On 10 June the CMS collaboration reached another major milestone when the heart of the detector, the beam pipe, was fully installed after 15 years of complex design and manufacture. This fragile, 44 m long component is one of the last elements of the CMS experiment to be installed.
The design of the beam pipe required compromising on numerous needs of the experiment, with the physicists calling for no material, no support and virtually nothing at the collision point, while the engineers wanted a thicker pipe for greater stability of the vacuum and better electrical conductivity. The compromise is a complex beam pipe made of changing thickness and materials. For 2 m on either side of the interaction point the pipe is of 0.8 mm thick beryllium, weighing less than 1.5 kg. Beyond that for 18 m on either side, and widening towards the ends, are sections of stainless steel, which is good for welding, assembly and precision alignment.
It is very important for both the LHC machine and the detector to have a good vacuum, and a recent “bake-out” should have cleaned out stray particles to ensure that this happens. During this process the beam pipe is heated to 200–250 °C for 48 hours. The length of the pipe is coated with non-evaporable getter material, made of titanium, zirconium and vanadium, which acts as a pump, constantly absorbing residual particles even at the interaction point where no pump would fit.
