Lyn Evans, the LHC Project Leader, gave a public talk on the status of the LHC last Wednesday. Although there are no slides covering the accident on September 19 and its aftermath, he actually spoke at some length on the subject during the talk. I’ve just learned about the talk today — slower, I admit, than some other bloggers — and it turns out that the video from the talk is part of CERN’s public records. He speaks starting about the incident at about the 9 minute mark:
I think these remarks are some of the most in-depth information on what happened, and how long it will take to fix it, that are available publicly from CERN. I certainly learned a lot from them, so I thought they would be worth pointing out to all of you. The short version is that the accident is still under investigation, and some magnets may require replacement or repair, but Dr. Evans is confident that we will be running again in the spring of next year.
For those who are interested in reading rather than listening — the sound isn’t always so good — I have transcribed his remarks “below the fold.” I have done my best to convey his informal remarks accurately, but there may be mistakes, which I apologize in advance for. I stress this very strongly: if you are interested in quoting Dr. Evans for whatever reason, please go back to the official CERN record (i.e. the video of the talk linked above) rather than relying on my transcription.
Dr. Evans said:
But while we were waiting for the cryogenics to recover [after the transformer failure], we were commissioning the very last sector, the sector 34. We’d already taken seven sectors up to 5.5 TeV, which we considered to be good for 5 TeV operation. And on the last sector, I think you know very well, we were ramping between 5 and 5.5 TeV, and we had a fault. The first manifestation was a massive quench, but the massive quench was quite normal, because the incident, which I will explain in a moment, provoked the quench protection system to fire the quench heaters in about a hundred magnets, and they all quenched and it behaved perfectly, I think it was a perfect test of our quench protection — if unsolicited, a perfect test of our quench protection system. And there there is no problem at all. I think the fact that the system behaved as it should and the magnets quenched, that would not have been a problem. The problem is the incident that caused it all was an electrical fault, which caused a penetration of the helium vessel, so helium went from the liquid into the vacuum of the cryostat, and from there through the quench relief valves into the tunnel. And we’ve now analyzed all the signals, and we know to within one magnet where the fault was. It’s got to be a big fault. I think at this level of current, there shouldn’t be any problem at all. There’s got to be a big electrical fault. We think that it’s in the busbar, in the busbar splice, but we cannot be sure until we open up and examine. So the consequence was the discharge of helium from the magnet into the insulation vacuum, and from there into the tunnel, which is where the big discharge of helium into the tunnel comes from. Now we are warming up the sector, I think the magnet which is at fault, or the busbar of that magnet which is at fault of course, we will open it up and examine it. There is collateral damage for sure, I think that high presure into the — like that, is certainly provoked some other damage which we will have to fix. Warming up will take at least three weeks, and then of course the repair we don’t know how long it will take, but I think it doesn’t need a rocket scientist to realize that this will take us into the winter shutdown, I mean into December, and one thing we cannot do is to run through the winter, I think, for many reasons. For basically as the DG [Director General of CERN] announced yesterday, we are certainly down until early Spring. So what we are doing now, apart from gearing up to make whatever repair is necessary, is to try to readjust the schedule of the maintenance of the machine, not the LHC but the injector chain, in order to get them up as early as possible next year, because up until this accident, we would only have come up in June of next year to start the LHC. So we want to come up as early as we possibly can, and this has now got to be reorganized, that we shut down the injectors, we perform the maintenance that we would have performed from January to March, we bring it much forward. So I think that’s about all I can say for now. I’m certainly willing to answer questions.
[…]
Question #1: So I have an obvious one. Do you plan to check whether this could happen in other sectors?
Evans: Well, all other sectors have been taken up to at least 5.5, in fact higher than that, so we have no problem at all running at 5 TeV. Now in this sector, this is a very hostile sector. This is the one underneath the Jura [mountains] where the conditions are not good, and if we find that it’s the busbar splice, then we will open up every single interconnect and [draw?] them. But for the other sectors, for the moment, there is no reason to do anything, we will keep them cold, so we can start up as smoothly as possible.
Question #2: Do you have an idea how many magnets could have been affected?
Evans: We have three short straight sections, quadrupoles, and certainly a few dipoles affected, which have to be changed. There may be, because of the high pressure when the break happened, there may be super insulation on other dipoles which have to be changed, which means they have to be taken to the surface, de-cryostated, and re-cryostated again. I think we have no worry at all about not having enough spares in order to make the repair. How many we will have to do, we don’t know yet.
[…]
Question #3: Is the damage restricted to the machine? What is the state of the tunnel?
Evans: Well, we have been up to now, this happened on Friday, so since Friday we have been very carefully going through various steps. Because we lost the electricity, we’ve reestablished that. Obviously, making things safe is the first priority. The civil engineers went in yesterday, and I think there is no visible damage to the tunnel at all, as far as I know today, I’m sure there isn’t.
Question #4: How big is the amount of helium that has escaped?
Evans: The estimate is between one and two tons, of the 15 tons in the sector. But getting more helium is not our biggest problem.
Question #5: So you mentioned trying to advance the maintenance schedule for the injector chain, is that likely to have consequences for the running of test beams and such things?
Evans: Well, I think Jos [Engelen, CERN’s Chief Scientific Officer] should answer that. I mean one thing you can imagine, the first thing I have to do is get the morale back up again, because the last sector, in fact the last circuit to be tested on the last sector, to get to 5 TeV, this was a kick in the teeth. I think Jos can maybe comment on the other question.
Dr. Engelen: A schedule is being worked out by AB at the moment. It would seem that the beams to the north area would have to be suppressed earlier than forseen, and there should be a possibility of keeping another beam going, but it is too early to say.
[…]
Question #6: After the original shutdown, there were 7 TeV collisions scheduled. So will the 5 TeV collisions be taking place?
Evans: I think this has now got to be discussed between the machine directorate and the experiments, how we start up next year. Obviously 5 TeV would be much faster than starting up at 7, but we will see.
