LS2 Report: the complex case of vacuums in the experiments
It takes a lot more than nothing to make a vacuum. The vacuum chambers of the LHC experiments, for example, are complex components, particularly those that are nestled in the heart of the detectors, which come in a variety of shapes and are made of special material. During the second long shutdown, the teams in the vacuum group are therefore hard at work replacing the beam tubes in the ALICE and CMS experiments.
ALICE will install a new inner tracking system (ITS) closer to the beam to improve the detection of short-lived particles. As a consequence, a beam tube with a smaller diameter must be installed to replace the current chamber. “We have developed a chamber 3.8 centimetres in diameter, compared to 5 before, and with a thickness of 0.8 millimetres, which is at the limit of what can be achieved with current technology,” explains Josef Sestak of the vacuum group, who is in charge of the project.
This central vacuum chamber is made of beryllium, a metal that is very light, very resistant and transparent to particles. To put it another way, it lets particles through without intercepting them, a quality essential to ensuring that the experiment can detect all the particles. However, beryllium is a very difficult metal to work with: it comes in the form of a powder that must be compressed at very high pressure to obtain a bar of metal that is then hollowed out. Only a few companies in the world can produce such components from beryllium.
ALICE’s central vacuum chamber, which is around one metre long, has just been tested and validated at CERN, following two years of development in collaboration with a company in the United States. It is now being prepared to receive a coating of non-evaporable getter (NEG), a material that is able to trap residual molecules once it is heated. “The experiments’ vacuum systems rely on this coating because conventional vacuum pumps cannot be installed near to the interaction point as they would disturb physics operations. The nearest vacuum pumps are actually placed at least 10 metres away from the interaction point,” explains Josef Sestak. A similar chamber is under development for CMS, but it is six metres long.
Aside from the central chamber, the vacuum teams are replacing all the peripheral parts of the vacuum chamber in the ALICE and CMS experiments. Stainless steel components will be replaced with aluminium parts, since aluminium displays a much lower induced radioactivity than stainless steel. Eight vacuum chambers of four different types, connected by bellows and other connecting components, must be replaced in CMS. Four spare chambers are also being produced. “Some of these components are conical, with a diameter of 200 millimetres reducing down to 45 millimetres,” explains Josef Sestak. The aluminium used is also special, with the finest grain possible. It must be machined with extreme precision in order to be almost perfectly aligned.
Once they have been validated and treated, the new vacuum chambers for ALICE and CMS will be installed in 2020.