Design and Performance Optimization of the LHC Collimation System

The Large Hadron Collider (LHC) is presently under construction at CERN. The LHC is a circular accelerator that stores proton beams and accelerates them to a 7 TeV beam energy. The required bending fields are achieved with super-conducting magnets. The stored proton beams are collided in experimental detectors and produce a design luminosity of 1E+34 cm-2.s-1. Every storage ring encounters unavoidable proton losses. The protons that diffuse into the so-called beam halo can touch accelerator components. In order to avoid quenches of the superconducting magnets, the halo protons must be removed before reaching the magnets. This is achieved with a multi-stage cleaning system, built out of two-sided collimators that are located at adequate positions in the machine. Due to the high stored beam intensity (required for high luminosity), the efficiency of the LHC beam cleaning must be much better than in any other exisiting machine: not more than 0.00002% of protons hitting the collimators may escape and impact on any meter of super-conducting magnet at 7 TeV. Detailed simulations of realistic operational conditions were performed to address the performance of the cleaning system. Beam loss maps show the distribution of proton losses around the machine down to the 10 cm level. The simulations were used for optimizing the system design and improving its overall efficiency by more than a factor of 10. The final performance of the so-called "phase 1" collimation system is discussed for the ideal LHC machine and for a case with a realistically perturbed orbit. For the phase 1 system, it is predicted that the allowable LHC intensity is limited to 43% (ideal case) or 27% (nominal orbit) of the nominal design value. The limitations and the assumptions used to derive theses limits are explained, including a list of characteristic loss locations around the ring. A prototype LHC collimator was tested in the SPS with LHC-like proton beam conditions (injection energy). The control and beam-based alignment of the collimator gap was demonstrated down to the 50 um level. Interesting results on the beam dynamics for halo particles were obtained, including slowly decaying beam losses after movement of collimator jaws. The robustness of the CFC (fiber-reinforced graphite) collimator jaws was experimentally confirmed and results were compared with predictions from numerical models.