Pickin’ up good vibrations
In preparation for the civil engineering work on the HL-LHC, vibration measurements were carried out at the LHC’s Point 1 last month. These measurements will help evaluate how civil engineering work could impact the beam, and will provide crucial details about the site’s geological make-up before construction begins.
A seismic truck at Point 1 generated wave-like vibrations measured by EN/MME.
From carrying out R&D to produce state-of-the-art magnets to developing innovative, robust materials capable of withstanding beam impact, the HL-LHC is a multi-faceted project involving many groups and teams across CERN’s departments. It was in this framework that the project management mandated CERN's Mechanical and Materials Engineering (EN/MME) group to measure the propagation of vibrations around Point 1. Their question: can civil engineering work for the HL-LHC – the bulk of which is scheduled for LS2 – begin while the LHC is running? Although the civil engineering work for the LHC was carried out during LEP operation, the LHC is much more sensitive to vibrations.
“While the main civil engineering work will, of course, take place during LS2, we would like to identify which parts of it could be carried out during LHC operation,” says Paolo Fessia, who is in charge of the HL-LHC integration. It is a tricky endeavour. Imagine a massive digger pounding away just 40 metres from the beam. Meanwhile, LHC beam stability would be needed at the micron level. Could this be feasible?
“Over this past year, we have performed a number of vibration studies,” says Michael Guinchard, who is in charge of the mechanical measurement lab (EN/MME). “At SM18, we generated artificial vibrations on the floor and looked at their effect on the active part of an LHC quadrupole magnet. We also carried out similar measurements close to AWAKE, where we used their parallel tunnels to our advantage. We placed a shaker in one tunnel – creating known vibrations – and then we looked at the response in the other tunnel. This allowed us to examine the attenuation through the earth separating the two tunnels.” With these measurements in hand, the team was well prepared to study vibration propagation at Point 1.
They began in the ATLAS UL16 tunnel, installing four geophone sensors to measure vibrations in the ground and convert the signal into an electronic signal. Further sensors were placed around the vibration sources at the surface, linked to the underground geophones using the LHC “White Rabbit” synchronisation network. This allowed the team to look at the effects of the vibrations at the same time.
“The first vibrations we studied were generated by a core-drilling machine, used to examine the site’s geological make-up,” says Paolo. “This information will be essential for designing and constructing the new underground caverns and technical galleries needed for the HL-LHC, as construction companies need to know exactly what they will find when they dig (hard rock, sand, water, etc.). While this is the main purpose of the drilling, it has also been used to study the effect of pulsed vibrations.” The drilling activities are the responsibility of the GS-SE group, which will also be in charge of the follow-up of the execution of all the civil engineering work for the HL-LHC.
A few days later, the seismic truck rolled in. This unique, 24-tonne machine uses its entire weight to push down on the ground, generating wave-like vibrations from 4 up to 100 times per second. “We created waves with a wide range of frequencies and looked at their attenuation,” says Michael. Working in collaboration with the BE-OP team, measurements with beam were also taken and will provide a valuable data set for more detailed analysis.
So, while the HL-LHC is still many years away from operation, its impact on the LHC can already be felt… in this case, quite literally!
