LHC portal

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.

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.

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 )

Sector 78 has been warmed back up to allow for a damaged dipole to be repaired/replaced. Luckily the machine schedule is so far ahead of the experiments that at worst it is an O(weeks) delay to the autumn beam tests.

Oh boy, they’re starting again! Magnet training will take until the end of this year:
https://lhc-commissioning.web.cern.ch/ ... g-term.htm

Need to speed up to save the world.

Sector 23 also had to be warmed up for a repair (diode replacement). S23 and 78 will be cooled back down over August. The 2-week beam test is scheduled for 18th October

While at least 3 sectors are fully trained to 7 TeV, the target beam energy from the training campaign will be 6.8 TeV. S23 and S78 are just about there (and warm now anyway). S56 is maybe a dozen quenches away.

The champagne bottle under summer student temple shall remain corked (if it's still there...)

I can't wait for the fun to happen again! Thanks to, among other things:

• Better beam quality thanks to Linac4
• Higher energy thanks to quench training
• Experiments with improved tooling

BTW does anyone know if it is possible to read the elogs? They appear to be blocked, and I lost my account (because certain people started bugging the operators with direct mail messages, we all got kicked out ).

I'm here!!! I am watching quietly from the sidelines!

And now, a few month after I thought it did, the final sectors are cooling down.
Now you can see it on the Vistars page at https://op-webtools.web.cern.ch/vistar/ ... p?usr=LHC2
Sectors 23 and 78 were above 300K (until last week, but now you can see that the cooling has begun. When I checked (Aug 9), they were 260K and 299K respectively. Although 299K still is room temperature, it is noticeably cooler than the 302 and hotter it has been for the last few weeks.
On this same page you can also see the quench training is still going on, giving frighening temperature peeks every few hours.

But of course you know that, since you are still coming to this forum while nobody is there: only the hard core is left.