Design and Performance Evaluation of Nonlinear Collimation Systems for CLIC and LHC

The beam collimation systems are an essential part of the high energy colliders. A collimation system should remove beam halo to reduce detector background and ensure the machine protection, thus minimizing the activation and damage of sensitive accelerator components. The mechanical and optics design of collimation systems is not simple, and they should fullfil some often conflicting constraints and requirements: high cleaning efficiency, high mechanical robustness, and low wakefields (impedances). The conventional collimation systems are generally based on linear optics. Nevertheless, several alternative advanced concepts on collimation have been proposed in the literature. In this thesis report we have studied in detail nonlinear collimation systems. These are based on a general scheme with a skew sextupole pair, which can be adapted to both linear and circular colliders. In particular we have designed a nonlinear collimation system for the Compact Linear Collider (CLIC). This system fullfils the function of machine protection against mis-steered or errant beams with energy offset higher than 1.5 % of the nonimal energy 1.5 TeV. The performance of this collimation system has been evaluated by means of tracking studies, and compared with that of the conventional baseline linear collimation system. Since the collimation requirements for linear colliders designed to operate at center-of-mass energy around TeV are similar to tho se for the Large Hadron Collider (LHC) at collision beam energy 7 TeV, it is thus close thought to apply a similar LHC nonlinear collimation scheme as that designed for CLIC. We have explored this possibility, and have proposed an alternative nonlinear system for the Phase-II betatron cleaning in the LHC. Its performance and cleaning efficiency have further been evaluated by tracking studies. Moreover a comparison of the features of the nonlinear collimation system and the linear collimation system has been made for the LHC.