The alignment of the LHC

The Large Hadron Collider (LHC) has been aligned using both classical and non-standard techniques. The results of these alignments were seen on September 10, 2008 when the beam made several turns in the machine with very few correctors activated. This paper will present the different steps of the alignment as well the techniques used to obtain the alignment accuracy required for beam operation. The correlation of these results with the position recorded by the beam position monitors (BPM) will be presented. THE GEODETIC NETWORK The geodetic network is composed of about 500 points sealed in the floor. Their position was determined from the alignment of the main quadrupole magnets (MQ) of the Large Electron Positron Collider (LEP), which was in operation between 1980 and 2000 and located in the same tunnel as the LHC. The MQ position was: • determined initially from a surface network of geodetic points using GPS, their co-ordinates being transferred to the tunnel via eight civil engineering shafts and propagated on tripods along the tunnel. • gradually improved during the LEP shut-downs and their position in 2000, before its dismantling, was considered by the Survey team and the physicists as the best that could be achieved and therefore a good reference for the alignment of the LHC. Levelling measurements, horizontal angles, offsets with respect to a stretched wire, gyroscopic measurements as well as mekometer range-finder were combined and compensated using the least square method in order to determine the co-ordinates of the LHC geodetic network. The absolute 1 σ accuracy of the geodetic points in X and Y is considered to be ±2 mm for points close to the shafts and ±4.5 mm for ones located in the middle of two shafts. In the Z direction, the 1 σ accuracy is ±2 mm. Eight deep references have been anchored in the rocks 20-25 m lower than the tunnel level and are located very close to each sector extremity. They provide stable references during levelling measurements. Their first height determination was performed during the complete levelling of the geodetic network in 2004. THE FIRST ALIGNMENT All components have been aligned with respect to the geodetic network in order to achieve relative accuracy of 0.25 mm [1]. It is realised in several steps: • in the vertical plane with direct levelling done with the NA2 optical level, • the roll angle is adjusted using a gauge equipped with an inclinometer and installed on two fiducials, • in the horizontal plane with offsets to a stretched wire and distances measured with the tacheometer TDA5005. To avoid steps between consecutive magnets, an initial local smoothing was performed prior to performing the magnet interconnection. THE SMOOTHING OPERATION A second smoothing is applied to the relative component positions to minimise steps between magnets that may create perturbations to the particle beam. Further measurements are thus done directly on the fiducials located on the components without using the geodetic network since the absolute position can be considered as established at this stage. The goal is to obtain a 1 σ deviation with respect to a smooth curve of 0.15 mm in a 150 m long sliding window [2]. This operation is important particularly for the aperture, and therefore concerns all magnets. There are three steps: a control and adjustment of the roll angle followed by a vertical and an horizontal measurement. Roll Angle Measurement The roll angles were measured for all the sectors under warm conditions except for sector 4-5 and 7-8. Table 1 shows the deviation from the nominal after the initial alignment and during the final smoothing. A slight degradation of the magnet roll angle between the initial and final alignment can be seen. The average deviation changes very little while the standard deviation changes from 0.045 to 0.085 mrad. Magnet deviations larger than 0.1 mrad were corrected. Table 1: roll angle deviations from nominal Initial alignment During smoothing Sector Avg (mrad) Stdev (mrad) Avg (mrad) Stdev (mrad) 1-2 0.00 0.04 -0.01 0.07 2-3 -0.01 0.04 0.00 0.06 3-4 0.00 0.06 -0.02 0.09 4-5 -0.01 0.04 0.03 0.09 5-6 0.00 0.04 0.01 0.08 6-7 0.00 0.04 -0.01 0.10 7-8 0.00 0.05 -0.04 0.08 8-1 0.01 0.05 0.05 0.11 Vertical Measurement The vertical measurements are made in a double run process, using the digital level DNA03 and a CERN made illuminated staff. The sequence applied to the arcs is illustrated in Figure 1. Proceedings of PAC09, Vancouver, BC, Canada WE4GRC04