Open heart surgery at the LHC
On 17 January this year there was a race against time in the CMS cavern. In order to replace a faulty LHC component, members of the Vacuums, Surfaces and Coatings (VSC) Group, in collaboration with the CMS experiment team, had to extract and then reinsert a 2-m long section of vacuum chamber. And they had one hour to do it.
At the start of the LHC's winter technical stop, an X-ray was done to check the position of the RF fingers at Point 5. The X-ray at the top confirmed that the RF fingers (in the red circle) were not in the correct position, unlike on the lower picture.
If the vacuum is insufficient, pressure mounts and the problems start. In the LHC the ideal pressure is around 10-10 mbar. Once this threshold is exceeded, the “noise”, which means the interference generated by the residual gas present in the machine, compromises physics measurements.
In early summer 2011, a pressure a hundred times in excess of the ideal pressure was observed at the connection between two sections of the vacuum chamber only a few metres from the CMS interaction point.
Outside in
One hypothesis on which attention was particularly concentrated was the possibility of faulty radiofrequency fingers. The fingers are designed to ensure continuous electrical contact between two sections of vacuum chamber while allowing them to slide over each other as the machine contracts and expands with temperature variations. If one of the fingers is distorted and loses contact with the adjacent section of vacuum chamber, perfect electrical contact is broken and this can induce heating. There is then a risk of gas being propagated inside the vacuum chamber, causing pressure peaks, and this is precisely what was observed.
So at the start of the LHC’s winter technical stop, this section of the accelerator was X-rayed and the hypothesis was confirmed: the RF fingers, which were supposed to be around the adjacent section of vacuum chamber, had slid inside it (see the photos below).
The checks showed that there was too great a distance between the two sections of vacuum chamber. Initial studies indicate that the equipment has moved a few millimetres since installation of the LHC. It is therefore conceivable that during a movement the two sections of vacuum chamber moved so far apart that contact was virtually lost. That's when things got outside in.
We open up!
The top photo shows the RF fingers inside the adjacent section of vacuum chamber, while on the lower photograph they are outside of it as they should be.
Although the cause had been established, the problem remained. After several discussions with the CMS team, the decision was taken to take action, but this entailed removing a 2-metre-long section of vacuum chamber! Naturally without compromising the vacuum quality of the rest of the detector. So, on 17 January, in a strictly time-limited operation in view of the proximity of a relatively radioactive component, and operating on scaffolding 10 m above the floor of the CMS cavern, several sections of the VSC Group and members of the CMS collaboration joined forces to perform the repair work. This was a challenge for the team since for the first time a 2-metre section of vacuum chamber had to be removed from the very heart of a particle physics experiment!
Luckily, there was a tried-and-tested technique to hand to address the vacuum problem as it had already been used in the LHC. During the operation, neon was continuously injected into the untouched adjacent sections of vacuum chamber. Pumped into these adjacent vacuum chambers at the opposite ends to the intervention zone, the neon acts as a barrier to the ambient air as it moves out of the vacuum chamber into the cavern, thereby preventing the air from penetrating into the vacuum chamber. Unlike the ambient air, this rare gas doesn't impair the coating inside the vacuum chambers and has no adverse effect on the speed of vacuum pumping.
After only one hour, the patient could be “stitched up” again. The operation went perfectly smoothly, as confirmed by a new X-ray. Vincent Baglin, member of the VSC Group, expressed his appreciation to all those involved for their extraordinary commitment and the quality of the work achieved.
