An Active Pre-Alignment System and Metrology Network for CLIC

The pre-alignment tolerance on the transverse positions of the components of the CLIC linacs is typically ten microns over distances of 200 m. Such tight tolerances cannot be obtained by a static one-time alignment because normal seismic ground movement and cultural noise associated with human and industrial activity quickly creates significant errors. It is therefore foreseen to maintain the components in place using an active-alignment system which will be linked to a permanent metrology and geodetic network. This report describes the overall philosophy and implementation of such a system and proposes one possible solution for active-alignment which uses stepping-motors to move components and stretched-wires as reference lines. Special sensors have been developed to measure the position of the components with respect to the reference lines, and to measure local tilt and relative vertical position. An in-depth analysis has been made of the repercussions on the alignment system of perturbing effects due to the attraction of the moon and the sun, and of the presence of nearby geological masses. The active-alignment system was used to maintain the components of the 30 GHz Two-Beam Test Accelerator in position in the CLIC Test Facility CTF2 as a practical demonstration of successful operation in an accelerator environment. The hardware and control system that was built for this application are described together with the results obtained.