Implementation and maintenance of the alignment of accelerators (presented as "sinking and ageing")

The precise alignment of the components of particle accelerators, and of their experimental equipment, requires special techniques derived from geodesy. The increasing size of these machines and the demand for tighter and tighter tolerances have led to the development of special instruments and methods. The present needs, in terms of relative accuracy along the beam lines, are around 0.1 mm (rms)—and they are fully satisfied by this geodetic metrology. But nothing is absolutely stable and perfectly rigid, neither the ground nor the structures : various geomechanical forces and progressive changes of mechanical properties in some material affect the positioning of the components (hence the subtitle), and the alignment is to be maintained regularly. This paper describes the basic concepts and techniques used for that purpose.