Transverse and longitudinal beam collimation in a high-energy proton collider (LHC)

In the Large Hadron Collider (LHC), particles from the beam halo might potentially impinge on the vacuum chamber, effecting harmful transitions of the superconducting magnets (quenches). This can be prevented by the collimation system which confines the particle losses to special, non superconducting sections of the machine. Due to the high energy and intensity of the LHC, any removal system must attain an unprecedented efficiency. The cleaning system was designed on the basis of purely geometric and optical models which neglect non linear effects and assume perfectly absorbing materials. In a second step, true scattering in matter is considered. A series of machine developments (MD) were carried out in 1996-7 with the principal aim of validating the design assumptions. A collimation system comparable to that of the LHC was employed. The predictions of the numerical model used to compute the LHC collimation system efficiency were compared with the data acquired during the measurement sessions. The experiments pointed to areas for further refinement of the design model and highlighted the physical constraints which one might expect in such an elaborated cleaning system. They revealed the critical importance to the simulation model of accurate values for the diffusion velocity of the halo particles and of the aperture limitations in the machine.In addition, the experiments confirmed the necessity of controlling the closed orbit excursions in the collimation sections. They also revealed the sensitivity of the cleaning to small deviations of the linear optic functions. Insights gained from the MD sessions allowed further refinements to the LHC collimation system model. The results of subsequent simulations indicate that the ultimate efficiency required for optimum operation of the LHC can be reached.