The last piece of the ring

In a sequel to the recent success at CMS, the last section of the LHC beam vacuum system was installed in ATLAS on Monday 16 June. The place where experiment and accelerator meet, the completion of the last part of the continuous 27 km beampipe signals how near the LHC is to the circulation of its first beam.

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The last section of the LHC beampipe being installed in ATLAS.

Although a seemingly simple piece of technology in comparison with the complexity of the detectors, the beampipe is a carefully designed and essential part of both the experiments and the LHC. The section of beam pipe that completed the LHC ring on Monday 16 June is about 20 metres from the centre of ATLAS, very close to where the first magnets of the LHC are located.

Unlike the central ATLAS beampipe, which is made of beryllium, the final section is made of stainless steel. An aluminium cone surrounds the stainless steel tube with the super slim LUCID detector (which measures the luminosity delivered by the LHC to ATLAS) squeezed in between.

The installation has been a feat not only of engineering but also of co-ordination. The beam pipe is extremely fragile and can only be installed when the other, larger, elements of the experiment have been completed. "We’ve been waiting on tenterhooks really," says Ray Veness, the Vacuum Engineering Section leader. "On our side we have to wait for them to finish other parts of the experiment. It does mean that now the beam pipe is in, it really is getting very close to the end of the LHC construction."

This final section of beam pipe also completes the LHC vacuum system. The vacuum team is now making sure that the beam pipe is leak free and ready for beam, so far with no cause for concern. Creating an ultra-high vacuum in the pipe is very important to prevent protons from hitting any stray gas particles, producing potentially damaging radiation.

The Vacuum Engineering Group has also been working very closely with the surveyors to precisely position the beam pipe through the centre of the experiment. "There’s no question that the beam will go in a straight line, and we want to make absolutely sure that our beam pipe is right in the middle of it. We’ve aligned it to within 0.5mm of the nominal position with the help of surveyors and targets," explains Ray.

Hard work over many months by not just AT-VAC, but also TS-MME, TS-SU and the installation and transport teams in ATLAS has contributed to this success. For Ray and his group this milestone in ATLAS represents the work of over a decade. "I started designing this section in 1994, so 14 years ago, starting really with sketches over coffee." And at CERN, what starts with coffee and a napkin often ends with champagne and a feat of engineering.

Comparing the beam pipes in ATLAS, CMS, ALICE and LHCb

In terms of materials the beampipes in ATLAS, CMS and ALICE are similar, with the central section around the interaction point made of beryllium and the forward parts made of stainless steel and aluminium.

However, all the beampipes have different designs. ATLAS and CMS’ beampipes are symmetric around the interaction point, with the same series of detectors on each side.

In ALICE and LHCb they are asymmetric. For example, the LHCb experiment has a 13 metre long beryllium cone-shaped pipe extending in just one direction.

(See Bulletin article BUL-NA-2007-128)