CLIC pre-alignment — status and remaining challenges

The Compact Linear Collider (CLIC) is a study of a 3 TeV linear electron–positron ($e^+e^-$) accelerator and is a successor candidate for CERN’s Large Hadron Collider. The CLIC luminosity target is $5.9 × 10^{34}$ cm$^{−2}$ s$^{−1}$, which causes unprecedented pre-alignment requirements of its main linear accelerator (main linac). Along the 50 km long tunnel, the main components of any 200 m long section have to be positioned within 10 µm from a straight reference line. The pre-alignment challenge has been studied at CERN since the 1990s, and the main technical challenges have been solved. This article summarizes the positioning strategy and presents it to an audience outside the particle accelerator community. The methods can be of interest especially in the field of large-scale metrology. The positioning strategy consists of several steps or subsystems. The development of a straight reference line over tens of kilometers allows absolute positioning of accelerator components, while a process called fiducialization defines component reference axes with regard to alignment targets. Emphasis is on a support pre-alignment network that acts as a link between the straight reference line and fiducialization. The subsystems and remaining challenges in their development are presented. The chosen strategy’s potential is demonstrated experimentally by building a short test setup.