet has quenched, the limit of what field strength is can produce without quenching again is slightly higher! So in order to push the limits of the LHC, magnets are "trained" in all sectors to increase their quench limit so that the whole machine can be run at even higher energies.I presume the training of magnets will go on until they realize that the limit can be pushed no further (on until a magnet explodes again :? ).
So finally, the limit of the entire LHC will be determined by the single magnet with the lowest quench limit, and that's what they are finding out now.
- Tau

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DCWhitworth
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Re: Cooling of last sector started! (About quench training)

Post by DCWhitworth » Wed Apr 28, 2021 3:42 pm

Thanks for the info. If I recall the 2008 incident was caused by substandard welding on the electrical interconnects and also inadequate pressure release on the helium containment, which blew at least one magnet of it's mounts.
DC

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Tau
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Re: Cooling of last sector started! (About quench training)

Post by Tau » Fri Apr 30, 2021 9:19 am

DCWhitworth wrote:
Wed Apr 28, 2021 3:42 pm
Thanks for the info. If I recall the 2008 incident was caused by substandard welding on the electrical interconnects and also inadequate pressure release on the helium containment, which blew at least one magnet of it's mounts.
Indeed. They underestimated the interconnection resistance.
It doesn't sound like much if a resistance is 50 nano ohm, but at 13 kA this still dissipates 8 W, which is enough to heat the magnet above its superconduction temperature. So after the incident they redid and checked all the weldings to be only a few nano ohm.
This number keeps blowing my mind: for me, resistant is expressed in kilo ohm. Talking about a resistance that is that small is almost surrealistic (a thick massive wire of 3 mm^2 has a resistance of 6 milliohms per meter, which means that even a sliver of that wire, the thickness of a hair, still has a resistance of 300 nano ohm. Go figure :-) )
- Tau

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