Success in the pipeline for CMS

The very heart of any LHC experiment is not a pixel detector, nor a vertex locator but a beam pipe. It is the site of each collision and the boundary where the accelerator and experiment meet. As an element of complex design and manufacture the CMS beam pipe was fifteen years in the making and finally fully installed on Tuesday 10 June.

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The compensation modules were the final pieces to take their places in the cavern at Point 5: "These are like bellows," says Wolfram Zeuner, Deputy Technical Co-ordinator for CMS. "They allow us to compensate for the change in length when we heat or cool the beam pipe. And they are the very last elements; beam pipe installation, which began last year, is now complete."

End cap beam pipe installation in the CMS detector.

Central beam pipe installation.

The beam pipe is neither too fragile nor too bulky, but just right to satisfy the conflicting needs of the machine and detector. "For the machine the thicker the better because if it were to break that would halt the entire LHC," explains Patrick Lepeule, the engineer responsible for the pipe’s design and installation. "And the beam has such high energy that instability of the vacuum is a real concern." On top of this, a more solid pipe gives better electrical conductivity.

However, the detector has very different priorities: "For CMS the ideal pipe is a transparent wall that particles can pass through without interacting. The experiment wants no material, no support, and virtually nothing at the collision point! We had to work through all these requirements and in the end there was a compromise," says Patrick. Designers of the CMS beam pipe also faced the additional challenge of creating a pipe that would allow for full opening of the detector.

The compromise is a complex beam pipe made of sections of varying thickness and materials. For two metres either side of the interaction point the pipe is incredibly thin yet vacuum-tight, made from 0.8 mm of beryllium, a metal of low atomic number and weighing under 1.5kg. Beyond that for 18 metres on each side and widening towards the ends, are more solid sections of stainless steel, good for welding, assembly and precision alignment.

Happy seeing the designs come to fruition, Patrick praises the installation team: "Nothing would have been possible without the availability and the professionalism of the team."

"We all certainly had a few sleepless nights," adds Wolfram. "But beam pipe building is an art as well as a science, so it was worth it. This is a great achievement for all."

The task of a beam pipe may seem simple but it is in fact a beautifully precise piece of machinery running through the very heart of another.

(A full version of this article is available in the 9 June issue of the ‘CMS Times’).