The LHC at level best
On 10 March, a team of CERN surveyors descended into the LHC tunnel. Their aim: to take measurements of the height of the LHC magnets to see how geological shifts might be affecting the machine and to take reference positions of the machine before the interconnects are opened.
CERN surveyors take levelling measurements of the LHC magnets during LS1.
The LHC tunnel is renowned for its geological stability: set between layers of sandstone and molasse, it has allowed the alignment of the world’s largest accelerators to be within sub-millimetre precision. But even the most stable of tunnels can be affected by geological events. To ensure the precise alignment of the LHC, the CERN survey team performs regular measurements of the vertical position of the magnets (a process known as “levelling”).
Over the past month, the team has been taking measurements of the LHC before the temperature of the magnets reaches 100 K, beyond which there may be some mechanical movements. As no data could be gathered while the machine was in operation, these measurements will provide the clearest picture yet of the machine’s position at the end of the run. The team used a so-called “fast levelling” technique, which involves measuring every second magnet in order to complete the survey as quickly as possible and to reduce the influence of the environmental conditions (temperature gradients, air currents, etc.) that affect the observations made with an optical level. At these points, technicians were able not only to measure the height of the magnets but also to make immediate height comparisons with the previous magnets. No magnet realignments are being carried out at this stage.
“By comparing these measurements with the base measurements taken during the 2008-2009 shutdown, we will soon have an accurate picture of how ground disturbances may have affected the machine,” explains Dominique Missiaen, leader of the BE Department section responsible for large-scale metrology. “This comparison will also help us predict possible future deviations and deterioration of the relative positions between magnets.”
The next series of levelling measurements will be taken at the end of the first long shutdown, once the work on the LHC interconnects has been completed. Technicians will then perform complete levelling measurements of the machine, measuring every LHC magnet and adjusting the magnet heights when they see significant variations. “The main aim of this levelling is not to have perfect measurements of the height of the machine, but rather to have an accurate evaluation of each magnet with respect to its neighbours,” says Dominique. “We will ensure the magnets are all smoothly aligned for the restart of the machine, as even the smallest of differences can affect the beam orbit.”
| Levelling LEP  Vertical movement of LEP magnets, which were corrected for during every annual shutdown. During the LEP era, levelling measurements were taken in the tunnel every year. In 1992, the first measurements revealed significant ground movement in the middle of Sector 7-8 that had shifted the LEP magnets down by some 18 mm. Over the course of 2-3 years, about 100 magnets had to be manually adjusted to compensate for this movement until the “fault” had been corrected and a “smoothed position” for the magnets had been achieved. The phenomena reappeared during the first measurement of the LHC magnets in 2007 and, consequently, this sector is measured and realigned every year during the winter technical stop. |
