Vacuum team race against time to fix LHC component

In summer 2011, an engineer on the Compact Muon Solenoid (CMS) experiment noticed a glitch in the pressure reading at a connection between two sections of the vacuum chamber, only a few metres from the CMS interaction point. The pressure was one hundred times too high.

Pressure mounts in the LHC if the vacuum in the beam pipe is too weak. The ideal pressure is around 10-10mbar. Above this threshold, “noise” – interference generated by residual gas in the machine – compromises physics measurements.

At the start of the LHC's winter technical stop, an X-ray confirmed that radiofrequency fingers at Point 5 were not connected properly. The fingers ensure a continuous electrical contact between two sections of vacuum chamber - they slide over each other as the LHC contracts and expands with temperature. If a finger is distorted and loses contact with the adjacent section of vacuum chamber, electrical contact is broken, which can induce heating. There is then a risk of gas propagation inside the vacuum chamber, which causes peaks in pressure.

Checks showed that there was too great a distance between the two sections of vacuum chamber. The equipment had moved a few millimetres since installation of the LHC, enough to lose contact between the two sections. After several discussions with the CMS, the VSC team decided on a challenging course of action: to remove a 2-metre section of the vacuum chamber from the very heart of the experiment without compromising the vacuum in adjacent sections. In a strictly time-limited operation in view of the proximity of a slightly radioactive component, and operating on scaffolding 10 metres above the floor of the CMS cavern, CMS collaboration members joined forces with several sections of the VSC group to repair the part.

The teams used a technique tried-and-tested at the LHC. They continuously injected neon into the adjacent sections of vacuum chamber at opposite ends of the intervention zone. Neon acts as a barrier to ambient air as it moves out of the vacuum chamber into the cavern, preventing any air from penetrating the chamber. Unlike ambient air, neon doesn't impair the coating on the inside of the vacuum chambers and has no adverse effect on the speed of vacuum pumping.

Replacing the part took one hour, and a new X-ray confirmed the success of the operation. Vincent Baglin of the VSC group expressed his appreciation to all involved for the quality of their work and their extraordinary commitment.

At the start of the LHC's winter technical stop, an X-ray of the RF fingers at Point 5 confirmed they improperly connected.