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Seth Zenz | Imperial College London | UK

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Growing Pains

Yesterday CNN reported on a transformer failure that temporarily shut down the LHC.  This happened over this past weekend, and the issues continued into this week.  What you should know about it, from our perspective here at CERN, is that it was absolutely not a big deal.  The LHC isn’t a big magic experiment machine, it’s actually made of a huge number of components.  We expect that some of those components will have problems as the accelerator gets going, because turning on the complex is nothing like switching on a light switch: we’ve never built an LHC before.  Swapping out these components is generally pretty easy; the transformer may have weighed forty tons, but replacing it was straightforward enough and only took a day or two.  So here at the experiments, we’re a tiny bit annoyed about the delay, but we really think it’s par for the course.

Today we’ve had another delay with the machine itself, which as far as I know hasn’t made the news yet.  According to one of the status pages for LHC, there has been a magnet quench in LHC sector 34; at this moment, it reads, “Investigating quench in S34, more news as available.” A magnet quench is when the temperature of a magnet section goes too high for it to be superconducting; when that happens, the resistance goes higher, and the current going through the magnet heats it up rapidly.  This leads to further loss of superconductivity, and more heat, which could damage the (very thin) wires that ultimately make up the LHC electromagnets.  The solution, oddly enough, is to install heaters that can spread the energy of a quench more uniformly over a larger area; thus puts the magnets out of action for a number of hours but prevents damage to the accelerator.  Quenches are expected to happen pretty routinely, and to be dealt with without any permanent damage; you can learn more about them in this article from Symmetry.

You can see the effect of the quench on the sector 34 section of the LHC cooldown status page, and get additional details from this page on S34.  I have no inside information, and I’m no more of an accelerator expert than you are, so the details aren’t entirely clear to me; but it is pretty clear that around 11 AM today, the temperature in S34 went up by quite a bit, and it still isn’t recovered completely.  Past that, we don’t know yet what is going on; I expect that the accelerator experts will make more details available once they have a handle on the situation.

A problem with the accelerator is potentially more serious than a problem with a transformer.  Transformers are big and expensive, but compared with the LHC they are small potatoes and quick to replace.  Damage to the LHC magnets can be a more serious business, because replacing and repairing them involves heating a large portion of the accelerator up to room temperature, which can take months.  However, let me stress: I’m not worried.  I have absolutely no reason to believe that any maintenance is needed which would require warming the machine, and it’s far more likely that this is a minor glitch than a major one.  We still have a lot to do to prepare the detectors for data taking, and we are continuing to work.   Collisions will happen when they happen: probably not as soon as we hope, but soon enough in any case.

Update:The CERN users’ page has a brief official statement:

During the commissioning of the final LHC sector (sector 3-4) for 5 TeV operation, an incident occurred at 12:05 today resulting in a large helium leak into the tunnel. Further details are not yet known. Investigations will continue over the weekend and more information will be made available as soon as possible.

That means the incident is more complicated than I realized when I was writing, and that there will certainly be a delay of a few days because of the incident.  Past that, I’d say we’re still waiting for information.

Update 2: The Times has an article about this.  It has an unidentified “CERN source,” vanishing logbook entries, and other intrigue–so it’s definitely worth a read if you’re into that sort of thing.

Update 3 (Sept 20): The BBC is now reporting that the magnets in sector 34 will have to be warmed for repairs, which will indeed take months.  I suspect this means that the work planned for the winter shutdown, in particular preparing for the full 7 TeV on 7 TeV collisions, will begin earlier than planned, as much as possible in parallel with the repairs.  Hopefully we’ll come online and go quickly to full energy a few months into 2009 — so in the long term, this may not end up being such a large delay in the physics program.  It’s obviously a short-term disappointment, though, and a lost opportunity to calibrate our detectors with physics data prior to the 2009 run.

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