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Surveying the surveyors

7 July 2020

The need at CERN to align components within a fraction of a millimetre demands skills and tools beyond the scope of normal surveyor jobs.

Alban Vieille

A career as a surveyor offers the best of two worlds, thinks Dominique Missiaen, a senior member of CERN’s survey, mechatronics and measurements (SMM) group: “I wanted to be a surveyor because I felt I would like to be inside part of the time and outside the other, though being at CERN is the opposite because the field is in the tunnels!” After qualifying as a surveyor and spending time doing metrology for a cement plant in Burma and for the Sorbonne in Paris, Missiaen arrived at CERN as a stagier in 1986. He never left, starting in a staff position working on the alignment of the pre-injector for LEP, then of LEP itself, and then leading the internal metrology of the magnets for the LHC. From 2009–2018 he was in charge of the whole survey section, and since last year has a new role as a coordinator for special projects, such as the development of a train to remotely survey the magnets in the arcs of the LHC.

“Being a surveyor at CERN is completely different to other surveying jobs,” explains Missiaen. “We are asked to align components within a couple of tenths of a millimetre, whereas in the normal world they tend to work with an accuracy of 1–2  cm, so we have to develop new and special techniques.”

A history of precision

When building the Proton Synchrotron in the 1950s, engineers needed an instrument to align components to 50 microns in the horizontal plane. A device to measure such distances did not exist on the market, so the early CERN team invented the “distinvar” – an instrument to ensure the nominal tension of an invar wire while measuring the small length to be added to obtain the distance between two points. It was still used as recently as 10 years ago, says Missiaen. Another “stretched wire” technique developed for the ISR in the 1960s and still in use today replaces small-angle measurements by a short-distance measurement: instead of measuring the angle between two directions, AB and AC, using a theodolite, it measures the distance between the point B and the line AC. The AC line is realised by a nylon wire, while the distance is measured using a device invented at CERN called the “ecartometer”.

Invention and innovation haven’t stopped. The SMM group recently adapted a metrology technique called frequency sweeping interferometry for use in a cryogenic environment to align components inside the sealed cryostats of the future High-Luminosity LHC (HL-LHC), which contract by up to 12 mm when cooled to operational temperatures. Another recent innovation, in collaboration with the Institute of Plasma Physics in Prague that came about while developing the challenging alignment system for HIE-ISOLDE, is a non-diffractive laser beam with a central axis that diverges by just a few mm over distances of several hundred metres and which can “reconstruct” itself after meeting an obstacle.

The specialised nature of surveying at CERN means the team has to spend a lot of time finding the right people and training newcomers. “It’s hard to measure at this level and to maintain the accuracy over long distances, so when we recruit we look for people who have a feeling for this level of precision,” says Missiaen, adding that a constant feed of students is important. “Every year I go back to my engineering school and give a talk about metrology, geodesy and topometry at CERN so that the students understand there is something special they can do in their career. Some are not interested at all, while others are very interested – I never find students in between!”

We see the physics results as a success that we share in too

CERN’s SMM group has more than 120 people, with around 35 staff members. Contractors push the numbers up further during periods such as the current long-shutdown two (LS2), during which the group is tasked with measuring all the components of the LHC in the radial and vertical direction. “It takes two years,” says Jean-Frederic Fuchs, who is section leader for accelerators, survey and geodesy. “During a technical stop, we are in charge of the 3D-position determination of the components in the tunnels and their alignment at the level of a few tenths of a millimetre. There is a huge number of various accelerator elements along the 63 km of beam lines at CERN.”

Fuchs did his master’s thesis at CERN in the domain of photogrammetry and then left to work in Portugal, where he was in charge of guiding a tunnel-boring machine for a railway project. He returned to CERN in the early 2000s as a fellow, followed by a position as a project associate working on the assembly and alignment of the CMS experiment. He then left to join EDF where he worked on metrology inside nuclear power plants, finally returning to CERN as a staff member in 2011 working on accelerator alignment. “I too sought a career in which I didn’t have to spend too much time in the office. I also liked the balance between measurements and calculations. Using theodolites and other equipment to get the data is just one aspect of a surveyor’s job – post-treatment of the data and planning for measurement campaigns is also a big part of what we do.”

With experience in both experiment and accelerator alignment, Fuchs knows all too well the importance of surveying at CERN. Some areas of the LHC tunnel are moving by about 1 mm per year due to underground movement inside the rock. The tunnel is rising at point 5 (where CMS is located) and falling between P7 and P8, near ATLAS, while the huge mass of the LHC experiments largely keeps them at the same vertical position, therefore requiring significant realignment of the LHC magnets. During LS2, the SMM group plans to lower the LHC at point 5 by 3 mm to better match the CMS interaction point by adjusting jacks that allow the LHC to be raised or lowered by around 20 mm in each direction. For newer installations, the movement can be much greater. For example, LINAC4 has moved up by 5 mm in the source area, leading to a slope that must be corrected. The new beam-dump tunnels in the LHC and the freshly excavated HL-LHC tunnels in points 1 and 5 are also moving slightly  compared to the main LHC tunnel. “Today we almost know all the places where it moves,” says Fuchs. “For sure, if you want to run the LHC for another 18 years there will be a lot of measurement and realignment work to be done.” His team also works closely with machine physicists to compare its measurements to those performed with the beams themselves.

It is clear that CERN’s accelerator infrastructure could not function at the level it does without the field and office work of surveyors. “We see the physics results as a success that we share in too,” says Missiaen. “When the LHC turned on you couldn’t know if a mistake had been made somewhere, so in seeing the beam go from one point to another, we take pride that we have made that possible.”

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