Starry nights over CLIC

To meet the alignment requirements for CLIC, the future linear accelerator project, CERN’s surveyors have started an unprecedented campaign of measurements.


Sébastien Guillaume during the zenith camera installation.

On a beautiful summer’s night, Sébastien Guillaume sets up his camera equipment in the middle of the countryside and turns his lens towards the heavens, ready to spend a night photographing stars. He is neither an amateur astronomer, nor a contemporary artist looking for inspiration for an unusual work of art. The young man is in fact a PhD student in geodesy from ETH Zurich, doing research in the domain of ultra-precise measurements. To this end, he has embarked on an unprecedented campaign of measurements, together with the CERN surveyors. His goal: to show that the components of CLIC, the future electron-positron collider project, can be aligned with a precision of 10 microns (one-hundredth of a millimetre) over a distance of 200 metres. This objective may sound a little bland, but in reality achieving such a level of precision will require incredibly complex technology. Take the LHC for example: its components are aligned with a precision of approximately 0.15 mm over 100 m, which is already a remarkable achievement. CLIC needs 15 times greater alignment precision. That is the challenge facing CERN’s surveying section (part of the BE/ABP Group).

One of the thorniest problems they have to face is that their measuring instruments are sensitive to the Earth’s gravity. That affects in particular the ultra-precise optical levels and theodolites used for aligning accelerator components. The alignment systems currently envisaged for CLIC are no exception: long wires for horizontal positioning, and hydrostatic levelling sensors for vertical positioning. Now, the accelerator design is defined in a system of Cartesian coordinates that is independent of gravity. It is therefore important to describe the gravitational field precisely within this purely mathematical system. The job would be straightforward if the gravitational field formed a perfect surface, e.g. an ellipsoid. But this is not the case; rather, the field varies due to the uneven way in which masses are distributed at the surface of the Earth and below. In order to correct their instruments’ measurements for the effects caused by these gravitational anomalies, surveyors therefore need to determine the shape of the "geoid", which is a surface formed by all the points at which the gravitational potential energy is the same (corresponding to the value of the potential energy at mean sea level). The shape of the geoid in this region is currently known sufficiently well to permit a relative precision of a few tenths of a millimetre for several hundred metres. "Our hope is to determine its shape to within a few microns," says Mark Jones, one of CERN’s surveyors, who is supervising the project. The necessary measurements will be made over a limited surface. To test the feasibility, the team decided to work directly above an 800-metre tunnel called TZ32. The tunnel, which was excavated as part of the LEP civil engineering preparations, connects to point 3 on the LHC’s underground ring.

But let’s get back to our stars: what are they doing in such a down-to-earth measurement project? The precise position of certain stars is known to within a millisecond of arc (that’s roughly the angle formed by a fly-sized object, seen from a distance of a thousand kilometres!). By photographing these stars with a zenith camera, it is possible to determine the precise direction of the local vertical (determined by gravity) for a point on the surface that can be located to within one or two centimetres by GPS. By comparing this local vertical (which is physical) with a mathematical model (an ellipsoid) of the gravitational field, it is possible to determine the difference between the two (the deviation of the vertical). This measurement can then be combined with gravity measurements that have already been carried out and with a geological density model to establish a new geoid. "There are only two cameras of this type in the world," notes Guillaume. "But we have tried to modify this one to make it even more precise."

The PhD student plans to take 80 measurements along the 800 metres, meaning quite a few sleepless nights over the summer period. Once the measurements are done, he will need to transfer them down to a point 100 metres below. In this way, he hopes to demonstrate that the required degree of precision can be achieved. "The experts are a bit dubious, "concedes Sébastien Guillaume, but he doesn’t let that concern him. It won’t be the first time that CERN’s surveyors have risen to a challenge, although it may be the first time that they aim for the stars!