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The Final Alignment of the LHC

The Large Hadron Collider (LHC) project is progressing well. All the cryomagnets and the elements of the long stretched sections (LSS) have been pre-aligned and are connected and the cooling of the machine is under way. The final alignment of the machine is planed when the machine is cold. This paper presents the methodology used for this phase of the alignment. The smoothing process is described, as well as the results obtained based on the various sectors already aligned. THE LHC PROJECT The LHC, a circular collider of 27 km circumference, is composed of more than 2000 “cold” components interconnected in which protons will circulate in two cold bores tubes and collide in four interaction points. It is divided in eight sectors, each one comprising a standard Arc and 2 half LSS as shown on figure 1. Figure 1: The LHC project THE FIRST ALIGNMENT The initial alignment of the components is done using a geodetic reference network which was determined from the position of the main quadrupoles magnets of the Large Electron Positon (LEP) located in the same tunnel as the LHC. The alignment of the LEP, which run from 1989 to 2000, was improved during the various shut-downs and its position in 2000, before its dismantling, was considered as the best that could be achieved. Levelling measurements, horizontal angles, offsets with respect to a stretched wire, gyroscopic measurements as well as mekometer distances were combined in order to determine the co-ordinates of the geodetic network. Eight deep references have also been anchored in the rocks 20-25 m lower than the tunnel level. They are located very close to each sector extremity and will be used as stable references during the levelling measurements. The first determination of their heights was done during the complete levelling of the geodetic network in 2004. All the components have been aligned from this geodetic network in order to achieve a relative accuracy of 0.2 mm at 1 σ [1]. It is realised in two steps: • In the vertical plane with direct levelling done with the NA2 optical level, • In the horizontal plane with offsets with respect to a stretched wire and distances measured with the TDA5005. In order to avoid any steps between consecutive magnets, a local smoothing was also realised. The alignment of the components of the arcs is now totally completed while it has been realised at 95% in the LSS. THE SMOOTHING OPERATION The smoothing is the operation during which the relative position of the components is improved in order to avoid any steps between magnets that could create perturbations to the particle beam. Therefore the measurements are done directly on targets located on the components and the geodetic reference network is no longer used, the absolute position being considered as already achieved. The goal is to obtain a deviation w.r.t a smooth line of 0.15 mm at 1 σ in a 150 m long sliding window [2]. Alice All the cold magnets are interconnected by many pipes and relatively rigid bellows and therefore any movement of magnet, due to ground motion for example, could produce big damage in this interconnection. For this reason it has been decided to measure, and realign if necessary, all the components and not only the main quadrupoles as it was the case for the previous accelerators like the LEP. The LHC final alignment is as important for mechanical constraints as for physics purposes. Initially, the smoothing measurements were planned to be done the magnets being at a temperature lower than 80K. At this temperature all the mechanical movements and constraints due to the cool-down have occurred. But due to planning and co-activities reasons some sectors were measured at cold while for some others, the measurements were anticipated and done under warm conditions. The operation is done in three steps: a control and adjustment of the roll angle, then a vertical and a horizontal measurement. THE ROLL ANGLE MESUREMENTS The roll angles, also called tilt angles, were measured for all the sectors under warm conditions except for sector LHC-b CMS Atlas Sector 56 Sector 45