The precise nature of CERNs planned 50-km particle beam
The next generation of particle colliders will demand unprecedented precision in the alignment of their components. EU-funded researchers have pushed the limits with new techniques for the accurate positioning of scientific equipment.
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One of the proposed successors to the Large Hadron Collider (LHC) at CERN, the European Laboratory for Particle Physics near Geneva, is the Compact Linear Collider (CLIC) that will study collisions between beams of electrons and positrons accelerated towards each other through a tunnel 50 kilometres long.
“Over that length there will be hundreds of thousands of components,” says CERN’s Hélène Mainaud Durand, who is coordinating the EU-funded PACMAN project. “The challenge is to pre-align them on the tunnel floor with sufficient accuracy that a first pilot beam can be sent through.”
PACMAN has been developing technologies for aligning the components to a precision not seen before. Such is the complexity of the undertaking that it has generated training for 10 PhD students funded by the EU’s Marie Skłodowska-Curie fellowship programme.
The components include devices for accelerating, focusing and monitoring the beam. They will be on adjustable mounts that can position them to a tolerance of a few micrometres.
At that level, the alignment can be affected by changes in temperature and ground motion, which have to be compensated by actively moving the supports.
Eighteen organisations are involved in the work, half of them in industry.
The researchers created a test bed where a copper-beryllium wire, 0.1 millimetres in diameter, took the place of the electron beam. They aligned the components to the wire and then used three different methods to measure the position of the wire with respect to the floor mountings.
The position was first measured with an industrial coordinate measuring machine (CMM) equipped with an optical sensor.
“We needed to adapt the CMM with a sensor to measure the form and position of the wire with no contact,” says Mainaud Durand. “This was the first time something like that has been done.”
They also adapted two other techniques frequency scanning interferometry and micro-triangulation that are cheaper and more portable than a CMM and could be used in the collider tunnel.
In addition, the team developed a nano-positioning device to adjust the position of components to sub-nanometric accuracy and a new kind of seismic sensor.
CLIC is still in the development phase and if approved will not be built before the 2030s, but the techniques developed in PACMAN will be used when the LHC is upgraded with new components in 2024 in the HL-LHC project.
Breadth of experience
Outside the world of particle physics there are possible applications in the construction of the ITER fusion reactor, large astronomical telescopes and spacecraft. Precise alignment is also required in medical accelerators for proton therapy machines. The stretched wire method could have applications in tunnelling and other civil engineering projects.
“The students now completing their PhDs have gained an extraordinary breadth of expertise,” says Mainaud Durand. “Most of them have been offered jobs, three of whom have already started, one in Fermilab the leading US particle physics laboratory and two in European industry.”
Four of the researchers are female and the promotion of physics and engineering as a career for women was a major part of the project’s outreach activities in schools and elsewhere.
“The work carried out in the PACMAN project was really a team effort in multidisciplinary fields,” she adds. “It was not only a challenging technical project but also a human adventure, a fantastic opportunity for the students, the partners and CERN.”