Transverse alignment tolerances of about 10 m m (r.m.s.) are required for the main linac in order to limit the emittance blow-up due to transversely deflecting wakefields to reasonable values. The entire accelerator is mounted on a concrete base grounded to the floor. This base runs the full length of the tunnel (Fig. 1.2). An active alignment system using precision micro-movers is proposed to achieve these tight tolerances. The accelerating structures and beam-position monitors of each linac are supported by precisely pre-aligned V-blocks sitting on SiC girders. The ends of two adjacent girders are connected by swivel-joint link rods to a common platform that is driven by three remote-controlled 0.1 m m resolution stepping motors. The length of one girder is 2.23 m. The quadrupole supports are independent of the girder supports and are driven by five identical stepping motors. The quadrupole support platforms are located above the girders. Aligning the linac in this case means aligning the articulation points of the girders, and the magnetic components. The articulation points are equipped with, and aligned by, an overlapping optical off-set measurement system (RASNIK). This system ensures a relative precision between articulation points of the order of about 1 m m over distances of a few metres. At intervals of about 36 m (16 girders), the articulation points are attached via sensors to a stretched wire positioning system (WPS) which assures a relative precision of 10 m m over distances of about 200 m. The WPS consists of two stretched wires running parallel to the linacs and overlapping over half their own length. To be able to consider the wire as a reference in the vertical plane it is necessary to determine the catenary of the wire with great accuracy. The RASNIK system is preferred to the WPS for local positioning of the girders (within the 36 m intervals) because it is considerably cheaper. All magnetic elements are equipped with two-axes tilt monitors [making up the tilt monitoring system (TMS)] and with WPS sensors, and are positioned using the stretched wire. The WPS itself is attached to precision support plates which sit on the concrete block at 16-girder intervals (35.68 m). These precision plates also support a hydrostatic levelling system (HLS) which provides the reference for all height measurements and in particular for the measurement of the catenary of the wire. The corresponding geometrical reference networks are schematically shown in Fig. 2.30.
Fig. 2.30 : Schematic plan view of the two geometrical reference networks.
The support plates have standard CERN reference bore holes which enable them to be positioned with classical surveying techniques to a relative precision of about 0.1 mm. This classical part is done during the civil engineering stage in the following way. First, a system of reference pillars is created outside the tunnel, on the surface along the projected line of the linacs, at intervals of about 3.6 km using global positioning surveying techniques (GPS) with an absolute precision of about 1cm and a relative precision of about 1mm. These surface pillars are then used to create a set of underground reference pillars with the same relative precision (~1 mm) using vertical drop techniques. The underground pillars are themselves then used to create a set of temporary civil engineering reference pillars using gyroscopic and distance techniques, which are built at intervals of about 36 m along the length of the linac as the tunnelling work progresses. This temporary reference system is used to position the above-mentioned reference support plates on the concrete base to a precision of about 1 cm. The last step is to use adjustment screws on the support plates to position the plates with respect to the underground reference pillars to a precision of about 1 mm and a relative precision of 0.1 mm. All the instruments measuring these networks during this pre-alignment stage continuously provide information to a computer system which, following a statistical analysis, calculates the actual positions of the components, and then deduces and controls the movements to be made. Once the linacs have been pre-aligned in this way, beam is injected into the machine, and the more accurate signals produced by the sub-micron-resolution beam-position monitors take over the alignment process. The measuring system used for pre-alignment, however, remains active in the background to provide a position memory of the components so that the pre-alignment condition can be reinstated for beam start-up after a shutdown.