Ground Motion in Lep and Lhc

It is well known that large colliders, linear or circular, with small beam sizes are prone to ground motion effects.The effect of seismic perturbations on machine performance has been the subject of several studies[1,2,3,4]. The object of this paper is to report on the results obtained with a dedicated beam position monitor, concerning ground motion effects on the LEP beams. Since the ultimate objective of this study is the LHC it is appropriate to say that LEP is used as a test bench for LHC. The organisation of the report is as follows. It starts with some properties of ground vibrations which are relevant to the observation of the phenomenon in LEP. Then the transformation of ground motion power into beam motion power is calculated. Based on the properties of the transverse monitor the magnitude of the phenomenon can be measured with sufficient precision. The results of the observations will be presented both at the betatronfrequencies and at very low frequencies centred around the common mode. They are compared with published data on direct ground motion to check the plausibility of the hypothesis that ground motion is indeed the primary cause of the observed signals. Finally a discussion on the influence of the effect on the LHC beams is presented.