Filling the vacuum at LHCb

Last month, the Vacuum, Surfaces and Coatings (VSC) group was tasked with an unusually delicate operation in the LHCb experiment cavern: removing the LHC beam pipe while keeping the sensitive Vertex Locator vacuum vessel (VELO) completely isolated from the action.

The VSC group seal off the VELO beam pipe with a flange. Image: Gloria Corti.

LHCb’s VELO detector is one of the crown jewels of the experiment. With detector elements surrounded by a vacuum, it gets as close as 5 cm from the beam. Fantastic for physics, but difficult for all-important access. “Because of the sensitivity of the VELO detector and its proximity to the beam, the collaboration decided not to bake (see box) its portion of the beam pipe,” says Giulia Lanza (TE-VSC-LBV), the expert in charge of the beam vacuum operation. “Our group was therefore asked to remove the rest of the LHC beam pipe while keeping the VELO portion of the pipe completely isolated. This work was carried out by members of VSC-LBV and VSC-EIV, under the supervision of Cedric Garion.” By removing the beam pipe, LHCb engineers have more space to work on LS1 activities. It also gave the VSC group an opportunity to replace one of the vacuum chambers – which, although leak tight, does not conform to design specifications – and to replace the supports of the vacuum chambers.

To remove the beam pipe, the VSC team used a special procedure in which neon is flushed into the beam pipe. The gas allows the team to divide the beam pipe into two parts, minimising air exposure. Also, as neon is a noble gas, it can be left in the pipe without affecting its special coating (see box). Once pressurised with neon gas, the VELO is simply sealed off with a flange.

But with the beam pipe separated from the VELO detectors by just a simple sheet of thin aluminium foil, even the slightest change in pressure can cause serious damage. “If there is a pressure imbalance between the beam pipe and the VELO detector vacuum, the thin foil can break,” says Giulia. “To avoid this, we kept the pressure difference as low as 4 mbar and flushed neon into both the beam pipe and the vacuum chamber of the VELO detectors, maintaining the balance of pressure on both sides.“ Because of the VELO’s unique construction, the VSC team had a VELO expert, Eddy Jans from NIKHEF, on hand to ensure the process went smoothly.

The neon-filled VELO will remain in place for the rest of the long shutdown. “We will keep it at atmospheric pressure while work continues nearby,” concludes Giulia. “However, we will bring it back under vacuum as soon as we can, as this will be better for the integrity of the beam pipe once the machine is back in operation.”

by Katarina Anthony