High Precision Survey and Alignment of Large Linear Accelerators

Future linear accelerators require new survey techniques to achieve the necessary alignment precision. For TESLA, the demanded accuracy for the alignment of the components is 0.5mm horizontal and 0.2mm vertical, both on each 600m section. Other proposed linear colliders require similar accuracies. These demands can not be fulfilled with common, open-air geodetic methods, mainly because of refraction in the tunnel. Therefore the RTRS (Rapid Tunnel Reference Surveyor), a 25m long measurement train performing overlapping multipoint alignment on a regular tunnel reference network is being developed. Two refraction-free realizations of this concept are being developed at the moment: GeLiS measures the horizontal co-ordinates using multipoint alignment with stretched wires as strightness reference. In areas of the tunnel where the accelerator is following the earth curvature GeLiS measures the vertical co-ordinate using a new hydrostatic levelling system (HLS). LiCAS is based on laser straightness monitors (LSM) combined with frequency scanning interferometry (FSI) in an evacuated system. LiCAS measures both coordinates with respect to its LSM-beam and thus is suitable for geometrically straight tunnel sections. Both measurement systems will be placed on a train, which could do the reference survey autonomous or semiautonomous. GEODETIC TASKS A very high accuracy is demanded for the alignment of all accelerator components to run the linear collider successfully. The standard deviation of every component is postulated to be σl,h = 0.2mm in lateral and height over the maximum betatron wavelength (e.g. 600m). Because of the influence of refraction, this requirement can not be achieved with any open-air optical survey. CONCEPT A basic network of reference points fixed to the wall in an equidistance of approximately 5m is installed in the tunnel. The alignment of the accelerator is split up in two major steps: 1) The reference points will be determined by an automated system (Rapid Tunnel Reference Surveyor, RTRS) in 3D space. The RTRS is described here. 2) The coordinates are transferred to the machine components with a tacheometer. This step is geodetic standard work and is not described in this paper. It is planned to integrate this second step into the RTRS. Multipoint Alignment With the technique of multipoint alignment the effects of refraction can be eliminated or reduced if a mechanical structure or a laser beam in vacuum is used as straightness reference. Multipoint alignment in a simplified 2D context means that the lateral distances si-1, si and si+1 (see Fig. 1) between the straightness reference and several reference points are measured. Together with the distances si-1,i and si,i+1 the angle β and the distances d can be calculated. This is done sequentially for every reference point in the tunnel (see Fig. 1). A traverse then is used to estimate the coordinates of the reference points. LiCAS extends this concept to a 3D-measurement si-1,i si,i+1 s i -1 s ii s i + 1 i di-1,i di,i+1 tunnel wall